513 copy The Meteoritical Society 2007 Printed in USA
Meteoritics amp Planetary Science 42 Nr 45 513ndash540 (2007)Abstract available online at httpmeteoriticsorg
Petrography geochemistry and alteration of country rocks from the Bosumtwi impact structure Ghana
Forson KARIKARI1 Ludovic FERRIEgraveRE1 Christian KOEBERL1Wolf Uwe REIMOLD2 and Dieter MADER1
1Center for Earth Sciences University of Vienna Althanstrasse 14 A-1090 Vienna Austria2Museum of Natural History (Mineralogy) Humboldt University Invalidenstrasse 43 D-10115 Berlin Germany
(Received 27 October 2006 revision accepted 18 January 2007)
AbstractndashSamples of the country rocks that likely constituted the target rocks at the 107 Myr oldBosumtwi impact structure in Ghana West Africa collected outside of the crater rim in the northernand southern parts of the structure were studied for their petrographic characteristics and analyzed fortheir major- and trace-element compositions The country rocks mainly meta-graywacke shale andphyllite of the Early Proterozoic Birimian Supergroup and some granites of similar age arecharacterized by two generations of alteration A pre-impact hydrothermal alteration often alongshear zones is characterized by new growth of secondary minerals such as chlorite sericite sulfidesand quartz or replacement of some primary minerals such as plagioclase and biotite by secondarysericite and chlorite A late argillic alteration mostly associated with the suevites is characterized byalteration of the meltglass clasts in the groundmass of suevites to phyllosilicates Suevite whichoccurs in restricted locations to the north and to the south-southwest of the crater rim contains meltfragments diaplectic quartz glass ballen quartz and clasts derived from the full variety of targetrocks No planar deformation features (PDFs) in quartz were found in the country rock samples andonly a few quartz grains in the suevite samples show PDFs and in rare cases two sets of PDFs Basedon a total alkali element-silica (TAS) plot the Bosumtwi granites have tonalitic to quartz-dioriticcompositions The Nb versus Y and Ta versus Yb discrimination plots show that these granites are ofvolcanic-arc tectonic provenance Provenance studies of the metasedimentary rocks at the Bosumtwicrater have also indicated that the metasediments are volcanic-arc related Compared to the averagesiderophile element contents of the upper continental crust both country rocks and impact breccias ofthe Bosumtwi structure show elevated siderophile element contents This however does not indicatethe presence of an extraterrestrial component in Bosumtwi suevite because the Birimian countryrocks also have elevated siderophile element contents which is thought to result from regionalhydrothermal alteration that is also related to widespread sulfide and gold mineralization
INTRODUCTION
The Bosumtwi crater in Ghana West Africa is centeredat 06deg30 N 01deg25 W The 107 Myr old impact structure(Koeberl et al 1997) is situated in the Ashanti region about32 km east of Kumasi the regional capital The Bosumtwiimpact structure is arguably the youngest and best-preservedterrestrial impact structure larger than 6 km in diameter(Scholz et al 2002 Earth Impact Database 2006) The craterhas a pronounced rim with a rim-to-rim diameter of about105 km The structure forms a hydrologically closed basinand is almost completely filled by Lake Bosumtwi(85 kilometers in diameter) The lake has a maximum depth
of about 80 m and the crater rim rises about 250ndash300 m abovethe lake level The area forms part of a tropical rain forestenvironment with warm climate high rainfall and highorganic activity Chemical weathering is intense leading tothe formation of locally thick lateritic soils
Studies over the past 50 years have confirmed that theBosumtwi crater structure was formed by meteorite impactThis is indicated by outcrops of suevitic breccia around thecrater (Chao 1968 Jones et al 1981) samples of which havebeen shown to contain the high-pressure quartz modificationcoesite (Littler et al 1961) as well as Ni-rich iron spherulesand baddeleyite in vesicular glass (El Goresy 1966 El Goresyet al 1968) In addition Koeberl et al (1998) described shock-
514 F Karikari et al
characteristic planar deformation features (PDFs) in quartzfrom suevitic breccia (see also several papers in this issue)
The Bosumtwi impact structure is also the likely sourcecrater for the Ivory Coast tektites (eg Gentner et al 1964Jones 1985 Koeberl et al 1997 1998) This correlation ismainly based on similarities in geochemical and isotopiccompositions of target rocks and tektites as well assimilarities in the ages of the impact melt from suevites and ofthe Ivory Coast tektites Boamah and Koeberl (2003) carriedout detailed petrographic and geochemical studies on suevitesfrom shallow drill cores obtained to the north of the craterResults of structural and geological mapping of the Bosumtwicrater rim were reported by Reimold et al (1998)Geochemical signatures of soils from north of the crater andtheir relationship to airborne radiometric geophysical datawere discussed by Boamah and Koeberl (2002)
Various geophysical studies of the Bosumtwi structurehave been carried out (Pesonen et al 1998 Karp et al 2002Scholz et al 2002) Koeberl and Reimold (2005) published anupdated and revised geological map of the Bosumtwistructure and environs with explanations containing moredetail about the impact structure The most recent research onthe crater was a multinational drilling project by theInternational Continental Scientific Drilling Program (ICDP)during the second half of 2004 when 16 continuous cores ofboth lake sediments (14) and impactites (2) were obtained forpaleoenvironmental and impact-cratering studies (papers inthis issue) respectively
Here we report the petrographic and geochemicalcharacteristics of the main country rock types that likelyconstituted the target rocks at Bosumtwi and of suevites at theBosumtwi impact structure and discuss their pre- and post-impact alteration
GEOLOGICAL SETTING
The Bosumtwi impact structure was excavated in rocksof the Early Proterozoic Birimian Supergroup (Jones at al1981) These rocks crop out extensively in Ghana IvoryCoast Burkina Faso and Mali The Birimian Supergroupconsists of two major units one unit composed dominantly ofmetamorphosed sedimentary rocks and the other unit mostlyrepresented by bimodal although largely tholeiitic volcanicsnow altered and metamorphosed to greenstones Thesegreenstones initially comprised mainly basalts and andesitesthat have now been metamorphosed to hornblende-actinoliteschist calcite-chlorite schist mica schist and amphibolitesThe metasedimentary unit comprises volcaniclastic rocksturbidite-related wackes argillitic rocks and chemicalsediments (Leube et al 1990) now metamorphosed intometa-graywackes phyllites schists and shales The volcanicactivity and sedimentation were submarine according toWoodfield (1966) with evidence provided by theinterfingering of and lateral variation between
metavolcanics and metasediments and the appearance ofrelict pillow structures in the metavolcanics
Traditionally in Ghana the metasedimentary unit wasconsidered older than the metavolcanic unit (Junner 1937Woodfield 1966 Moon and Mason 1967) This however wasin contrast to the view held in the neighboring francophonecountries (Hirdes et al 1996) Recent studies have shown thatthe two units were formed contemporaneously (eg Leubeet al 1990) On the basis of Sm-Nd isochron and model agesTaylor et al (1992) constrained the age of Birimiansupracrustal rocks of western Ghana to between 20 and23 Gyr Feybesse et al (2006) placed somewhat tighterconstraints on the age of these rocks around 210 to 215 Gyr
Two types of granitoid intrusions into the volcanicpackage and the basin sediments occur which are referred toas the Belt (Dixcove) granitoids and the Basin (Cape Coastand Winneba) granitoids respectively (Wright et al 1985)According to Wright et al (1985) the Belt-type granitoids aregenerally discordant to regional structures often unfoliatedand of predominant dioritic to monzonitic compositionwhereas the Basin-type granitoids are generally concordant toregional structures often foliated and of predominantgranodioritic composition Hirdes et al (1992) characterizedthe Belt-type granitoids as a) small- to medium-sized plutonsmainly restricted to the Birimian volcanic terrane b) seldomfoliated (except for local intense shearing) with nocompositional banding c) metaluminous d) typically dioriticto granodioritic in composition e) higher in Na2O and CaOcontents than the Basin-type granitoids f) displayingpronounced retrograde alteration g) containing similargeochemical characteristics as the metavolcanics in the beltfor some elements and h) bounded by contact aureoles of afew tens of meters maximum width The Basin-typegranitoids on the other hand were characterized as a) largebatholiths restricted to Birimian sedimentary basins b)having a foliation with compositional banding ubiquitous andpronounced c) peraluminous d) typically granodioritic incomposition e) higher in K2O and Rb contents than the Belt-type granitoids f) showing little mineral alteration g)displaying no evidence for geochemical similarity to themetavolcanics in the belts and h) having extensive contact-metamorphic aureoles On the basis of U-Pb zircon andmonazite dating Hirdes et al (1992) determined the age ofthe Belt-type granitoids in the Ashanti belt to be about2175 Myr and the age of the Kumasi Basin-type granitoids atabout 2116 Myr
The majority of Birimian gold deposits and occurrencesin Ghana is located at or along the boundaries between themetasediment units and five Birimian volcanic belts Thegold mineralization is associated with shear zones andextensive hydrothermal alteration (Wright et al 1985Melcher and Stumpfl 1994 Oberthuumlr et al 1994 Feybesseet al 2006) Appiah (1991) reported formation of pyritearsenopyrite sericite chlorite quartz and carbonates both in
Petrography geochemistry and alteration of country rocks from Bosumtwi 515
the wall rocks and quartz reefs Melcher and Stumpfl (1994)also described hydrothermal alteration of the wall rock to goldreefs involving chlorite muscovite graphite carbonatesepidote quartz (silicification) and sericite Mineralogical andgeochemical changes associated with alteration in Birimianrocks were also described by Pelig-Ba et al (2004) The exacttiming of hydrothermal alteration and gold mineralization isstill not known (Dzigbodi-Adjimah 1993 Oberthuumlr et al1994) On the basis of Pb isotope compositions of galena andbournonite from gold-quartz veins Oberthuumlr et al (1994)obtained model ages between 2122 and 1940 Myr The Pbisotope compositions of arsenopyrite from sulfide ores inBirimian host rocks gave an isochron age of 2224 plusmn 20 MyrFine-grained muscovite (sericite) from Birimian sediment-hosted ore yielded K-Ar ages ranging from 1867 plusmn 42 to 1893plusmn 43 Myr In view of the uncertainties inherent to these dataOberthuumlr et al (1994) established an upper age limit of2132 Myr which is relatively well constrained and anuncertain lower age limit of 2100 Myr In their Birimianevolution model Feybesse et al (2006) suggested the goldmineralization was emplaced during the late stage of theEburnean event (Eburnean D2 tectonism between 2095ndash1980 Myr) and was associated with NE-SW ductiletranscurrent faults
The Bosumtwi impact structure is located withinBirimian rocks and near several localized granite intrusionsThe geology of the Bosumtwi impact structure and environshas been described by several authors (eg Junner 1937Woodfield 1966 Moon and Mason 1967 Jones et al 1981Reimold et al 1998 Koeberl and Reimold 2005) The craterstructure also occurs in the proximity of a contact betweenmetasedimentary and metavolcanic units of the Ashanti belt(Fig 1) The corridor along the flanks of this contactelsewhere in the Ashanti belt is well known for the occurrenceof shear zones and associated gold mineralization (eg Leubeet al 1990 Milesi et al 1992 Melcher and Stumpfl 1994 Yaoand Robb 2000)
The Birimian rocks in which the crater was excavatedcomprise shales phyllites schists meta-graywackes andgranitoids (eg Junner 1937) The Birimian metavolcanicrocks extend to the southeast of the crater On the westand northwest sides of the lake phyllites shalesmeta-graywackes and quartzites are exposed whereas theother parts show mainly exposures of phyllites Extensivegraphitic phyllite and shale occur in the southern sector occurThe metasedimentary rocks are intruded by microgranitedikes up to 60 m wide In the northeast sector of the craterrocks of the Pepiakese intrusion are found comprising avariety of rock types ranging from hornblende- to biotite-muscovite granite (Jones 1985) and diorite (Koeberl et al1998) The Pepiakese intrusion is thought to be related to theKumasi (Basin-type) granitoids
Impact breccias are widely exposed at and around thecrater These breccias are monomict lithic breccia
(autochthonous) and polymict lithic breccia (allochthonous)from the various target rocks as well as suevites (polymictimpact melt- or glass-bearing breccias) which occur to thenorth and south of the crater The suevite contains target rockfragments representative of all stages of shockmetamorphism as well as vitreous and devitrified impactglasses (Reimold et al 1998 Boamah 2001)
SAMPLES AND EXPERIMENTAL METHODS
A suite of samples previously collected by two authors(C K and W U R) during fieldwork in 1997 in the directvicinity of the crater and on the crater rim was selected forboth petrographic and geochemical studies Thirty-one thinsections (representing 4 shalephyllite 4 meta-graywacke8 granite a siltstone a quartz-rich schist 3 suevite and10 meltglass fragments from suevite samples) were preparedand investigated by optical microscopy For each sample therock type minerals present and shock effects were studied(Table 1)
For geochemical studies 36 samples (6 shalephyllite3 meta-graywacke 9 granite an arkose a siltstone a quartz-rich schist a quartz vein 8 suevite and 6 meltglass fragmentsfrom suevite samples) were crushed in polyethylene wrappersand powdered in a mechanical agate mill for bulk chemicalanalysis
The contents of major elements (Si Ti Al Mn Mg Caand P) and trace elements (V Cu Zn Y and Nb) of 26samples were determined by standard X-ray fluorescence(XRF) spectrometry at the University of the WitwatersrandJohannesburg South Africa Reimold et al (1994) describedthe procedures precision and accuracy of the XRF analyticalmethod Ten other samples (LB-2 LB-5 LB-9 LB-11LB-13 LB-24 LB-33 LB-34 LB-40 and LB-46) wereanalyzed by XRF spectrometry at the Department ofGeological Sciences University of Vienna AustriaHowever due to inadequate sample amounts available thetrace-element contents of LB-9 and LB-13 could not bedetermined by XRF (see Tables 2 and 3)
The contents of major elements (Fe Na and K) and traceelements (Sc Cr Co Ni As Se Br Rb Sr Zr Sb Cs BaHf Ta Th and U) including the rare earth elements (REE)were determined for all 36 samples by instrumental neutronactivation analysis (INAA) at the Department of GeologicalSciences University of Vienna Austria For details of theINAA method including precision and accuracy see Koeberl(1993)
RESULTS
Petrography
The sample locations rock types and petrographicobservations are summarized in Table 1 Microphotographs of
516 F Karikari et al
some typical alteration and other characteristics of thecountry rock and suevite samples are shown in Figs 2ndash8 Ageneralized description of the main country rock types andthe suevites is given below
ShaleIn hand specimen two types of shale differing in color
and appearance can be distinguished a) banded shale which
is a soft highly argillaceous rock consisting of alternatingvery thin beds of light gray dark gray or black color and b)graphitic shale which is also a soft but dull blackargillaceous rock that stains the fingers when handled Thereis minor disseminated sulfide in some of the shale samplesThe thin sections show quartz feldspar iron oxides opaqueminerals (sulfides) and very fine-grained opticallyunidentifiable phyllosilicates (Fig 2a) Some shale samples
Fig 1 A geological map of the area of the Bosumtwi impact structure in Ghana (inset) Modified after Jones et al (1981) and Reimold et al(1998) The locations of sampling sites are also shown
Petrography geochemistry and alteration of country rocks from Bosumtwi 517
Table 1 Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
LB-2 6deg328 N1deg264 W
Siltstone Massive very fine-grained greenish gray siltstone with shear fabric quartz dominant rich in biotite (locally altered to chlorite) some feldspar muscovite and large nodules of opaque minerals a few veinlets of Fe oxides occur no evidence of shock
LB-3A 6deg3305 N1deg259 W
Quartz-rich schist Quartzite bands (about 2ndash3 mm thick) with thin intercalated biotite-rich bands (up to 1 mm) quartz is well sutured and frequently displays undulatory extinction some fine-grained biotite is partially aligned parallel to the schistosity partially discordant and much biotite is deformed (kink banding) no characteristic evidence of shock
LB-5 6deg3305 N1deg259 W
Shale Very fine-grained gray-black shale with some even darker (carbon-rich) bands composed of quartz feldspar and Fe oxides a few thin veinlets of quartz cross-cut the section no evidence of shock
LB-7 6deg3315 N1deg2575 W
Meta-graywacke(biotite rich)
Medium-grained mylonitized meta-graywacke composed of quartz (47 vol) plagioclase (6 vol) K-feldspar (9 vol) biotite (7 vol) and opaque minerals and other traces (2 vol) large biotite grains are partially oxidized fine-grained chlorite grains appear unaltered feldspar is partially altered to sericite no evidence of shock Matrix (mainly biotite chlorite and sericite) is 29 vol
LB-8 6deg3312 N1deg2563 W
Meta-graywacke Medium-grained gray meta-graywacke composed of quartz (39 vol) K-feldspar (6 vol) plagioclase (4 vol) sericite chlorite muscovite and opaque minerals (1 vol) most of the feldspar is altered to sericite and biotite is completely altered to chlorite Matrix (mainly sericite chlorite and quartz) is 40 vol no evidence of shock
LB-10 6deg3306 N1deg256 W
Microgranite Altered fine- to medium-grained microgranite composed of plagioclase (26 vol) quartz (24 vol) K-feldspar (14 vol) biotite (13 vol) muscovite (12 vol) Fe oxides (9 vol) and accessory minerals (2 vol) biotite grains are mostly oxidized no evidence of shock
LB-11 6deg3306 N1deg256 W
Mylonitic shale Fine-grained dark gray mylonitic shale composed of quartz phyllosilicates feldspar and opaque minerals some wide quartz ribbons no evidence of shock
LB-18 6deg327 N1deg257 W
Microgranite Fine- to medium-grained slightly sheared microgranite consists of quartz (40 vol) plagioclase (20 vol) K-feldspar (10 vol) biotite (10 vol) muscovitesericite (13 vol) chlorite (6 vol) and accessory minerals (mainly sphene) (1 vol) most of the biotite is oxidized and occurs in ldquoclustersrdquo some granophyric intergrowths of quartz in K-feldspar no evidence of shock
LB-19A 6deg327 N1deg257 W
Granite Altered medium-grained granite consists of quartz (27 vol) K-feldspar (22 vol) plagioclase (11 vol) biotite (5 vol) muscovite (5 vol) chlorite (27 vol) Fe oxides and accessory minerals (eg sphene) (3 vol) granophyric intergrowth of quartz and K-feldspar is abundant nice spherulites of feldspar most of the biotite is altered micro-fractures partially filled with Fe oxides cross-cut the section no evidence of shock
LB-19B 6deg327 N1deg257 W
Meta-graywacke Altered medium-grained sheared meta-graywacke (biotite-rich similar to sample LB-7) composed of quartz (36 vol) plagioclase (7 vol) K-feldspar (9 vol) biotite (5 vol) sericite and opaque minerals (mainly Fe oxides) (3 vol) some biotite grains are partially oxidized some feldspar grains are partially altered to sericite no evidence of shock Matrix (mainly sericite quartz chert and chlorite) amounts to 42 vol
LB-24 6deg330 N1deg256 W
Granite Medium-grained granite (very few oxides in this sample compared to LB-25 and very little granophyric intergrowth no spherulites observed) consists of plagioclase (42 vol) K-feldspar (30 vol) quartz (8 vol) biotite (some completely oxidizedaltered to chlorite) (13 vol) muscovite (5 vol) and Fe oxide and accessory minerals (2 vol) most feldspar is altered to sericite no evidence of shock
LB-25 6deg330 N1deg256 W
Granite Altered medium-grained granite (similar to LB-19A but with more oxides) consists of quartz (35 vol) K-feldspar (15 vol) plagioclase (10 vol) secondary phyllosilicate (25 vol) biotite completely altered to chlorite (5 vol) and Fe oxide (10 vol) granophyric intergrowth of quartz K-feldspar is abundant some spherulites of feldspar no evidence of shock
LB-26 6deg330 N1deg256 W
Granite Altered fine- to medium-grained granite composed of quartz (38 vol) K-feldspar (10 vol) plagioclase (5 vol) biotite (completely altered to chlorite 10 vol) Fe oxides (4 vol) and sericite (37 vol) some well-developed spherulites of feldspar granophyric intergrowth of quartz and K-feldspar occurs mostly at the edges of the spherulites no evidence of shock
LB-32 6deg331 N1deg229 W
Mylonitic shalephyllite
Very fine- to fine-grained mylonitic shale phyllite dark gray in color (sample locally similar to LB-11) composed of quartz phyllosilicates (biotite identified other phyllosilicates optically not identifiable) feldspar and opaque minerals thin quartz veinlets cross-cut the section no evidence of shock
LB-33 6deg3290 N1deg2228 W
Meta-graywacke Medium-grained meta-graywacke (similar to LB-8 but matrix poor) composed of quartz (64 vol) K-feldspar (12 vol) plagioclase (9 vol) biotite (mostly oxidized) (1 vol) opaque minerals (1 vol) and accessory minerals (zircon sphene epidote) (2 vol) a few feldspar grains are partially altered to sericite some quartz grains show undulatory extinction Matrix (mainly sericite quartz and chlorite) amounts to 11 vol no evidence of shock
518 F Karikari et al
LB-34 6deg3313 N1deg2264 W
Granite Granite composed mainly of quartz (38 vol) plagioclase (29 vol) K-feldspar (15 vol) biotite (5 vol) sericite (12 vol) and traces of Fe oxides and other opaque minerals (1 vol) very large biotite grains (partially oxidized) feldspar is partially altered to sericite no evidence of shock
LB-38 6deg3079 N1deg2052 W
Granite Coarse-grained granite muscovite-rich composed of quartz K-feldspar plagioclase muscovite sericite chlorite and opaque minerals feldspar is intensely altered to sericite no evidence of shock
LB-39a 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color) with angular to subrounded lithic clasts (up to 2 cm size) set into a clastic matrix (40 vol) clasts include graphitic shale (13 vol) phyllite (7 vol) meta-graywacke (13 vol) microgranite (2 vol) quartz and quartzitic grains (10 vol) chert (4 vol) melt (altered andor recrystallized) fragments and diaplectic quartz glass (together 11 vol) A few quartz grains show PDFs some clasts and matrix are altered (brownish oxides chlorite and other phyllosilicates)
LB-39c 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color similar to sample LB-39a) with angular to subrounded clasts (up to 15 cm) in a clastic matrix clasts include phyllite meta-graywacke microgranite glassmelt (mostly vesicular and fresh) diaplectic quartz glass quartz quartzite and feldspar A few quartz grains show PDFs (up to 2 sets) some clasts and matrix are altered to phyllosillicates (argillic alteration)
LB-40 6deg3388 N1deg2388 W
Large melt fragment from suevite
Large gray melt fragment from suevite (gray) that consists of melt matrix and melted or vitrified clasts (88 vol) very few clasts are unmelted unshocked meta-graywacke clasts (9 vol) few quartz (3 vol) and feldspar clasts (lt1 vol) meltglass (some fragments very vesicular and others partially recrystallized) diaplectic quartz and ballen quartz are dominant are dominant and a few vitrified metasediment clasts are present as well
LB-43 6deg3388 N1deg2388 W
Suevite Suevite (brownish in color very similar to sample LB-39a) with some angular to subrounded clasts in a clastic matrix (42 vol) clast population includes shale (15 vol) microgranite (5 vol) vitrified phyllite (6 vol) meltglass (mostly fresh vesicular glass diaplectic quartz glass some altered melt) (12 vol) meta-graywacke (13 vol) and quartz and quartzitic grains (7 vol) some clasts and matrix are altered (brownish oxides chlorite) a few quartz grains with PDFs (up to 2 sets)
LB-44A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt clast from suevite highly vesicular and very clast-poor clast population includes diaplectic quartz glass ballen quartz unshocked quartz and one vitrified meta-graywacke
LB-44B 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite (similar to sample LB-44A) with rounded 2 cm wide ballen quartz inclusion
LB-45 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite highly vesicular and very clast-poor (similar to sample LB-44A) clast population includes diaplectic quartz glass ballen quartz vitrified meta-graywacke and unshocked quartz
LB-45A 6deg3388 N1deg2388 W
Melt clast from suevite
Melt clast from suevite highly vesicular and very clast-poor (lt5 vol similar to sample LB-45) clast population includes diaplectic quartz glass ballen quartz and unshocked quartz
LB-45B 6deg3388 N1deg2388 W
Melt fragmentfrom suevite
Melt fragment from bulk suevite gray in color similar to sample LB-47A consists of melt matrix and melted or vitrified clasts (only a few quartz meta-graywacke quartzite and feldspar clasts are unmelted) meltglass (some highly vesicular others partially recrystallized) diaplectic quartz and ballen quartz and a few vitrified metasediment (mainly graywacke) clasts are present
LB-46 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment sample gray in color similar to sample LB-45B
LB-47A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-40) that consists of melt matrix and melted or vitrified clasts (few quartz meta-graywacke and quartzite clasts are unmelted or unvitrified) meltglass (some fragments are very vesicular andor with flow structure others are partially recrystallized) diaplectic quartz and ballen quartz are dominant but also some vitrified metasediment clasts
LB-47B 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-47A) that consists of melt matrix and melted or vitrified clasts (few quartzite quartz and feldspar are unmelted or unvitrified) meltglass (with well-developed flow structure some fragments are highly vesicular others are partially recrystallized) diaplectic quartz and ballen quartz are dominant
LB-48 6deg3388 N1deg2388 W
Melt clast from suevite
Large gt4 cm in diameter melt clast in suevite (light gray in color) well-developed flow structures are visible some parts of the clast are highly vesicular others are partially recrystallized few unmelted or unvitrified quartz and quartzite clasts are also preserved inside the melt fragment diaplectic quartz and ballen quartz are also present
LB-51 6deg2674 N1deg2262 W
Graphitic shale Well-laminated fine-grained graphitic shale (black gray in color) composed mainly of quartz optically unidentifiable phyllosilicates and carbon (graphite) local development of crenulation cleavage no trace of shock deformation
Table 1 Continued Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
514 F Karikari et al
characteristic planar deformation features (PDFs) in quartzfrom suevitic breccia (see also several papers in this issue)
The Bosumtwi impact structure is also the likely sourcecrater for the Ivory Coast tektites (eg Gentner et al 1964Jones 1985 Koeberl et al 1997 1998) This correlation ismainly based on similarities in geochemical and isotopiccompositions of target rocks and tektites as well assimilarities in the ages of the impact melt from suevites and ofthe Ivory Coast tektites Boamah and Koeberl (2003) carriedout detailed petrographic and geochemical studies on suevitesfrom shallow drill cores obtained to the north of the craterResults of structural and geological mapping of the Bosumtwicrater rim were reported by Reimold et al (1998)Geochemical signatures of soils from north of the crater andtheir relationship to airborne radiometric geophysical datawere discussed by Boamah and Koeberl (2002)
Various geophysical studies of the Bosumtwi structurehave been carried out (Pesonen et al 1998 Karp et al 2002Scholz et al 2002) Koeberl and Reimold (2005) published anupdated and revised geological map of the Bosumtwistructure and environs with explanations containing moredetail about the impact structure The most recent research onthe crater was a multinational drilling project by theInternational Continental Scientific Drilling Program (ICDP)during the second half of 2004 when 16 continuous cores ofboth lake sediments (14) and impactites (2) were obtained forpaleoenvironmental and impact-cratering studies (papers inthis issue) respectively
Here we report the petrographic and geochemicalcharacteristics of the main country rock types that likelyconstituted the target rocks at Bosumtwi and of suevites at theBosumtwi impact structure and discuss their pre- and post-impact alteration
GEOLOGICAL SETTING
The Bosumtwi impact structure was excavated in rocksof the Early Proterozoic Birimian Supergroup (Jones at al1981) These rocks crop out extensively in Ghana IvoryCoast Burkina Faso and Mali The Birimian Supergroupconsists of two major units one unit composed dominantly ofmetamorphosed sedimentary rocks and the other unit mostlyrepresented by bimodal although largely tholeiitic volcanicsnow altered and metamorphosed to greenstones Thesegreenstones initially comprised mainly basalts and andesitesthat have now been metamorphosed to hornblende-actinoliteschist calcite-chlorite schist mica schist and amphibolitesThe metasedimentary unit comprises volcaniclastic rocksturbidite-related wackes argillitic rocks and chemicalsediments (Leube et al 1990) now metamorphosed intometa-graywackes phyllites schists and shales The volcanicactivity and sedimentation were submarine according toWoodfield (1966) with evidence provided by theinterfingering of and lateral variation between
metavolcanics and metasediments and the appearance ofrelict pillow structures in the metavolcanics
Traditionally in Ghana the metasedimentary unit wasconsidered older than the metavolcanic unit (Junner 1937Woodfield 1966 Moon and Mason 1967) This however wasin contrast to the view held in the neighboring francophonecountries (Hirdes et al 1996) Recent studies have shown thatthe two units were formed contemporaneously (eg Leubeet al 1990) On the basis of Sm-Nd isochron and model agesTaylor et al (1992) constrained the age of Birimiansupracrustal rocks of western Ghana to between 20 and23 Gyr Feybesse et al (2006) placed somewhat tighterconstraints on the age of these rocks around 210 to 215 Gyr
Two types of granitoid intrusions into the volcanicpackage and the basin sediments occur which are referred toas the Belt (Dixcove) granitoids and the Basin (Cape Coastand Winneba) granitoids respectively (Wright et al 1985)According to Wright et al (1985) the Belt-type granitoids aregenerally discordant to regional structures often unfoliatedand of predominant dioritic to monzonitic compositionwhereas the Basin-type granitoids are generally concordant toregional structures often foliated and of predominantgranodioritic composition Hirdes et al (1992) characterizedthe Belt-type granitoids as a) small- to medium-sized plutonsmainly restricted to the Birimian volcanic terrane b) seldomfoliated (except for local intense shearing) with nocompositional banding c) metaluminous d) typically dioriticto granodioritic in composition e) higher in Na2O and CaOcontents than the Basin-type granitoids f) displayingpronounced retrograde alteration g) containing similargeochemical characteristics as the metavolcanics in the beltfor some elements and h) bounded by contact aureoles of afew tens of meters maximum width The Basin-typegranitoids on the other hand were characterized as a) largebatholiths restricted to Birimian sedimentary basins b)having a foliation with compositional banding ubiquitous andpronounced c) peraluminous d) typically granodioritic incomposition e) higher in K2O and Rb contents than the Belt-type granitoids f) showing little mineral alteration g)displaying no evidence for geochemical similarity to themetavolcanics in the belts and h) having extensive contact-metamorphic aureoles On the basis of U-Pb zircon andmonazite dating Hirdes et al (1992) determined the age ofthe Belt-type granitoids in the Ashanti belt to be about2175 Myr and the age of the Kumasi Basin-type granitoids atabout 2116 Myr
The majority of Birimian gold deposits and occurrencesin Ghana is located at or along the boundaries between themetasediment units and five Birimian volcanic belts Thegold mineralization is associated with shear zones andextensive hydrothermal alteration (Wright et al 1985Melcher and Stumpfl 1994 Oberthuumlr et al 1994 Feybesseet al 2006) Appiah (1991) reported formation of pyritearsenopyrite sericite chlorite quartz and carbonates both in
Petrography geochemistry and alteration of country rocks from Bosumtwi 515
the wall rocks and quartz reefs Melcher and Stumpfl (1994)also described hydrothermal alteration of the wall rock to goldreefs involving chlorite muscovite graphite carbonatesepidote quartz (silicification) and sericite Mineralogical andgeochemical changes associated with alteration in Birimianrocks were also described by Pelig-Ba et al (2004) The exacttiming of hydrothermal alteration and gold mineralization isstill not known (Dzigbodi-Adjimah 1993 Oberthuumlr et al1994) On the basis of Pb isotope compositions of galena andbournonite from gold-quartz veins Oberthuumlr et al (1994)obtained model ages between 2122 and 1940 Myr The Pbisotope compositions of arsenopyrite from sulfide ores inBirimian host rocks gave an isochron age of 2224 plusmn 20 MyrFine-grained muscovite (sericite) from Birimian sediment-hosted ore yielded K-Ar ages ranging from 1867 plusmn 42 to 1893plusmn 43 Myr In view of the uncertainties inherent to these dataOberthuumlr et al (1994) established an upper age limit of2132 Myr which is relatively well constrained and anuncertain lower age limit of 2100 Myr In their Birimianevolution model Feybesse et al (2006) suggested the goldmineralization was emplaced during the late stage of theEburnean event (Eburnean D2 tectonism between 2095ndash1980 Myr) and was associated with NE-SW ductiletranscurrent faults
The Bosumtwi impact structure is located withinBirimian rocks and near several localized granite intrusionsThe geology of the Bosumtwi impact structure and environshas been described by several authors (eg Junner 1937Woodfield 1966 Moon and Mason 1967 Jones et al 1981Reimold et al 1998 Koeberl and Reimold 2005) The craterstructure also occurs in the proximity of a contact betweenmetasedimentary and metavolcanic units of the Ashanti belt(Fig 1) The corridor along the flanks of this contactelsewhere in the Ashanti belt is well known for the occurrenceof shear zones and associated gold mineralization (eg Leubeet al 1990 Milesi et al 1992 Melcher and Stumpfl 1994 Yaoand Robb 2000)
The Birimian rocks in which the crater was excavatedcomprise shales phyllites schists meta-graywackes andgranitoids (eg Junner 1937) The Birimian metavolcanicrocks extend to the southeast of the crater On the westand northwest sides of the lake phyllites shalesmeta-graywackes and quartzites are exposed whereas theother parts show mainly exposures of phyllites Extensivegraphitic phyllite and shale occur in the southern sector occurThe metasedimentary rocks are intruded by microgranitedikes up to 60 m wide In the northeast sector of the craterrocks of the Pepiakese intrusion are found comprising avariety of rock types ranging from hornblende- to biotite-muscovite granite (Jones 1985) and diorite (Koeberl et al1998) The Pepiakese intrusion is thought to be related to theKumasi (Basin-type) granitoids
Impact breccias are widely exposed at and around thecrater These breccias are monomict lithic breccia
(autochthonous) and polymict lithic breccia (allochthonous)from the various target rocks as well as suevites (polymictimpact melt- or glass-bearing breccias) which occur to thenorth and south of the crater The suevite contains target rockfragments representative of all stages of shockmetamorphism as well as vitreous and devitrified impactglasses (Reimold et al 1998 Boamah 2001)
SAMPLES AND EXPERIMENTAL METHODS
A suite of samples previously collected by two authors(C K and W U R) during fieldwork in 1997 in the directvicinity of the crater and on the crater rim was selected forboth petrographic and geochemical studies Thirty-one thinsections (representing 4 shalephyllite 4 meta-graywacke8 granite a siltstone a quartz-rich schist 3 suevite and10 meltglass fragments from suevite samples) were preparedand investigated by optical microscopy For each sample therock type minerals present and shock effects were studied(Table 1)
For geochemical studies 36 samples (6 shalephyllite3 meta-graywacke 9 granite an arkose a siltstone a quartz-rich schist a quartz vein 8 suevite and 6 meltglass fragmentsfrom suevite samples) were crushed in polyethylene wrappersand powdered in a mechanical agate mill for bulk chemicalanalysis
The contents of major elements (Si Ti Al Mn Mg Caand P) and trace elements (V Cu Zn Y and Nb) of 26samples were determined by standard X-ray fluorescence(XRF) spectrometry at the University of the WitwatersrandJohannesburg South Africa Reimold et al (1994) describedthe procedures precision and accuracy of the XRF analyticalmethod Ten other samples (LB-2 LB-5 LB-9 LB-11LB-13 LB-24 LB-33 LB-34 LB-40 and LB-46) wereanalyzed by XRF spectrometry at the Department ofGeological Sciences University of Vienna AustriaHowever due to inadequate sample amounts available thetrace-element contents of LB-9 and LB-13 could not bedetermined by XRF (see Tables 2 and 3)
The contents of major elements (Fe Na and K) and traceelements (Sc Cr Co Ni As Se Br Rb Sr Zr Sb Cs BaHf Ta Th and U) including the rare earth elements (REE)were determined for all 36 samples by instrumental neutronactivation analysis (INAA) at the Department of GeologicalSciences University of Vienna Austria For details of theINAA method including precision and accuracy see Koeberl(1993)
RESULTS
Petrography
The sample locations rock types and petrographicobservations are summarized in Table 1 Microphotographs of
516 F Karikari et al
some typical alteration and other characteristics of thecountry rock and suevite samples are shown in Figs 2ndash8 Ageneralized description of the main country rock types andthe suevites is given below
ShaleIn hand specimen two types of shale differing in color
and appearance can be distinguished a) banded shale which
is a soft highly argillaceous rock consisting of alternatingvery thin beds of light gray dark gray or black color and b)graphitic shale which is also a soft but dull blackargillaceous rock that stains the fingers when handled Thereis minor disseminated sulfide in some of the shale samplesThe thin sections show quartz feldspar iron oxides opaqueminerals (sulfides) and very fine-grained opticallyunidentifiable phyllosilicates (Fig 2a) Some shale samples
Fig 1 A geological map of the area of the Bosumtwi impact structure in Ghana (inset) Modified after Jones et al (1981) and Reimold et al(1998) The locations of sampling sites are also shown
Petrography geochemistry and alteration of country rocks from Bosumtwi 517
Table 1 Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
LB-2 6deg328 N1deg264 W
Siltstone Massive very fine-grained greenish gray siltstone with shear fabric quartz dominant rich in biotite (locally altered to chlorite) some feldspar muscovite and large nodules of opaque minerals a few veinlets of Fe oxides occur no evidence of shock
LB-3A 6deg3305 N1deg259 W
Quartz-rich schist Quartzite bands (about 2ndash3 mm thick) with thin intercalated biotite-rich bands (up to 1 mm) quartz is well sutured and frequently displays undulatory extinction some fine-grained biotite is partially aligned parallel to the schistosity partially discordant and much biotite is deformed (kink banding) no characteristic evidence of shock
LB-5 6deg3305 N1deg259 W
Shale Very fine-grained gray-black shale with some even darker (carbon-rich) bands composed of quartz feldspar and Fe oxides a few thin veinlets of quartz cross-cut the section no evidence of shock
LB-7 6deg3315 N1deg2575 W
Meta-graywacke(biotite rich)
Medium-grained mylonitized meta-graywacke composed of quartz (47 vol) plagioclase (6 vol) K-feldspar (9 vol) biotite (7 vol) and opaque minerals and other traces (2 vol) large biotite grains are partially oxidized fine-grained chlorite grains appear unaltered feldspar is partially altered to sericite no evidence of shock Matrix (mainly biotite chlorite and sericite) is 29 vol
LB-8 6deg3312 N1deg2563 W
Meta-graywacke Medium-grained gray meta-graywacke composed of quartz (39 vol) K-feldspar (6 vol) plagioclase (4 vol) sericite chlorite muscovite and opaque minerals (1 vol) most of the feldspar is altered to sericite and biotite is completely altered to chlorite Matrix (mainly sericite chlorite and quartz) is 40 vol no evidence of shock
LB-10 6deg3306 N1deg256 W
Microgranite Altered fine- to medium-grained microgranite composed of plagioclase (26 vol) quartz (24 vol) K-feldspar (14 vol) biotite (13 vol) muscovite (12 vol) Fe oxides (9 vol) and accessory minerals (2 vol) biotite grains are mostly oxidized no evidence of shock
LB-11 6deg3306 N1deg256 W
Mylonitic shale Fine-grained dark gray mylonitic shale composed of quartz phyllosilicates feldspar and opaque minerals some wide quartz ribbons no evidence of shock
LB-18 6deg327 N1deg257 W
Microgranite Fine- to medium-grained slightly sheared microgranite consists of quartz (40 vol) plagioclase (20 vol) K-feldspar (10 vol) biotite (10 vol) muscovitesericite (13 vol) chlorite (6 vol) and accessory minerals (mainly sphene) (1 vol) most of the biotite is oxidized and occurs in ldquoclustersrdquo some granophyric intergrowths of quartz in K-feldspar no evidence of shock
LB-19A 6deg327 N1deg257 W
Granite Altered medium-grained granite consists of quartz (27 vol) K-feldspar (22 vol) plagioclase (11 vol) biotite (5 vol) muscovite (5 vol) chlorite (27 vol) Fe oxides and accessory minerals (eg sphene) (3 vol) granophyric intergrowth of quartz and K-feldspar is abundant nice spherulites of feldspar most of the biotite is altered micro-fractures partially filled with Fe oxides cross-cut the section no evidence of shock
LB-19B 6deg327 N1deg257 W
Meta-graywacke Altered medium-grained sheared meta-graywacke (biotite-rich similar to sample LB-7) composed of quartz (36 vol) plagioclase (7 vol) K-feldspar (9 vol) biotite (5 vol) sericite and opaque minerals (mainly Fe oxides) (3 vol) some biotite grains are partially oxidized some feldspar grains are partially altered to sericite no evidence of shock Matrix (mainly sericite quartz chert and chlorite) amounts to 42 vol
LB-24 6deg330 N1deg256 W
Granite Medium-grained granite (very few oxides in this sample compared to LB-25 and very little granophyric intergrowth no spherulites observed) consists of plagioclase (42 vol) K-feldspar (30 vol) quartz (8 vol) biotite (some completely oxidizedaltered to chlorite) (13 vol) muscovite (5 vol) and Fe oxide and accessory minerals (2 vol) most feldspar is altered to sericite no evidence of shock
LB-25 6deg330 N1deg256 W
Granite Altered medium-grained granite (similar to LB-19A but with more oxides) consists of quartz (35 vol) K-feldspar (15 vol) plagioclase (10 vol) secondary phyllosilicate (25 vol) biotite completely altered to chlorite (5 vol) and Fe oxide (10 vol) granophyric intergrowth of quartz K-feldspar is abundant some spherulites of feldspar no evidence of shock
LB-26 6deg330 N1deg256 W
Granite Altered fine- to medium-grained granite composed of quartz (38 vol) K-feldspar (10 vol) plagioclase (5 vol) biotite (completely altered to chlorite 10 vol) Fe oxides (4 vol) and sericite (37 vol) some well-developed spherulites of feldspar granophyric intergrowth of quartz and K-feldspar occurs mostly at the edges of the spherulites no evidence of shock
LB-32 6deg331 N1deg229 W
Mylonitic shalephyllite
Very fine- to fine-grained mylonitic shale phyllite dark gray in color (sample locally similar to LB-11) composed of quartz phyllosilicates (biotite identified other phyllosilicates optically not identifiable) feldspar and opaque minerals thin quartz veinlets cross-cut the section no evidence of shock
LB-33 6deg3290 N1deg2228 W
Meta-graywacke Medium-grained meta-graywacke (similar to LB-8 but matrix poor) composed of quartz (64 vol) K-feldspar (12 vol) plagioclase (9 vol) biotite (mostly oxidized) (1 vol) opaque minerals (1 vol) and accessory minerals (zircon sphene epidote) (2 vol) a few feldspar grains are partially altered to sericite some quartz grains show undulatory extinction Matrix (mainly sericite quartz and chlorite) amounts to 11 vol no evidence of shock
518 F Karikari et al
LB-34 6deg3313 N1deg2264 W
Granite Granite composed mainly of quartz (38 vol) plagioclase (29 vol) K-feldspar (15 vol) biotite (5 vol) sericite (12 vol) and traces of Fe oxides and other opaque minerals (1 vol) very large biotite grains (partially oxidized) feldspar is partially altered to sericite no evidence of shock
LB-38 6deg3079 N1deg2052 W
Granite Coarse-grained granite muscovite-rich composed of quartz K-feldspar plagioclase muscovite sericite chlorite and opaque minerals feldspar is intensely altered to sericite no evidence of shock
LB-39a 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color) with angular to subrounded lithic clasts (up to 2 cm size) set into a clastic matrix (40 vol) clasts include graphitic shale (13 vol) phyllite (7 vol) meta-graywacke (13 vol) microgranite (2 vol) quartz and quartzitic grains (10 vol) chert (4 vol) melt (altered andor recrystallized) fragments and diaplectic quartz glass (together 11 vol) A few quartz grains show PDFs some clasts and matrix are altered (brownish oxides chlorite and other phyllosilicates)
LB-39c 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color similar to sample LB-39a) with angular to subrounded clasts (up to 15 cm) in a clastic matrix clasts include phyllite meta-graywacke microgranite glassmelt (mostly vesicular and fresh) diaplectic quartz glass quartz quartzite and feldspar A few quartz grains show PDFs (up to 2 sets) some clasts and matrix are altered to phyllosillicates (argillic alteration)
LB-40 6deg3388 N1deg2388 W
Large melt fragment from suevite
Large gray melt fragment from suevite (gray) that consists of melt matrix and melted or vitrified clasts (88 vol) very few clasts are unmelted unshocked meta-graywacke clasts (9 vol) few quartz (3 vol) and feldspar clasts (lt1 vol) meltglass (some fragments very vesicular and others partially recrystallized) diaplectic quartz and ballen quartz are dominant are dominant and a few vitrified metasediment clasts are present as well
LB-43 6deg3388 N1deg2388 W
Suevite Suevite (brownish in color very similar to sample LB-39a) with some angular to subrounded clasts in a clastic matrix (42 vol) clast population includes shale (15 vol) microgranite (5 vol) vitrified phyllite (6 vol) meltglass (mostly fresh vesicular glass diaplectic quartz glass some altered melt) (12 vol) meta-graywacke (13 vol) and quartz and quartzitic grains (7 vol) some clasts and matrix are altered (brownish oxides chlorite) a few quartz grains with PDFs (up to 2 sets)
LB-44A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt clast from suevite highly vesicular and very clast-poor clast population includes diaplectic quartz glass ballen quartz unshocked quartz and one vitrified meta-graywacke
LB-44B 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite (similar to sample LB-44A) with rounded 2 cm wide ballen quartz inclusion
LB-45 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite highly vesicular and very clast-poor (similar to sample LB-44A) clast population includes diaplectic quartz glass ballen quartz vitrified meta-graywacke and unshocked quartz
LB-45A 6deg3388 N1deg2388 W
Melt clast from suevite
Melt clast from suevite highly vesicular and very clast-poor (lt5 vol similar to sample LB-45) clast population includes diaplectic quartz glass ballen quartz and unshocked quartz
LB-45B 6deg3388 N1deg2388 W
Melt fragmentfrom suevite
Melt fragment from bulk suevite gray in color similar to sample LB-47A consists of melt matrix and melted or vitrified clasts (only a few quartz meta-graywacke quartzite and feldspar clasts are unmelted) meltglass (some highly vesicular others partially recrystallized) diaplectic quartz and ballen quartz and a few vitrified metasediment (mainly graywacke) clasts are present
LB-46 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment sample gray in color similar to sample LB-45B
LB-47A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-40) that consists of melt matrix and melted or vitrified clasts (few quartz meta-graywacke and quartzite clasts are unmelted or unvitrified) meltglass (some fragments are very vesicular andor with flow structure others are partially recrystallized) diaplectic quartz and ballen quartz are dominant but also some vitrified metasediment clasts
LB-47B 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-47A) that consists of melt matrix and melted or vitrified clasts (few quartzite quartz and feldspar are unmelted or unvitrified) meltglass (with well-developed flow structure some fragments are highly vesicular others are partially recrystallized) diaplectic quartz and ballen quartz are dominant
LB-48 6deg3388 N1deg2388 W
Melt clast from suevite
Large gt4 cm in diameter melt clast in suevite (light gray in color) well-developed flow structures are visible some parts of the clast are highly vesicular others are partially recrystallized few unmelted or unvitrified quartz and quartzite clasts are also preserved inside the melt fragment diaplectic quartz and ballen quartz are also present
LB-51 6deg2674 N1deg2262 W
Graphitic shale Well-laminated fine-grained graphitic shale (black gray in color) composed mainly of quartz optically unidentifiable phyllosilicates and carbon (graphite) local development of crenulation cleavage no trace of shock deformation
Table 1 Continued Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 515
the wall rocks and quartz reefs Melcher and Stumpfl (1994)also described hydrothermal alteration of the wall rock to goldreefs involving chlorite muscovite graphite carbonatesepidote quartz (silicification) and sericite Mineralogical andgeochemical changes associated with alteration in Birimianrocks were also described by Pelig-Ba et al (2004) The exacttiming of hydrothermal alteration and gold mineralization isstill not known (Dzigbodi-Adjimah 1993 Oberthuumlr et al1994) On the basis of Pb isotope compositions of galena andbournonite from gold-quartz veins Oberthuumlr et al (1994)obtained model ages between 2122 and 1940 Myr The Pbisotope compositions of arsenopyrite from sulfide ores inBirimian host rocks gave an isochron age of 2224 plusmn 20 MyrFine-grained muscovite (sericite) from Birimian sediment-hosted ore yielded K-Ar ages ranging from 1867 plusmn 42 to 1893plusmn 43 Myr In view of the uncertainties inherent to these dataOberthuumlr et al (1994) established an upper age limit of2132 Myr which is relatively well constrained and anuncertain lower age limit of 2100 Myr In their Birimianevolution model Feybesse et al (2006) suggested the goldmineralization was emplaced during the late stage of theEburnean event (Eburnean D2 tectonism between 2095ndash1980 Myr) and was associated with NE-SW ductiletranscurrent faults
The Bosumtwi impact structure is located withinBirimian rocks and near several localized granite intrusionsThe geology of the Bosumtwi impact structure and environshas been described by several authors (eg Junner 1937Woodfield 1966 Moon and Mason 1967 Jones et al 1981Reimold et al 1998 Koeberl and Reimold 2005) The craterstructure also occurs in the proximity of a contact betweenmetasedimentary and metavolcanic units of the Ashanti belt(Fig 1) The corridor along the flanks of this contactelsewhere in the Ashanti belt is well known for the occurrenceof shear zones and associated gold mineralization (eg Leubeet al 1990 Milesi et al 1992 Melcher and Stumpfl 1994 Yaoand Robb 2000)
The Birimian rocks in which the crater was excavatedcomprise shales phyllites schists meta-graywackes andgranitoids (eg Junner 1937) The Birimian metavolcanicrocks extend to the southeast of the crater On the westand northwest sides of the lake phyllites shalesmeta-graywackes and quartzites are exposed whereas theother parts show mainly exposures of phyllites Extensivegraphitic phyllite and shale occur in the southern sector occurThe metasedimentary rocks are intruded by microgranitedikes up to 60 m wide In the northeast sector of the craterrocks of the Pepiakese intrusion are found comprising avariety of rock types ranging from hornblende- to biotite-muscovite granite (Jones 1985) and diorite (Koeberl et al1998) The Pepiakese intrusion is thought to be related to theKumasi (Basin-type) granitoids
Impact breccias are widely exposed at and around thecrater These breccias are monomict lithic breccia
(autochthonous) and polymict lithic breccia (allochthonous)from the various target rocks as well as suevites (polymictimpact melt- or glass-bearing breccias) which occur to thenorth and south of the crater The suevite contains target rockfragments representative of all stages of shockmetamorphism as well as vitreous and devitrified impactglasses (Reimold et al 1998 Boamah 2001)
SAMPLES AND EXPERIMENTAL METHODS
A suite of samples previously collected by two authors(C K and W U R) during fieldwork in 1997 in the directvicinity of the crater and on the crater rim was selected forboth petrographic and geochemical studies Thirty-one thinsections (representing 4 shalephyllite 4 meta-graywacke8 granite a siltstone a quartz-rich schist 3 suevite and10 meltglass fragments from suevite samples) were preparedand investigated by optical microscopy For each sample therock type minerals present and shock effects were studied(Table 1)
For geochemical studies 36 samples (6 shalephyllite3 meta-graywacke 9 granite an arkose a siltstone a quartz-rich schist a quartz vein 8 suevite and 6 meltglass fragmentsfrom suevite samples) were crushed in polyethylene wrappersand powdered in a mechanical agate mill for bulk chemicalanalysis
The contents of major elements (Si Ti Al Mn Mg Caand P) and trace elements (V Cu Zn Y and Nb) of 26samples were determined by standard X-ray fluorescence(XRF) spectrometry at the University of the WitwatersrandJohannesburg South Africa Reimold et al (1994) describedthe procedures precision and accuracy of the XRF analyticalmethod Ten other samples (LB-2 LB-5 LB-9 LB-11LB-13 LB-24 LB-33 LB-34 LB-40 and LB-46) wereanalyzed by XRF spectrometry at the Department ofGeological Sciences University of Vienna AustriaHowever due to inadequate sample amounts available thetrace-element contents of LB-9 and LB-13 could not bedetermined by XRF (see Tables 2 and 3)
The contents of major elements (Fe Na and K) and traceelements (Sc Cr Co Ni As Se Br Rb Sr Zr Sb Cs BaHf Ta Th and U) including the rare earth elements (REE)were determined for all 36 samples by instrumental neutronactivation analysis (INAA) at the Department of GeologicalSciences University of Vienna Austria For details of theINAA method including precision and accuracy see Koeberl(1993)
RESULTS
Petrography
The sample locations rock types and petrographicobservations are summarized in Table 1 Microphotographs of
516 F Karikari et al
some typical alteration and other characteristics of thecountry rock and suevite samples are shown in Figs 2ndash8 Ageneralized description of the main country rock types andthe suevites is given below
ShaleIn hand specimen two types of shale differing in color
and appearance can be distinguished a) banded shale which
is a soft highly argillaceous rock consisting of alternatingvery thin beds of light gray dark gray or black color and b)graphitic shale which is also a soft but dull blackargillaceous rock that stains the fingers when handled Thereis minor disseminated sulfide in some of the shale samplesThe thin sections show quartz feldspar iron oxides opaqueminerals (sulfides) and very fine-grained opticallyunidentifiable phyllosilicates (Fig 2a) Some shale samples
Fig 1 A geological map of the area of the Bosumtwi impact structure in Ghana (inset) Modified after Jones et al (1981) and Reimold et al(1998) The locations of sampling sites are also shown
Petrography geochemistry and alteration of country rocks from Bosumtwi 517
Table 1 Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
LB-2 6deg328 N1deg264 W
Siltstone Massive very fine-grained greenish gray siltstone with shear fabric quartz dominant rich in biotite (locally altered to chlorite) some feldspar muscovite and large nodules of opaque minerals a few veinlets of Fe oxides occur no evidence of shock
LB-3A 6deg3305 N1deg259 W
Quartz-rich schist Quartzite bands (about 2ndash3 mm thick) with thin intercalated biotite-rich bands (up to 1 mm) quartz is well sutured and frequently displays undulatory extinction some fine-grained biotite is partially aligned parallel to the schistosity partially discordant and much biotite is deformed (kink banding) no characteristic evidence of shock
LB-5 6deg3305 N1deg259 W
Shale Very fine-grained gray-black shale with some even darker (carbon-rich) bands composed of quartz feldspar and Fe oxides a few thin veinlets of quartz cross-cut the section no evidence of shock
LB-7 6deg3315 N1deg2575 W
Meta-graywacke(biotite rich)
Medium-grained mylonitized meta-graywacke composed of quartz (47 vol) plagioclase (6 vol) K-feldspar (9 vol) biotite (7 vol) and opaque minerals and other traces (2 vol) large biotite grains are partially oxidized fine-grained chlorite grains appear unaltered feldspar is partially altered to sericite no evidence of shock Matrix (mainly biotite chlorite and sericite) is 29 vol
LB-8 6deg3312 N1deg2563 W
Meta-graywacke Medium-grained gray meta-graywacke composed of quartz (39 vol) K-feldspar (6 vol) plagioclase (4 vol) sericite chlorite muscovite and opaque minerals (1 vol) most of the feldspar is altered to sericite and biotite is completely altered to chlorite Matrix (mainly sericite chlorite and quartz) is 40 vol no evidence of shock
LB-10 6deg3306 N1deg256 W
Microgranite Altered fine- to medium-grained microgranite composed of plagioclase (26 vol) quartz (24 vol) K-feldspar (14 vol) biotite (13 vol) muscovite (12 vol) Fe oxides (9 vol) and accessory minerals (2 vol) biotite grains are mostly oxidized no evidence of shock
LB-11 6deg3306 N1deg256 W
Mylonitic shale Fine-grained dark gray mylonitic shale composed of quartz phyllosilicates feldspar and opaque minerals some wide quartz ribbons no evidence of shock
LB-18 6deg327 N1deg257 W
Microgranite Fine- to medium-grained slightly sheared microgranite consists of quartz (40 vol) plagioclase (20 vol) K-feldspar (10 vol) biotite (10 vol) muscovitesericite (13 vol) chlorite (6 vol) and accessory minerals (mainly sphene) (1 vol) most of the biotite is oxidized and occurs in ldquoclustersrdquo some granophyric intergrowths of quartz in K-feldspar no evidence of shock
LB-19A 6deg327 N1deg257 W
Granite Altered medium-grained granite consists of quartz (27 vol) K-feldspar (22 vol) plagioclase (11 vol) biotite (5 vol) muscovite (5 vol) chlorite (27 vol) Fe oxides and accessory minerals (eg sphene) (3 vol) granophyric intergrowth of quartz and K-feldspar is abundant nice spherulites of feldspar most of the biotite is altered micro-fractures partially filled with Fe oxides cross-cut the section no evidence of shock
LB-19B 6deg327 N1deg257 W
Meta-graywacke Altered medium-grained sheared meta-graywacke (biotite-rich similar to sample LB-7) composed of quartz (36 vol) plagioclase (7 vol) K-feldspar (9 vol) biotite (5 vol) sericite and opaque minerals (mainly Fe oxides) (3 vol) some biotite grains are partially oxidized some feldspar grains are partially altered to sericite no evidence of shock Matrix (mainly sericite quartz chert and chlorite) amounts to 42 vol
LB-24 6deg330 N1deg256 W
Granite Medium-grained granite (very few oxides in this sample compared to LB-25 and very little granophyric intergrowth no spherulites observed) consists of plagioclase (42 vol) K-feldspar (30 vol) quartz (8 vol) biotite (some completely oxidizedaltered to chlorite) (13 vol) muscovite (5 vol) and Fe oxide and accessory minerals (2 vol) most feldspar is altered to sericite no evidence of shock
LB-25 6deg330 N1deg256 W
Granite Altered medium-grained granite (similar to LB-19A but with more oxides) consists of quartz (35 vol) K-feldspar (15 vol) plagioclase (10 vol) secondary phyllosilicate (25 vol) biotite completely altered to chlorite (5 vol) and Fe oxide (10 vol) granophyric intergrowth of quartz K-feldspar is abundant some spherulites of feldspar no evidence of shock
LB-26 6deg330 N1deg256 W
Granite Altered fine- to medium-grained granite composed of quartz (38 vol) K-feldspar (10 vol) plagioclase (5 vol) biotite (completely altered to chlorite 10 vol) Fe oxides (4 vol) and sericite (37 vol) some well-developed spherulites of feldspar granophyric intergrowth of quartz and K-feldspar occurs mostly at the edges of the spherulites no evidence of shock
LB-32 6deg331 N1deg229 W
Mylonitic shalephyllite
Very fine- to fine-grained mylonitic shale phyllite dark gray in color (sample locally similar to LB-11) composed of quartz phyllosilicates (biotite identified other phyllosilicates optically not identifiable) feldspar and opaque minerals thin quartz veinlets cross-cut the section no evidence of shock
LB-33 6deg3290 N1deg2228 W
Meta-graywacke Medium-grained meta-graywacke (similar to LB-8 but matrix poor) composed of quartz (64 vol) K-feldspar (12 vol) plagioclase (9 vol) biotite (mostly oxidized) (1 vol) opaque minerals (1 vol) and accessory minerals (zircon sphene epidote) (2 vol) a few feldspar grains are partially altered to sericite some quartz grains show undulatory extinction Matrix (mainly sericite quartz and chlorite) amounts to 11 vol no evidence of shock
518 F Karikari et al
LB-34 6deg3313 N1deg2264 W
Granite Granite composed mainly of quartz (38 vol) plagioclase (29 vol) K-feldspar (15 vol) biotite (5 vol) sericite (12 vol) and traces of Fe oxides and other opaque minerals (1 vol) very large biotite grains (partially oxidized) feldspar is partially altered to sericite no evidence of shock
LB-38 6deg3079 N1deg2052 W
Granite Coarse-grained granite muscovite-rich composed of quartz K-feldspar plagioclase muscovite sericite chlorite and opaque minerals feldspar is intensely altered to sericite no evidence of shock
LB-39a 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color) with angular to subrounded lithic clasts (up to 2 cm size) set into a clastic matrix (40 vol) clasts include graphitic shale (13 vol) phyllite (7 vol) meta-graywacke (13 vol) microgranite (2 vol) quartz and quartzitic grains (10 vol) chert (4 vol) melt (altered andor recrystallized) fragments and diaplectic quartz glass (together 11 vol) A few quartz grains show PDFs some clasts and matrix are altered (brownish oxides chlorite and other phyllosilicates)
LB-39c 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color similar to sample LB-39a) with angular to subrounded clasts (up to 15 cm) in a clastic matrix clasts include phyllite meta-graywacke microgranite glassmelt (mostly vesicular and fresh) diaplectic quartz glass quartz quartzite and feldspar A few quartz grains show PDFs (up to 2 sets) some clasts and matrix are altered to phyllosillicates (argillic alteration)
LB-40 6deg3388 N1deg2388 W
Large melt fragment from suevite
Large gray melt fragment from suevite (gray) that consists of melt matrix and melted or vitrified clasts (88 vol) very few clasts are unmelted unshocked meta-graywacke clasts (9 vol) few quartz (3 vol) and feldspar clasts (lt1 vol) meltglass (some fragments very vesicular and others partially recrystallized) diaplectic quartz and ballen quartz are dominant are dominant and a few vitrified metasediment clasts are present as well
LB-43 6deg3388 N1deg2388 W
Suevite Suevite (brownish in color very similar to sample LB-39a) with some angular to subrounded clasts in a clastic matrix (42 vol) clast population includes shale (15 vol) microgranite (5 vol) vitrified phyllite (6 vol) meltglass (mostly fresh vesicular glass diaplectic quartz glass some altered melt) (12 vol) meta-graywacke (13 vol) and quartz and quartzitic grains (7 vol) some clasts and matrix are altered (brownish oxides chlorite) a few quartz grains with PDFs (up to 2 sets)
LB-44A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt clast from suevite highly vesicular and very clast-poor clast population includes diaplectic quartz glass ballen quartz unshocked quartz and one vitrified meta-graywacke
LB-44B 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite (similar to sample LB-44A) with rounded 2 cm wide ballen quartz inclusion
LB-45 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite highly vesicular and very clast-poor (similar to sample LB-44A) clast population includes diaplectic quartz glass ballen quartz vitrified meta-graywacke and unshocked quartz
LB-45A 6deg3388 N1deg2388 W
Melt clast from suevite
Melt clast from suevite highly vesicular and very clast-poor (lt5 vol similar to sample LB-45) clast population includes diaplectic quartz glass ballen quartz and unshocked quartz
LB-45B 6deg3388 N1deg2388 W
Melt fragmentfrom suevite
Melt fragment from bulk suevite gray in color similar to sample LB-47A consists of melt matrix and melted or vitrified clasts (only a few quartz meta-graywacke quartzite and feldspar clasts are unmelted) meltglass (some highly vesicular others partially recrystallized) diaplectic quartz and ballen quartz and a few vitrified metasediment (mainly graywacke) clasts are present
LB-46 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment sample gray in color similar to sample LB-45B
LB-47A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-40) that consists of melt matrix and melted or vitrified clasts (few quartz meta-graywacke and quartzite clasts are unmelted or unvitrified) meltglass (some fragments are very vesicular andor with flow structure others are partially recrystallized) diaplectic quartz and ballen quartz are dominant but also some vitrified metasediment clasts
LB-47B 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-47A) that consists of melt matrix and melted or vitrified clasts (few quartzite quartz and feldspar are unmelted or unvitrified) meltglass (with well-developed flow structure some fragments are highly vesicular others are partially recrystallized) diaplectic quartz and ballen quartz are dominant
LB-48 6deg3388 N1deg2388 W
Melt clast from suevite
Large gt4 cm in diameter melt clast in suevite (light gray in color) well-developed flow structures are visible some parts of the clast are highly vesicular others are partially recrystallized few unmelted or unvitrified quartz and quartzite clasts are also preserved inside the melt fragment diaplectic quartz and ballen quartz are also present
LB-51 6deg2674 N1deg2262 W
Graphitic shale Well-laminated fine-grained graphitic shale (black gray in color) composed mainly of quartz optically unidentifiable phyllosilicates and carbon (graphite) local development of crenulation cleavage no trace of shock deformation
Table 1 Continued Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
516 F Karikari et al
some typical alteration and other characteristics of thecountry rock and suevite samples are shown in Figs 2ndash8 Ageneralized description of the main country rock types andthe suevites is given below
ShaleIn hand specimen two types of shale differing in color
and appearance can be distinguished a) banded shale which
is a soft highly argillaceous rock consisting of alternatingvery thin beds of light gray dark gray or black color and b)graphitic shale which is also a soft but dull blackargillaceous rock that stains the fingers when handled Thereis minor disseminated sulfide in some of the shale samplesThe thin sections show quartz feldspar iron oxides opaqueminerals (sulfides) and very fine-grained opticallyunidentifiable phyllosilicates (Fig 2a) Some shale samples
Fig 1 A geological map of the area of the Bosumtwi impact structure in Ghana (inset) Modified after Jones et al (1981) and Reimold et al(1998) The locations of sampling sites are also shown
Petrography geochemistry and alteration of country rocks from Bosumtwi 517
Table 1 Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
LB-2 6deg328 N1deg264 W
Siltstone Massive very fine-grained greenish gray siltstone with shear fabric quartz dominant rich in biotite (locally altered to chlorite) some feldspar muscovite and large nodules of opaque minerals a few veinlets of Fe oxides occur no evidence of shock
LB-3A 6deg3305 N1deg259 W
Quartz-rich schist Quartzite bands (about 2ndash3 mm thick) with thin intercalated biotite-rich bands (up to 1 mm) quartz is well sutured and frequently displays undulatory extinction some fine-grained biotite is partially aligned parallel to the schistosity partially discordant and much biotite is deformed (kink banding) no characteristic evidence of shock
LB-5 6deg3305 N1deg259 W
Shale Very fine-grained gray-black shale with some even darker (carbon-rich) bands composed of quartz feldspar and Fe oxides a few thin veinlets of quartz cross-cut the section no evidence of shock
LB-7 6deg3315 N1deg2575 W
Meta-graywacke(biotite rich)
Medium-grained mylonitized meta-graywacke composed of quartz (47 vol) plagioclase (6 vol) K-feldspar (9 vol) biotite (7 vol) and opaque minerals and other traces (2 vol) large biotite grains are partially oxidized fine-grained chlorite grains appear unaltered feldspar is partially altered to sericite no evidence of shock Matrix (mainly biotite chlorite and sericite) is 29 vol
LB-8 6deg3312 N1deg2563 W
Meta-graywacke Medium-grained gray meta-graywacke composed of quartz (39 vol) K-feldspar (6 vol) plagioclase (4 vol) sericite chlorite muscovite and opaque minerals (1 vol) most of the feldspar is altered to sericite and biotite is completely altered to chlorite Matrix (mainly sericite chlorite and quartz) is 40 vol no evidence of shock
LB-10 6deg3306 N1deg256 W
Microgranite Altered fine- to medium-grained microgranite composed of plagioclase (26 vol) quartz (24 vol) K-feldspar (14 vol) biotite (13 vol) muscovite (12 vol) Fe oxides (9 vol) and accessory minerals (2 vol) biotite grains are mostly oxidized no evidence of shock
LB-11 6deg3306 N1deg256 W
Mylonitic shale Fine-grained dark gray mylonitic shale composed of quartz phyllosilicates feldspar and opaque minerals some wide quartz ribbons no evidence of shock
LB-18 6deg327 N1deg257 W
Microgranite Fine- to medium-grained slightly sheared microgranite consists of quartz (40 vol) plagioclase (20 vol) K-feldspar (10 vol) biotite (10 vol) muscovitesericite (13 vol) chlorite (6 vol) and accessory minerals (mainly sphene) (1 vol) most of the biotite is oxidized and occurs in ldquoclustersrdquo some granophyric intergrowths of quartz in K-feldspar no evidence of shock
LB-19A 6deg327 N1deg257 W
Granite Altered medium-grained granite consists of quartz (27 vol) K-feldspar (22 vol) plagioclase (11 vol) biotite (5 vol) muscovite (5 vol) chlorite (27 vol) Fe oxides and accessory minerals (eg sphene) (3 vol) granophyric intergrowth of quartz and K-feldspar is abundant nice spherulites of feldspar most of the biotite is altered micro-fractures partially filled with Fe oxides cross-cut the section no evidence of shock
LB-19B 6deg327 N1deg257 W
Meta-graywacke Altered medium-grained sheared meta-graywacke (biotite-rich similar to sample LB-7) composed of quartz (36 vol) plagioclase (7 vol) K-feldspar (9 vol) biotite (5 vol) sericite and opaque minerals (mainly Fe oxides) (3 vol) some biotite grains are partially oxidized some feldspar grains are partially altered to sericite no evidence of shock Matrix (mainly sericite quartz chert and chlorite) amounts to 42 vol
LB-24 6deg330 N1deg256 W
Granite Medium-grained granite (very few oxides in this sample compared to LB-25 and very little granophyric intergrowth no spherulites observed) consists of plagioclase (42 vol) K-feldspar (30 vol) quartz (8 vol) biotite (some completely oxidizedaltered to chlorite) (13 vol) muscovite (5 vol) and Fe oxide and accessory minerals (2 vol) most feldspar is altered to sericite no evidence of shock
LB-25 6deg330 N1deg256 W
Granite Altered medium-grained granite (similar to LB-19A but with more oxides) consists of quartz (35 vol) K-feldspar (15 vol) plagioclase (10 vol) secondary phyllosilicate (25 vol) biotite completely altered to chlorite (5 vol) and Fe oxide (10 vol) granophyric intergrowth of quartz K-feldspar is abundant some spherulites of feldspar no evidence of shock
LB-26 6deg330 N1deg256 W
Granite Altered fine- to medium-grained granite composed of quartz (38 vol) K-feldspar (10 vol) plagioclase (5 vol) biotite (completely altered to chlorite 10 vol) Fe oxides (4 vol) and sericite (37 vol) some well-developed spherulites of feldspar granophyric intergrowth of quartz and K-feldspar occurs mostly at the edges of the spherulites no evidence of shock
LB-32 6deg331 N1deg229 W
Mylonitic shalephyllite
Very fine- to fine-grained mylonitic shale phyllite dark gray in color (sample locally similar to LB-11) composed of quartz phyllosilicates (biotite identified other phyllosilicates optically not identifiable) feldspar and opaque minerals thin quartz veinlets cross-cut the section no evidence of shock
LB-33 6deg3290 N1deg2228 W
Meta-graywacke Medium-grained meta-graywacke (similar to LB-8 but matrix poor) composed of quartz (64 vol) K-feldspar (12 vol) plagioclase (9 vol) biotite (mostly oxidized) (1 vol) opaque minerals (1 vol) and accessory minerals (zircon sphene epidote) (2 vol) a few feldspar grains are partially altered to sericite some quartz grains show undulatory extinction Matrix (mainly sericite quartz and chlorite) amounts to 11 vol no evidence of shock
518 F Karikari et al
LB-34 6deg3313 N1deg2264 W
Granite Granite composed mainly of quartz (38 vol) plagioclase (29 vol) K-feldspar (15 vol) biotite (5 vol) sericite (12 vol) and traces of Fe oxides and other opaque minerals (1 vol) very large biotite grains (partially oxidized) feldspar is partially altered to sericite no evidence of shock
LB-38 6deg3079 N1deg2052 W
Granite Coarse-grained granite muscovite-rich composed of quartz K-feldspar plagioclase muscovite sericite chlorite and opaque minerals feldspar is intensely altered to sericite no evidence of shock
LB-39a 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color) with angular to subrounded lithic clasts (up to 2 cm size) set into a clastic matrix (40 vol) clasts include graphitic shale (13 vol) phyllite (7 vol) meta-graywacke (13 vol) microgranite (2 vol) quartz and quartzitic grains (10 vol) chert (4 vol) melt (altered andor recrystallized) fragments and diaplectic quartz glass (together 11 vol) A few quartz grains show PDFs some clasts and matrix are altered (brownish oxides chlorite and other phyllosilicates)
LB-39c 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color similar to sample LB-39a) with angular to subrounded clasts (up to 15 cm) in a clastic matrix clasts include phyllite meta-graywacke microgranite glassmelt (mostly vesicular and fresh) diaplectic quartz glass quartz quartzite and feldspar A few quartz grains show PDFs (up to 2 sets) some clasts and matrix are altered to phyllosillicates (argillic alteration)
LB-40 6deg3388 N1deg2388 W
Large melt fragment from suevite
Large gray melt fragment from suevite (gray) that consists of melt matrix and melted or vitrified clasts (88 vol) very few clasts are unmelted unshocked meta-graywacke clasts (9 vol) few quartz (3 vol) and feldspar clasts (lt1 vol) meltglass (some fragments very vesicular and others partially recrystallized) diaplectic quartz and ballen quartz are dominant are dominant and a few vitrified metasediment clasts are present as well
LB-43 6deg3388 N1deg2388 W
Suevite Suevite (brownish in color very similar to sample LB-39a) with some angular to subrounded clasts in a clastic matrix (42 vol) clast population includes shale (15 vol) microgranite (5 vol) vitrified phyllite (6 vol) meltglass (mostly fresh vesicular glass diaplectic quartz glass some altered melt) (12 vol) meta-graywacke (13 vol) and quartz and quartzitic grains (7 vol) some clasts and matrix are altered (brownish oxides chlorite) a few quartz grains with PDFs (up to 2 sets)
LB-44A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt clast from suevite highly vesicular and very clast-poor clast population includes diaplectic quartz glass ballen quartz unshocked quartz and one vitrified meta-graywacke
LB-44B 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite (similar to sample LB-44A) with rounded 2 cm wide ballen quartz inclusion
LB-45 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite highly vesicular and very clast-poor (similar to sample LB-44A) clast population includes diaplectic quartz glass ballen quartz vitrified meta-graywacke and unshocked quartz
LB-45A 6deg3388 N1deg2388 W
Melt clast from suevite
Melt clast from suevite highly vesicular and very clast-poor (lt5 vol similar to sample LB-45) clast population includes diaplectic quartz glass ballen quartz and unshocked quartz
LB-45B 6deg3388 N1deg2388 W
Melt fragmentfrom suevite
Melt fragment from bulk suevite gray in color similar to sample LB-47A consists of melt matrix and melted or vitrified clasts (only a few quartz meta-graywacke quartzite and feldspar clasts are unmelted) meltglass (some highly vesicular others partially recrystallized) diaplectic quartz and ballen quartz and a few vitrified metasediment (mainly graywacke) clasts are present
LB-46 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment sample gray in color similar to sample LB-45B
LB-47A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-40) that consists of melt matrix and melted or vitrified clasts (few quartz meta-graywacke and quartzite clasts are unmelted or unvitrified) meltglass (some fragments are very vesicular andor with flow structure others are partially recrystallized) diaplectic quartz and ballen quartz are dominant but also some vitrified metasediment clasts
LB-47B 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-47A) that consists of melt matrix and melted or vitrified clasts (few quartzite quartz and feldspar are unmelted or unvitrified) meltglass (with well-developed flow structure some fragments are highly vesicular others are partially recrystallized) diaplectic quartz and ballen quartz are dominant
LB-48 6deg3388 N1deg2388 W
Melt clast from suevite
Large gt4 cm in diameter melt clast in suevite (light gray in color) well-developed flow structures are visible some parts of the clast are highly vesicular others are partially recrystallized few unmelted or unvitrified quartz and quartzite clasts are also preserved inside the melt fragment diaplectic quartz and ballen quartz are also present
LB-51 6deg2674 N1deg2262 W
Graphitic shale Well-laminated fine-grained graphitic shale (black gray in color) composed mainly of quartz optically unidentifiable phyllosilicates and carbon (graphite) local development of crenulation cleavage no trace of shock deformation
Table 1 Continued Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
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Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
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Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 517
Table 1 Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
LB-2 6deg328 N1deg264 W
Siltstone Massive very fine-grained greenish gray siltstone with shear fabric quartz dominant rich in biotite (locally altered to chlorite) some feldspar muscovite and large nodules of opaque minerals a few veinlets of Fe oxides occur no evidence of shock
LB-3A 6deg3305 N1deg259 W
Quartz-rich schist Quartzite bands (about 2ndash3 mm thick) with thin intercalated biotite-rich bands (up to 1 mm) quartz is well sutured and frequently displays undulatory extinction some fine-grained biotite is partially aligned parallel to the schistosity partially discordant and much biotite is deformed (kink banding) no characteristic evidence of shock
LB-5 6deg3305 N1deg259 W
Shale Very fine-grained gray-black shale with some even darker (carbon-rich) bands composed of quartz feldspar and Fe oxides a few thin veinlets of quartz cross-cut the section no evidence of shock
LB-7 6deg3315 N1deg2575 W
Meta-graywacke(biotite rich)
Medium-grained mylonitized meta-graywacke composed of quartz (47 vol) plagioclase (6 vol) K-feldspar (9 vol) biotite (7 vol) and opaque minerals and other traces (2 vol) large biotite grains are partially oxidized fine-grained chlorite grains appear unaltered feldspar is partially altered to sericite no evidence of shock Matrix (mainly biotite chlorite and sericite) is 29 vol
LB-8 6deg3312 N1deg2563 W
Meta-graywacke Medium-grained gray meta-graywacke composed of quartz (39 vol) K-feldspar (6 vol) plagioclase (4 vol) sericite chlorite muscovite and opaque minerals (1 vol) most of the feldspar is altered to sericite and biotite is completely altered to chlorite Matrix (mainly sericite chlorite and quartz) is 40 vol no evidence of shock
LB-10 6deg3306 N1deg256 W
Microgranite Altered fine- to medium-grained microgranite composed of plagioclase (26 vol) quartz (24 vol) K-feldspar (14 vol) biotite (13 vol) muscovite (12 vol) Fe oxides (9 vol) and accessory minerals (2 vol) biotite grains are mostly oxidized no evidence of shock
LB-11 6deg3306 N1deg256 W
Mylonitic shale Fine-grained dark gray mylonitic shale composed of quartz phyllosilicates feldspar and opaque minerals some wide quartz ribbons no evidence of shock
LB-18 6deg327 N1deg257 W
Microgranite Fine- to medium-grained slightly sheared microgranite consists of quartz (40 vol) plagioclase (20 vol) K-feldspar (10 vol) biotite (10 vol) muscovitesericite (13 vol) chlorite (6 vol) and accessory minerals (mainly sphene) (1 vol) most of the biotite is oxidized and occurs in ldquoclustersrdquo some granophyric intergrowths of quartz in K-feldspar no evidence of shock
LB-19A 6deg327 N1deg257 W
Granite Altered medium-grained granite consists of quartz (27 vol) K-feldspar (22 vol) plagioclase (11 vol) biotite (5 vol) muscovite (5 vol) chlorite (27 vol) Fe oxides and accessory minerals (eg sphene) (3 vol) granophyric intergrowth of quartz and K-feldspar is abundant nice spherulites of feldspar most of the biotite is altered micro-fractures partially filled with Fe oxides cross-cut the section no evidence of shock
LB-19B 6deg327 N1deg257 W
Meta-graywacke Altered medium-grained sheared meta-graywacke (biotite-rich similar to sample LB-7) composed of quartz (36 vol) plagioclase (7 vol) K-feldspar (9 vol) biotite (5 vol) sericite and opaque minerals (mainly Fe oxides) (3 vol) some biotite grains are partially oxidized some feldspar grains are partially altered to sericite no evidence of shock Matrix (mainly sericite quartz chert and chlorite) amounts to 42 vol
LB-24 6deg330 N1deg256 W
Granite Medium-grained granite (very few oxides in this sample compared to LB-25 and very little granophyric intergrowth no spherulites observed) consists of plagioclase (42 vol) K-feldspar (30 vol) quartz (8 vol) biotite (some completely oxidizedaltered to chlorite) (13 vol) muscovite (5 vol) and Fe oxide and accessory minerals (2 vol) most feldspar is altered to sericite no evidence of shock
LB-25 6deg330 N1deg256 W
Granite Altered medium-grained granite (similar to LB-19A but with more oxides) consists of quartz (35 vol) K-feldspar (15 vol) plagioclase (10 vol) secondary phyllosilicate (25 vol) biotite completely altered to chlorite (5 vol) and Fe oxide (10 vol) granophyric intergrowth of quartz K-feldspar is abundant some spherulites of feldspar no evidence of shock
LB-26 6deg330 N1deg256 W
Granite Altered fine- to medium-grained granite composed of quartz (38 vol) K-feldspar (10 vol) plagioclase (5 vol) biotite (completely altered to chlorite 10 vol) Fe oxides (4 vol) and sericite (37 vol) some well-developed spherulites of feldspar granophyric intergrowth of quartz and K-feldspar occurs mostly at the edges of the spherulites no evidence of shock
LB-32 6deg331 N1deg229 W
Mylonitic shalephyllite
Very fine- to fine-grained mylonitic shale phyllite dark gray in color (sample locally similar to LB-11) composed of quartz phyllosilicates (biotite identified other phyllosilicates optically not identifiable) feldspar and opaque minerals thin quartz veinlets cross-cut the section no evidence of shock
LB-33 6deg3290 N1deg2228 W
Meta-graywacke Medium-grained meta-graywacke (similar to LB-8 but matrix poor) composed of quartz (64 vol) K-feldspar (12 vol) plagioclase (9 vol) biotite (mostly oxidized) (1 vol) opaque minerals (1 vol) and accessory minerals (zircon sphene epidote) (2 vol) a few feldspar grains are partially altered to sericite some quartz grains show undulatory extinction Matrix (mainly sericite quartz and chlorite) amounts to 11 vol no evidence of shock
518 F Karikari et al
LB-34 6deg3313 N1deg2264 W
Granite Granite composed mainly of quartz (38 vol) plagioclase (29 vol) K-feldspar (15 vol) biotite (5 vol) sericite (12 vol) and traces of Fe oxides and other opaque minerals (1 vol) very large biotite grains (partially oxidized) feldspar is partially altered to sericite no evidence of shock
LB-38 6deg3079 N1deg2052 W
Granite Coarse-grained granite muscovite-rich composed of quartz K-feldspar plagioclase muscovite sericite chlorite and opaque minerals feldspar is intensely altered to sericite no evidence of shock
LB-39a 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color) with angular to subrounded lithic clasts (up to 2 cm size) set into a clastic matrix (40 vol) clasts include graphitic shale (13 vol) phyllite (7 vol) meta-graywacke (13 vol) microgranite (2 vol) quartz and quartzitic grains (10 vol) chert (4 vol) melt (altered andor recrystallized) fragments and diaplectic quartz glass (together 11 vol) A few quartz grains show PDFs some clasts and matrix are altered (brownish oxides chlorite and other phyllosilicates)
LB-39c 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color similar to sample LB-39a) with angular to subrounded clasts (up to 15 cm) in a clastic matrix clasts include phyllite meta-graywacke microgranite glassmelt (mostly vesicular and fresh) diaplectic quartz glass quartz quartzite and feldspar A few quartz grains show PDFs (up to 2 sets) some clasts and matrix are altered to phyllosillicates (argillic alteration)
LB-40 6deg3388 N1deg2388 W
Large melt fragment from suevite
Large gray melt fragment from suevite (gray) that consists of melt matrix and melted or vitrified clasts (88 vol) very few clasts are unmelted unshocked meta-graywacke clasts (9 vol) few quartz (3 vol) and feldspar clasts (lt1 vol) meltglass (some fragments very vesicular and others partially recrystallized) diaplectic quartz and ballen quartz are dominant are dominant and a few vitrified metasediment clasts are present as well
LB-43 6deg3388 N1deg2388 W
Suevite Suevite (brownish in color very similar to sample LB-39a) with some angular to subrounded clasts in a clastic matrix (42 vol) clast population includes shale (15 vol) microgranite (5 vol) vitrified phyllite (6 vol) meltglass (mostly fresh vesicular glass diaplectic quartz glass some altered melt) (12 vol) meta-graywacke (13 vol) and quartz and quartzitic grains (7 vol) some clasts and matrix are altered (brownish oxides chlorite) a few quartz grains with PDFs (up to 2 sets)
LB-44A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt clast from suevite highly vesicular and very clast-poor clast population includes diaplectic quartz glass ballen quartz unshocked quartz and one vitrified meta-graywacke
LB-44B 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite (similar to sample LB-44A) with rounded 2 cm wide ballen quartz inclusion
LB-45 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite highly vesicular and very clast-poor (similar to sample LB-44A) clast population includes diaplectic quartz glass ballen quartz vitrified meta-graywacke and unshocked quartz
LB-45A 6deg3388 N1deg2388 W
Melt clast from suevite
Melt clast from suevite highly vesicular and very clast-poor (lt5 vol similar to sample LB-45) clast population includes diaplectic quartz glass ballen quartz and unshocked quartz
LB-45B 6deg3388 N1deg2388 W
Melt fragmentfrom suevite
Melt fragment from bulk suevite gray in color similar to sample LB-47A consists of melt matrix and melted or vitrified clasts (only a few quartz meta-graywacke quartzite and feldspar clasts are unmelted) meltglass (some highly vesicular others partially recrystallized) diaplectic quartz and ballen quartz and a few vitrified metasediment (mainly graywacke) clasts are present
LB-46 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment sample gray in color similar to sample LB-45B
LB-47A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-40) that consists of melt matrix and melted or vitrified clasts (few quartz meta-graywacke and quartzite clasts are unmelted or unvitrified) meltglass (some fragments are very vesicular andor with flow structure others are partially recrystallized) diaplectic quartz and ballen quartz are dominant but also some vitrified metasediment clasts
LB-47B 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-47A) that consists of melt matrix and melted or vitrified clasts (few quartzite quartz and feldspar are unmelted or unvitrified) meltglass (with well-developed flow structure some fragments are highly vesicular others are partially recrystallized) diaplectic quartz and ballen quartz are dominant
LB-48 6deg3388 N1deg2388 W
Melt clast from suevite
Large gt4 cm in diameter melt clast in suevite (light gray in color) well-developed flow structures are visible some parts of the clast are highly vesicular others are partially recrystallized few unmelted or unvitrified quartz and quartzite clasts are also preserved inside the melt fragment diaplectic quartz and ballen quartz are also present
LB-51 6deg2674 N1deg2262 W
Graphitic shale Well-laminated fine-grained graphitic shale (black gray in color) composed mainly of quartz optically unidentifiable phyllosilicates and carbon (graphite) local development of crenulation cleavage no trace of shock deformation
Table 1 Continued Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
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Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
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Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
518 F Karikari et al
LB-34 6deg3313 N1deg2264 W
Granite Granite composed mainly of quartz (38 vol) plagioclase (29 vol) K-feldspar (15 vol) biotite (5 vol) sericite (12 vol) and traces of Fe oxides and other opaque minerals (1 vol) very large biotite grains (partially oxidized) feldspar is partially altered to sericite no evidence of shock
LB-38 6deg3079 N1deg2052 W
Granite Coarse-grained granite muscovite-rich composed of quartz K-feldspar plagioclase muscovite sericite chlorite and opaque minerals feldspar is intensely altered to sericite no evidence of shock
LB-39a 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color) with angular to subrounded lithic clasts (up to 2 cm size) set into a clastic matrix (40 vol) clasts include graphitic shale (13 vol) phyllite (7 vol) meta-graywacke (13 vol) microgranite (2 vol) quartz and quartzitic grains (10 vol) chert (4 vol) melt (altered andor recrystallized) fragments and diaplectic quartz glass (together 11 vol) A few quartz grains show PDFs some clasts and matrix are altered (brownish oxides chlorite and other phyllosilicates)
LB-39c 6deg2698 N1deg2588 W
Suevite Suevite (brownish in color similar to sample LB-39a) with angular to subrounded clasts (up to 15 cm) in a clastic matrix clasts include phyllite meta-graywacke microgranite glassmelt (mostly vesicular and fresh) diaplectic quartz glass quartz quartzite and feldspar A few quartz grains show PDFs (up to 2 sets) some clasts and matrix are altered to phyllosillicates (argillic alteration)
LB-40 6deg3388 N1deg2388 W
Large melt fragment from suevite
Large gray melt fragment from suevite (gray) that consists of melt matrix and melted or vitrified clasts (88 vol) very few clasts are unmelted unshocked meta-graywacke clasts (9 vol) few quartz (3 vol) and feldspar clasts (lt1 vol) meltglass (some fragments very vesicular and others partially recrystallized) diaplectic quartz and ballen quartz are dominant are dominant and a few vitrified metasediment clasts are present as well
LB-43 6deg3388 N1deg2388 W
Suevite Suevite (brownish in color very similar to sample LB-39a) with some angular to subrounded clasts in a clastic matrix (42 vol) clast population includes shale (15 vol) microgranite (5 vol) vitrified phyllite (6 vol) meltglass (mostly fresh vesicular glass diaplectic quartz glass some altered melt) (12 vol) meta-graywacke (13 vol) and quartz and quartzitic grains (7 vol) some clasts and matrix are altered (brownish oxides chlorite) a few quartz grains with PDFs (up to 2 sets)
LB-44A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt clast from suevite highly vesicular and very clast-poor clast population includes diaplectic quartz glass ballen quartz unshocked quartz and one vitrified meta-graywacke
LB-44B 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite (similar to sample LB-44A) with rounded 2 cm wide ballen quartz inclusion
LB-45 6deg3388 N1deg2388 W
Melt clastfrom suevite
Melt clast from suevite highly vesicular and very clast-poor (similar to sample LB-44A) clast population includes diaplectic quartz glass ballen quartz vitrified meta-graywacke and unshocked quartz
LB-45A 6deg3388 N1deg2388 W
Melt clast from suevite
Melt clast from suevite highly vesicular and very clast-poor (lt5 vol similar to sample LB-45) clast population includes diaplectic quartz glass ballen quartz and unshocked quartz
LB-45B 6deg3388 N1deg2388 W
Melt fragmentfrom suevite
Melt fragment from bulk suevite gray in color similar to sample LB-47A consists of melt matrix and melted or vitrified clasts (only a few quartz meta-graywacke quartzite and feldspar clasts are unmelted) meltglass (some highly vesicular others partially recrystallized) diaplectic quartz and ballen quartz and a few vitrified metasediment (mainly graywacke) clasts are present
LB-46 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment sample gray in color similar to sample LB-45B
LB-47A 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-40) that consists of melt matrix and melted or vitrified clasts (few quartz meta-graywacke and quartzite clasts are unmelted or unvitrified) meltglass (some fragments are very vesicular andor with flow structure others are partially recrystallized) diaplectic quartz and ballen quartz are dominant but also some vitrified metasediment clasts
LB-47B 6deg3388 N1deg2388 W
Melt fragment from suevite
Melt fragment from suevite (gray in color very similar to sample LB-47A) that consists of melt matrix and melted or vitrified clasts (few quartzite quartz and feldspar are unmelted or unvitrified) meltglass (with well-developed flow structure some fragments are highly vesicular others are partially recrystallized) diaplectic quartz and ballen quartz are dominant
LB-48 6deg3388 N1deg2388 W
Melt clast from suevite
Large gt4 cm in diameter melt clast in suevite (light gray in color) well-developed flow structures are visible some parts of the clast are highly vesicular others are partially recrystallized few unmelted or unvitrified quartz and quartzite clasts are also preserved inside the melt fragment diaplectic quartz and ballen quartz are also present
LB-51 6deg2674 N1deg2262 W
Graphitic shale Well-laminated fine-grained graphitic shale (black gray in color) composed mainly of quartz optically unidentifiable phyllosilicates and carbon (graphite) local development of crenulation cleavage no trace of shock deformation
Table 1 Continued Petrographic description of rock samples from the Bosumtwi impact structure collected in 1997Sample no Location Rock type Petrographic description
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
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1 s
tand
ard
devi
atio
ns) o
f ana
lyze
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untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
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ontin
enta
l cru
st
Shal
e-ph
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e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
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ktite
aver
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Upp
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cr
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Ave
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Ave
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Ran
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ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 519Ta
ble
2 M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
SiO
271
358
1 6
35
66
659
4 6
51
71
0 6
73
747
66
169
2 8
78
100
4Ti
O2
081
013
06
4 0
72
081
06
3 0
43
05
90
34 0
58
053
02
70
10A
l 2O3
970
144
17
2 1
59
180
16
8 1
26
15
610
8 1
60
144
45
1lt0
01
Fe2O
37
456
83 6
50
65
110
5 5
52
45
5 5
75
337
58
94
36 3
05
040
MnO
007
006
00
5 0
03
013
00
3 0
12
00
30
05 0
04
005
00
40
01M
gO3
201
84 2
22
21
40
44 2
23
11
6 1
87
106
19
52
03 0
87
lt00
1C
aOlt0
01
099
04
4 0
14
lt00
1 0
44
10
1 0
88
078
07
20
91 0
19
001
Na 2
O0
211
49 2
03
03
51
52 2
20
32
2 3
11
357
27
04
39 0
73
002
K2O
056
254
27
4 2
75
250
21
2 0
90
16
20
37 2
75
069
05
80
01P 2
O5
005
047
01
3 0
07
012
01
3 0
12
00
90
04 0
18
011
00
3lt0
01
LoI
691
124
40
8 5
70
533
42
3 3
96
34
11
37 3
34
295
21
5lt0
01
Tota
l10
03
992
4 9
947
101
098
69
99
41 9
908
100
396
39
100
299
63
100
210
09
SiO
2Al 2O
37
354
04 3
70
41
83
30 3
88
56
2 4
31
693
41
44
79 1
95
K2O
Na 2
O2
641
71 1
35
78
21
64 0
97
02
8 0
52
010
10
20
16 0
78
042
Sc15
723
6 1
92
16
620
1 1
65
92
3 1
22
674
14
311
5 6
00
008
V
126
95 1
30 1
2413
1 1
1177
98
115
52
lt5C
r11
895
7 8
55
162
940
80
0 4
65
79
045
5 8
16
714
47
35
40C
o15
612
3 1
46
24
031
2 4
29
21
4 1
31
107
12
29
33 1
06
019
Ni
256
70 5
2 8
863
23
37
35
17 4
334
20
3C
u11
443
34
18
43 1
324
95
lt2 lt
2lt2
Zn10
366
74
105
153
6349
84
38 5
0lt9
As
524
106
23
1 3
39
658
38
7 2
63
04
11
13 0
12
309
06
90
72Se
02
121
18
02
02
15
15
lt1
41
5 2
42
0 lt
12
02
Rb
223
541
94
5 9
10
921
808
33
0 6
17
165
76
727
1 2
59
083
Sr65
220
0 2
06 1
0319
432
0 3
02 3
2728
2 3
1346
9 1
0515
4Y
3364
19
522
11
14 2
311
5lt3
Zr18
111
1 1
21 1
2316
492
7 1
3511
613
2 1
8315
1 3
75
493
Nb
106
9 1
012
98
10
8 7
5Sb
402
015
01
1 0
30
138
01
6 0
13
01
40
10 0
16
017
01
10
10C
s0
811
84 3
66
29
42
74 3
13
15
7 2
59
092
32
41
47 1
35
006
Ba
344
1170
836
587
639
498
363
661
146
110
218
9 1
9929
5La
173
110
52
8 2
03
273
23
4 3
50
99
423
8 1
26
160
52
00
09C
e28
322
3 2
28
40
460
6 7
44
13
7 2
327
468
25
622
9 1
68
016
Nd
170
116
74
4 2
15
252
41
4 5
93
11
9719
0 1
34
107
51
50
27Sm
432
237
15
2 0
52
505
09
0 1
33
25
43
30 3
16
235
11
50
02Eu
116
563
05
0 0
17
136
02
9 0
30
07
61
03 0
94
096
03
50
01G
d4
5819
2 1
66
08
04
35 1
11
13
0 2
30
293
26
02
62 1
06
056
Tb0
962
55 0
34
01
40
75 0
18
03
0 0
36
037
04
80
39 0
17
002
Tm0
470
74 0
27
01
60
40 0
16
01
4 0
22
017
02
90
22 0
10
006
Yb
322
496
19
3 1
28
238
13
0 1
01
15
71
01 2
29
153
07
00
05Lu
054
067
02
8 0
20
038
02
1 0
16
02
60
17 0
33
022
01
00
00
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
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untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
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red
to a
vera
ge c
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sitio
n of
Iv
ory
Coa
st te
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s an
d up
per c
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enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
520 F Karikari et al
Hf
250
236
27
3 3
52
419
24
3 2
40
31
02
61 4
19
316
07
60
01Ta
045
008
04
3 0
54
057
03
9 0
26
03
00
28 0
47
034
01
50
03A
u (p
pb)
155
00 1
4 0
2lt1
2 1
5 1
6 lt
11
08
10
13
03
01
Th2
872
44 3
22
37
64
64 2
63
17
9 3
08
332
32
23
06 1
00
002
U2
536
20 1
58
14
12
72 1
12
05
3 0
59
078
06
30
63 0
39
002
CIA
9167
71
81
78 7
8 6
165
5865
60 6
7K
U18
4234
0514
376
161
8976
3415
683
141
5422
995
3939
364
5291
9312
390
3195
ThU
114
039
20
3
267
171
23
4 3
40
52
64
28 5
13
489
25
91
30La
Th
602
449
16
4 0
54
587
08
9 1
96
32
27
17 3
91
524
52
13
85Zr
Hf
723
470
44
1 3
48
391
38
1 5
60
375
508
43
647
8 4
96
460
HfT
a5
6228
4 6
32
65
87
36 6
17
93
5 1
02
931
89
19
34 4
95
033
LaN
Yb N
363
149
18
5 1
07
773
12
2 2
33
42
916
0 3
71
709
50
31
08G
dNY
b N1
153
14 0
70
05
11
48 0
69
10
4 1
19
236
09
21
39 1
23
847
EuE
u0
800
81 0
95
08
00
89 0
87
06
9 0
96
101
10
01
18 0
97
037
a Sam
ple n
ot an
alyz
ed b
y X
RF
for t
race
elem
ents
(lac
k of
mat
eria
l) b
lank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
) = (A
l 2O3[
Al 2O
3+
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
u N(S
mN
times G
d N)0
5 M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as n
oted
all
Fe a
s Fe 2
O3
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
SiO
262
865
6
672
613
74
3
664
71
468
4
636
TiO
20
690
62
045
060
0
13
067
0
990
58
052
Al 2O
317
615
0
164
144
14
9
169
17
315
1
149
Fe2O
35
585
51
436
776
1
10
472
0
983
19
604
MnO
005
008
0
060
11
002
0
05
001
008
0
08M
gO2
593
98
120
572
0
30
174
0
341
69
588
CaO
081
097
2
160
12
080
1
09
016
314
0
62N
a 2O
351
318
4
581
57
521
4
37
467
486
2
89K
2O1
460
85
082
120
2
25
163
1
812
57
093
P 2O
50
170
19
015
010
0
03
024
0
020
23
017
LO
I4
954
07
234
736
1
07
325
2
340
53
528
Tota
l10
01
100
1
997
410
03
10
00
10
10
10
01
100
3
100
9
SiO
2Al 2O
33
574
36
409
426
4
99
392
4
124
54
427
K2O
Na 2
O0
420
27
018
076
0
43
037
0
390
53
032
Sc15
015
9
868
207
3
58
130
3
616
11
164
V
113
124
83
139
14
64
67
50
105
Cr
571
506
7
0155
0
900
36
5
225
395
54
0C
o13
117
9
943
240
0
98
913
5
638
67
230
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of c
ount
ry ro
cks
from
the
Bos
umtw
i im
pact
stru
ctur
e
Shal
eph
yllit
eM
eta-
gray
wac
keSi
ltsto
neA
rkos
eQ
uartz
-ric
h sc
hist
Qua
rtz(v
ein
)G
raph
itic
shal
eSh
ale
LB-5
1LB
-5LB
-11
LB-3
2LB
-37
LB-1
3aLB
-9a
LB-2
2LB
-33
LB-2
LB-2
0LB
-3A
LB-4
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 521
Ni
3420
19
124
9
18
13
27
135
Cu
lt2lt2
19
lt2
15
lt2
lt28
lt2
Zn78
70
5796
35
59
25
69
78A
s13
20
93
136
356
2
60
095
4
865
73
114
Se1
11
5
13
23
1
5
lt13
0
4lt1
2
lt18
Rb
467
460
29
445
7
505
59
7
609
796
19
4Sr
202
390
48
815
7
256
36
1
566
1205
24
1Y
1011
13
13
13
18
1113
10
Zr17
315
5
130
105
78
2
131
23
224
7
105
Nb
108
7
8
9
9
2010
8
Sb0
360
25
022
012
0
14
031
lt0
11
010
0
02C
s2
052
09
135
198
2
10
296
2
854
42
077
Ba
254
323
29
734
8
624
67
6
536
1420
16
8La
761
275
10
421
7
131
19
4
238
712
16
0C
e18
556
6
243
432
23
3
360
50
312
7
322
Nd
617
289
10
720
3
113
23
0
276
617
16
9Sm
115
495
2
303
94
170
4
25
519
103
3
61Eu
032
133
0
871
12
050
1
26
140
277
1
12G
d1
753
40
217
326
1
50
355
3
406
13
300
Tb0
320
47
040
052
0
25
063
0
390
68
041
Tm0
190
21
017
027
0
17
026
0
110
17
018
Yb
147
133
1
051
89
116
2
11
065
090
1
02Lu
027
019
0
140
24
015
0
33
006
014
0
15H
f6
722
77
236
250
2
28
329
5
574
91
236
Ta1
070
29
024
024
0
50
029
1
300
41
020
Au
(ppb
)0
50
7
15
00
1
2
lt14
1
90
6
05
Th4
365
06
148
211
2
55
276
3
618
37
221
U1
600
93
065
102
1
38
072
1
402
72
068
CIA
6765
57
78
54
61
6448
68
KU
7619
7622
105
6997
2613
550
187
8510
722
7859
114
22Th
U2
735
45
230
206
1
86
383
2
573
08
325
LaT
h1
745
45
700
103
5
13
704
6
598
50
724
ZrH
f25
755
8
552
420
34
3
399
41
750
4
444
HfT
a6
269
59
994
106
4
55
114
4
3012
0
117
LaN
Yb N
350
140
6
667
76
762
6
22
249
534
10
6G
d NY
b N0
972
07
167
140
1
05
137
4
275
52
239
EuE
u0
700
99
119
095
0
95
099
1
021
07
104
Maj
or el
emen
ts in
wt
tra
ce el
emen
ts in
ppm
exc
ept a
s not
ed A
ll Fe
as F
e 2O
3 bl
ank
spac
es =
not
det
erm
ined
N =
chon
drite
-nor
mal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
al in
dex
of al
tera
tion
(CIA
)=
(Al 2O
3[A
l 2O3 +
CaO
+ N
a 2O
+ K
2O])
times 1
00 in
mol
ecul
ar p
ropo
rtion
s E
uEu
= E
uN(S
mN
times G
d N)0
5
Tabl
e 2
Con
tinue
d M
ajor
and
trac
e el
emen
t com
posi
tion
of ta
rget
rock
s fr
om th
e B
osum
twi i
mpa
ct s
truct
ure
Mic
rogr
anite
Mic
rogr
anite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
Gra
nite
LB-1
0LB
-18
LB-2
4LB
-26
LB-3
4LB
-36
LB-3
8LB
-50
LB-5
7
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
522 F Karikari et alTa
ble
3 M
ajor
- and
trac
e-el
emen
t com
posi
tion
of s
uevi
tes
and
mel
tgla
ss fr
agm
ents
from
the
Bos
umtw
i im
pact
stru
ctur
eSu
evite
Mel
tgla
ss fr
agm
ent
LB-3
0aLB
-30b
LB-3
1bLB
-31a
-6a
LB-3
9aLB
-39c
LB-4
1LB
-43
LB-4
0LB
-44
LB-4
5LB
-46
LB-4
7LB
-48
SiO
2
633
53
1
602
59
3
628
630
729
65
8
633
643
68
1
674
61
3Ti
O2
0
66
079
0
82
075
0
710
660
50
067
0
670
67
056
0
58
075
Al 2O
3
154
21
1
190
17
8
154
172
123
17
3
167
164
15
6
158
16
5Fe
2O3
6
29
997
7
03
491
7
49
714
592
492
6
59
611
618
6
15
462
6
41M
nO
005
0
06
005
0
10
013
004
005
0
03
004
003
0
04
007
0
04M
gO
079
2
02
171
2
61
248
091
228
0
83
125
099
0
77
167
1
25C
aO
117
1
06
094
0
87
051
090
026
0
98
104
137
1
32
315
1
34N
a 2O
1
86
162
2
09
247
1
78
196
185
291
1
69
200
239
2
60
378
2
63K
2O
134
3
10
252
1
88
193
1
731
111
68
177
1
691
38
165
2
63
178
P 2O
5
006
0
10
008
0
11
015
006
010
0
07
005
006
0
06
022
0
09L
OI
8
75
663
4
52
708
6
508
183
43
415
7
405
61
280
0
53
674
Tota
l
996
7
996
0
989
1
997
8
994
699
87
101
2
999
2
100
399
36
99
61
10
04
98
79
SiO
2Al 2O
3
411
2
51
317
3
34
409
366
594
3
81
378
392
4
37
427
3
71K
2ON
a 2O
0
72
191
1
20
076
1
08
088
060
058
1
04
085
058
0
63
070
0
67
Sc
163
25
5
173
14
0
180
17
215
915
3
170
16
117
8
150
15
7
175
V
92
15
0
129
14
4
146
110
86
104
11
810
5
97
48
113
Cr
14
0
170
13
9
104
17
7
101
118
124
19
4
948
163
94
1
100
15
8C
o
227
30
7
210
20
1
187
19
723
216
5
208
29
024
4
220
17
6
255
Ni
70
95
58
49
73
86
5641
79
0
7272
17
3
39
69C
u
32
29
7
32
3327
lt2
520
25
36
48
8
lt2Zn
82
14
1
118
85
83
91 9
044
84
93
84
69
77
67A
s
31
3
6
36
3
2
83
3
124
24
376
3
98
324
288
4
22
486
4
09Se
lt1
4
lt18
lt1
5
lt12
lt1
8
22
lt15
02
lt1
9
20
lt19
1
6
lt18
lt1
8R
b
414
12
56
91
1
721
62
5
701
345
571
64
3
600
559
46
2
654
53
8Sr
27
7
300
25
3
308
19
5
245
252
271
22
2
295
304
28
3
773
29
5Y
9
29
19
19
15
1210
20
19
16
20
12
21Zr
13
1
156
13
2
168
14
2
169
136
148
16
3
173
165
14
5
192
15
5N
b
10
11
10
10
910
9
10
1010
9
9
10
Sb
028
0
36
030
0
28
037
0
360
250
28
041
0
240
29
025
0
22
032
Cs
2
49
608
5
32
420
3
62
412
224
398
3
72
340
263
2
91
325
3
64B
a
605
94
7
792
58
3
543
54
250
669
6
530
58
868
1
584
115
8
657
La
263
62
7
255
31
2
223
20
728
128
8
283
29
141
5
316
28
7
329
Ce
41
2
815
42
9
509
42
2
423
593
564
45
6
810
755
48
4
503
46
5N
d
197
52
9
226
29
0
168
19
324
623
9
202
22
133
0
243
23
2
246
Sm
334
9
57
419
4
69
410
4
354
064
17
367
4
126
43
423
4
21
422
Eu
105
2
39
121
1
18
124
1
051
091
25
109
1
181
70
135
1
35
135
Gd
2
43
696
3
09
342
4
34
355
340
400
3
11
308
495
3
42
372
4
13Tb
0
39
108
0
55
053
0
66
059
047
059
0
48
051
076
0
49
053
0
57Tm
0
16
046
0
30
021
0
31
029
023
028
0
24
026
033
0
26
025
0
21Y
b
103
2
80
187
1
37
222
2
231
241
75
159
1
602
13
165
1
48
150
Lu
017
0
45
030
0
21
030
0
300
200
25
022
0
210
30
021
0
24
021
Hf
3
12
404
3
46
357
3
20
327
366
323
4
12
293
342
2
90
341
3
38Ta
0
42
043
0
45
034
0
40
053
039
038
0
46
046
048
0
40
042
0
45
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 523
are characterized by the presence of cross-cutting quartzveinlets Much of the metasediment occurring at Bosumtwihas been sheared and especially the graphitic shales oftencontain quartz ribbons (Figs 2b and 2c) For example sampleLB-3a is composed of quartz bands intercalated with thinbiotite-rich bands (Fig 3a)
Meta-graywackesThe meta-graywackes are more massive and harder than
the shales They are medium-grained light to dark grayclastic rocks Some samples have a weak foliation and someare strongly mylonitized Pyrite grains occur dispersed insome samples
In thin section these rocks are mainly composed ofquartz K-feldspar plagioclase mica chlorite and carbonate(Figs 3b and 3c) The abundance of feldspar and poor sortingin the samples suggests the original sediments had not beentransported too far from their source and therefore couldrepresent turbidites The plagioclase in some samples hasbeen partially to completely altered to sericite it may occur asrelatively large porphyroclasts in some samples Biotite ispartially to completely altered to chlorite (Fig 4) Noevidence of shock deformation was found in any of thesamples from this suite
GranitesThere are two types of granite samples in our suite a
fine- to medium-grained type (eg LB-10 and LB-18) whichhas been referred to as microgranite by some authors (egWoodfield 1966) and a medium- to coarse-grainedleucogranite (Fig 5a) In thin section the samples consist ofquartz feldspar (plagioclase and alkali feldspar) biotite andmuscovite as well as some secondary sericite and chloriteMost of the granites are altered with most feldspar altered tosericite (Fig 5b) and biotite to chlorite (Fig 5c) Some othergranite samples display seemingly oxidized biotite (egsample LB-24 Fig 6) Several granite samples (eg LB-19Aand LB-25) display abundant graphic intergrowth of quartzand K- or alkali feldspar (Fig 5c) and some spheruliticgrowths of feldspar No evidence of shock deformation wasfound
SuevitesThe suevites are composed of melt clasts (including some
partially devitrified glass) and clasts of the aforementionedcountry rock types in an optically unresolvable groundmass oftarget rock fragments quartz and phyllosilicates (includingchlorite and sericite) (Figs 7a and 7b) Whether or not thefine-grained groundmass contains small melt fragments is thesubject of ongoing research The clast population of suevitesfrom the southern crater rim is comparatively more polymictwith both the banded and graphitic shales forming dominantclast types This has imparted relatively darker gray color tothe suevites from the south Clast populations of suevites from
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
524 F Karikari et al
Fig 2 a) Very fine-grained shale with some narrow somewhatdarker (carbon-rich) layers and some relatively coarser-grainedoxide grains (eg circle) Two thin secondary veinlets of quartzcross-cut the S1 foliation (sample LB-5 plane-polarized light) b) Amicrophotograph (cross-polarized light) of well-banded graphiticshale with a mylonitic quartz ribbon (light colored) sample LB-51c) A microphotograph of pervasive crenulation and microfoldinggraphitic shale sample LB-51
Fig 3 a) Quartz-rich schist comprising quartz bands and relativelythinner biotite-rich bands quartz is well sutured (sample LB-3across-polarized light) b) Sheared medium-grained meta-graywackecomposed mainly of quartz and feldspar clasts and minor biotiteclasts (upper left) (sample LB-7 plane-polarized light) c) Barelydeformed (note cross-cutting microfracture in central part of image)medium-grained meta-graywacke dominated by quartz (somerecrystallized) and feldspar clasts in a fine-grained matrix ofphyllosilicates quartz and feldspar (sample LB-33 cross-polarizedlight)
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
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Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
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Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 525
northern locations contain mostly meta-graywacke and thesesamples are light gray in color
The clasts in the suevites show different stages of shockmetamorphism associated with the impact as well asalteration of melt particles and some rock fragments In thinsection some suevites show fresh glass clasts (highlyvesicular or with flow structures) (Fig 8a) Planardeformation features in quartz grains occur in one or two setsper grain (Fig 8b) Crystals of quartz and feldspar and evenlarger lithic clasts such as shale or schist also show differentstages of isotropization the majority of the quartz grains inlithic clasts within suevite occur as diaplectic glass and somehave ballen texture The suevites are characterized byalteration of the meltglass clasts in the groundmass tophyllosilicates that so far have not been identified Figures 7aand 7b show the argillic alteration of the groundmass ofsuevites to phyllosilicate minerals This alteration of suevitecomponents represents post-impact alteration and thedetailed study of these alteration effects in suevite usingX-ray diffraction (XRD) and infrared spectroscopy will bediscussed in a separate paper
MeltGlass FragmentsMelt and glass fragments from suevites are highly
vesicular and very clast-poor They usually consist of meltmatrix and melted or vitrified clasts with few (lt5 vol)crystalline clasts of quartz meta-graywacke phyllite shalegranite and quartzite Some melt fragments show flowstructures and others are partially recrystallized Diaplecticquartz and ballen quartz (Fig 8c) are common in these meltglass fragments
Geochemistry
The results of major- and trace-element analyses as wellas some characteristic geochemical ratios of the 36 analyzedsamples are given in Tables 2 and 3 The averagecompositions of the various rock types are given in Table 4together with the average composition of Ivory Coast tektites(with data from Koeberl et al 1997 1998 Boamah andKoeberl 2003) and upper continental crust rocks (Taylor andMcLennan 1985)
Major ElementsThe main country rocks (shalephyllite meta-graywacke
and granite) and the suevites and meltglass fragmentsgenerally show some variation in their major elementcomposition between the groups There is also wide variationin the major element composition within the groups of themain country rocks as well as some variation in the suevitesand meltglass fragments (Tables 2 and 3) In the Harkervariation diagrams of Fig 9 the quartz schist has the highestSiO2 content with a value of 878 wt The SiO2 contents ofthe granites with an average value of 668 wt and a range
from 613 to 743 wt are higher than the contents of boththe shales and the suevites The suevites have an average SiO2content of 621 wt and a range from 531 to 729 wtwhich is slightly lower than the SiO2 content of the shalesamples The shale-phyllite average SiO2 content is640 wt with a range from 581 to 713 wt The meltfragments have an average SiO2 content of 650 wt whichis slightly higher than the SiO2 content of the bulk suevitesand also have a more limited variation of SiO2 content (from613 to 681 wt) than the bulk suevites The CaO contents ofthe granites are slightly higher than those of the metasedimentsamples (shalephyllite arkose and schist) with an averagevalue of 110 wt (plusmn097 wt) and a range from 012 to314 wt The shales have an average CaO content of050 wt with a range from lt001 to 099 wt The sueviteshave an average CaO content of 082 wt with a range from026 to 117 wt whereas the melt fragments have a muchhigher average CaO content of 153 wt with a range from098 to 315 wt The loss on ignition (LoI) values of suevitesare higher than the LoI values of the melt fragments with anaverage value of 644 wt (plusmn190 wt) and a range from 343to 875 wt compared to the melt fragment average LoI of454 wt (plusmn259 wt) with a range from 053 to 740 wtAmong the country rocks the granite samples have lower LoIvalues than the metasediment samples the shale sampleshave the highest LoI contents with an average LoI value of645 wt (plusmn311 wt) and a range from 408 to 124 wtThe granites have an average LoI of 347 wt (plusmn218 wt)with a range from 053 to 736 wt The Fe2O3 (total Fe asFe2O3) contents of suevite samples are slightly higher thanthose of the country rocks (meta-graywacke and granites)with an average content in suevite of 671 wt (plusmn164 wt)and a range from 491 to 997 wt compared to the granitesthat have an average Fe2O3 content of 436 wt (plusmn226 wt)and a range from 098 to 776 wt The shale-phyllitesamples however have the highest Fe2O3 contents among the
Fig 4 Extensive alteration of biotite to chlorite (Chl) and of feldspar(mainly plagioclase = Pl) to sericite (see circle and ellipse) in meta-graywacke (sample LB-8 cross-polarized light)
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
526 F Karikari et al
analyzed samples with an average content of 722 wt and arange from 552 to 105 wt The melt fragments from thesuevites have much higher Fe2O3 contents than the bulksuevites with an average content of 601 wt (plusmn071 wt)and a more limited variation in the Fe2O3 contents (from 462to 659 wt) than the bulk suevites
The bulk suevites have low SiO2Al2O3 ratios with anaverage value of 383 and a range from 251 to 594 and alsorelatively low K2ONa2O ratios with an average value of 097and a range from 058 to 191 The melt fragments haveslightly higher average SiO2Al2O3 and lower K2ONa2O
ratios than the bulk suevite The country rocks have variableSiO2Al2O3 ratios with the shale-phyllite samples havingaverage SiO2Al2O ratio of 441 (plusmn147) and the graniteshaving an average SiO2Al2O ratio of 424 (plusmn039) The shale-phyllite samples also have an average K2ONa2O ratio of 269(plusmn258) which is higher than the average suevite K2ONa2Oratio of 097 (plusmn044) The degree of alteration in the countryrocks and suevites may be inferred using chemical index ofalteration (CIA) values (Rollinson 1993) The shale-pyllitesgranites melt fragments and bulk suevites have average CIAvalues of 76 (range from 67 to 91) 62 (range from 48 to 78)
Fig 5 Hydrothermally altered granite samples a) Medium-grained granite with large feldspar (mostly plagioclase = Pl) and quartz (Qtz)(sample LB-26 cross-polarized light) b) Enlarged region (rectangle in [a]) containing a large euhedral crystal of alkali feldspar with a corealtered to sericite a second plagioclase grain (Pl) is also indicated c) Strong alteration in a fine-grained leucogranite indicated by chlorite(Chl) after biotite and sericite (ellipse) in the interstices between larger granophyric intergrowths of quartz and albite and muscovite (Ms)(sample LB-25 cross-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 527
65 (range from 52 to 73) and 71 (range from 63 to 75)respectively
Trace ElementsThe country rocks and suevites show limited variation in
trace element contents between the groups but have somevariability within groups The siderophile and chalcophileelements namely Cr Co Ni Cu and V are enriched in bothcountry rocks and suevites by a factor of about 2 relative totheir abundances in average upper crust (Taylor andMcLennan 1985) The average Ni content in suevites(66 ppm) and average Ni content in shales (92 ppm) are aboutfour times higher than the Ni abundance (20 ppm) in averageupper continental crust (Taylor and McLennan 1985) Nickelcontents in meltglass fragments from suevites are somewhathigher than in bulk suevites (84 versus 66 ppm) Co contentsare also slightly higher in the melt fragments (232 versus216 ppm) but Cr contents are very similar (134 (plusmn43) versus134 (plusmn28) ppm) The Ni values of bulk suevites and meltfragments are similar to the Ni contents reported for Birimianvolcanic rocks by Sylvester and Attoh (1992) and thosereported for some sulfide-mineralized samples from theAshanti and Tarkwa mines by Dai et al (2005) In thesuevites the contents of the high field strength elements(HFSE) Zr Hf Ta Nb U and Th are not significantlydifferent from values for the shallow-drilled suevites reportedby Boamah and Koeberl (2003) except that Zr contentsobtained in this study are slightly higher than those of thesuevites from the shallow drilling outside the northern craterrim The HFSE contents of the country rocks especially theshales are essentially similar to the values for Birimiangraywackes and metapelites reported by Dai et al (2005)
Trace-element ratios also show some variability betweenthe suevites and the country rocks as well as variabilitywithin groups The KU ThU LaTh ZrHf and HfTa ratiosof the suevites show limited variability compared to thevariability within the country rocks The ThU ZrHf and HfTa values for suevites have the following ranges 242ndash472372ndash516 and 620ndash106 ppm respectively whereas theThU ZrHf and HfTa values of shale-phyllites are 039ndash267 348ndash723 and 562ndash284 respectively
Rare Earth Elements (REE)The C1 chondrite-normalized REE distribution patterns
of the suevites and the various country rocks are shown inFig 10 They generally show patterns typical of Archeancrustal rocks (Taylor and McLennan 1985) with light REE
Fig 6 Granite sample LB-24 (plane-polarized light) showing apartially oxidized biotite blast Bt-1 and a smaller lath of unoxidizedbiotite Bt-2 This sample is composed mainly of feldspar (mostlyplagioclase = Pl) quartz (Qtz) biotite and muscovite
Fig 7 a) Suevite with a variety of lithic clasts mostly shale (S)phyllite (P) with crenulation mylonitic fine-grained meta-graywacke(G) in an optically unresolvable phyllosilicate-rich groundmass(sample LB-39c plane-polarized light) b) Mylonitic fine-grainedmeta-graywacke clasts (G) in groundmass of mostly phyllosilicates(formed by the argillic alteration of melt clasts and smaller rockfragments) quartz grains and opaque minerals (sample LB-39aplane-polarized light)
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
528 F Karikari et al
(LREE) enrichment lack of Eu anomaly or slightly negativeslightly positive Eu anomalies and depleted heavy REE(HREE) Compared to the country rocks the suevites show avery limited variation in their REE enrichment with theirchondrite-normalized patterns showing LREE enrichments(LaNYbN ratios ranging from 627 to 173) and depletion inHREE (GdNYbN ratio ranging from 129 to 223) Thesuevite patterns do not show significant Eu anomalies withEuEu values ranging from 082 to 112 (average 094) Theshale-phyllite samples have a rather wide variation in theirREE abundance and the patterns are characterized by LREEenrichment (LaNYbN ratio ranging from 107 to 149)depletion in HREE (GdNYbN ratio ranging from 051 to314) and slightly negative Eu anomalies (EuEu valuesranging from 080 to 095 with an average of 085) There isalso no significant difference in the chondrite-normalizedREE distribution pattern between the studied groups ofsamples and the average Ivory Coast tektites
Provenance of the Main Country Rocks
In order to understand the effect of the high-energyBosumtwi impact cratering event on the country rocks it isimportant to understand not only the fundamental petrologyand geochemistry of the country rocks but also theirprovenance or tectonic setting Here we present theprovenance studies of the country rocks focusing mainly onthe granites and meta-graywacke
Granite Classification and ProvenanceAccording to Leube et al (1990) Na2O K2O CaO and
Rb are significant parameters in separating granitoidsbelonging to the Belt (Dixcove) type from those of the Basin(Cape Coast and Winneba) type with the Belt-type havinghigher Na2O and CaO contents and lower K2O and Rbcontents than the Basin-type The analyzed granite sampleshave average Na2O and CaO contents of 387 (plusmn117) wtand 110 (plusmn097) wt respectively and average K2O and Rbcontents of 150 (plusmn062) wt and 487 (plusmn176) ppmrespectively In comparison with the average Na2O CaOK2O and Rb contents of Basin granitoids (Winneba type)reported by Leube et al (1990)mdash377 230 389 wt and152 ppm respectively and the average Na2O CaO K2O andRb contents of Belt granitoids (Dixcove type)mdash453 324213 wt and 534 ppm respectivelymdashmost of the analyzedgranite samples have high Na2O contents For example theNa2O content of LB-24 is 458 wt for LB-34 is 521 wtfor LB-38 is 467 wt and for LB-50 the Na2O content is486 wt The CaO contents of these samples (eg LB-38[016 wt] and LB-50 [314 wt]) however are lower thanthe reported average Belt granitoid CaO content of 324 wtThe analyzed granite samples have low K2O and Rb contentsin comparison to the average K2O and Rb contents reportedfor the Belt granitoids (Leube et al 1990) of 389 wt and
Fig 8 a) A vesicular glass fragment in suevite groundmass mineralsinclude phyllosilicates and quartz (sample LB-43 plane-polarizedlight) b) Planar deformation features (2 sets) in quartz (clast insuevite sample LB-43 cross-polarized light) c) Ballen quartz insuevite (sample LB-40 plane-polarized light)
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
ble
4 A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge c
ompo
sitio
n of
Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
ndash73
5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
ndash59
43
98 plusmn
02
83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
ndash78
2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
ndash19
10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
ndash118
60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
ndash52
25
Zn10
0 plusmn
3466
ndash153
6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
ndash13
25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
ndash71
230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
ndash41
5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 529Ta
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= 6
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(n =
8)
Mel
tgla
ss fr
agm
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(n
= 6
)
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ktite
aver
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Ave
rage
Ran
geA
vera
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Ave
rage
Ran
geA
vera
geR
ange
SiO
264
0 plusmn
48
858
1ndash7
13
66
8 plusmn
414
613
ndash74
362
1 plusmn
59
253
1ndash7
29
650
plusmn 2
661
3ndash6
81
67
6
660
TiO
20
62 plusmn
02
50
13ndash0
81
05
8 plusmn
023
013
ndash09
90
70 plusmn
01
10
50ndash0
82
065
plusmn 0
07
056
ndash07
5
056
0
50A
l 2O3
153
plusmn 3
01
970
ndash18
0 1
58
plusmn 1
2214
4ndash1
76
169
plusmn 2
86
123
ndash21
116
4 plusmn
06
156
ndash17
3
167
15
2Fe
2O3
722
plusmn 1
73
552
ndash10
5 4
36
plusmn 2
260
98ndash7
76
671
plusmn 1
64
491
ndash99
76
01 plusmn
07
14
62ndash6
59
6
16
450
MnO
006
plusmn 0
04
003
ndash01
3 0
06
plusmn 0
030
01ndash0
11
007
plusmn 0
03
004
ndash01
30
04 plusmn
00
10
03ndash0
07
0
06M
gO2
01 plusmn
09
00
44ndash3
20
26
0 plusmn
213
030
ndash58
81
83 plusmn
07
30
79ndash2
61
113
plusmn 0
33
077
ndash16
7
346
2
20C
aO0
50 plusmn
03
5lt0
01ndash
099
11
0 plusmn
097
012
ndash31
40
82 plusmn
03
20
26ndash1
17
153
plusmn 0
81
098
ndash31
5
138
4
20N
a 2O
130
plusmn 0
84
021
ndash22
0 3
87
plusmn 1
171
57ndash5
21
207
plusmn 0
42
162
ndash29
12
52 plusmn
07
21
69ndash3
78
1
90
390
K2O
220
plusmn 0
84
056
ndash27
5 1
50
plusmn 0
620
82ndash2
57
191
plusmn 0
64
111
ndash31
01
82 plusmn
04
31
38ndash2
63
1
95
340
P 2O
50
16 plusmn
01
50
05ndash0
47
01
4 plusmn
008
002
ndash02
40
09 plusmn
00
30
06ndash0
15
009
plusmn 0
06
005
ndash02
2L
OI
645
plusmn 3
11
408
ndash12
4 3
47
plusmn 2
180
53ndash7
36
644
plusmn 1
90
343
ndash87
54
54 plusmn
25
90
53ndash7
40
0
002
Tota
l99
810
03
996
997
99
8
SiO
2A
l 2O3
441
plusmn 1
47
330
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5 4
24
plusmn 0
393
57ndash4
99
383
plusmn 1
08
251
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43
98 plusmn
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83
71ndash4
37
4
04
434
K2O
N
a 2O
269
plusmn 2
58
097
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2 0
41
plusmn 01
70
18ndash0
76
097
plusmn 0
44
058
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10
75 plusmn
01
70
58ndash1
04
1
03
087
Sc18
6 plusmn
30
157
ndash23
6 1
14
plusmn 6
173
58ndash2
07
174
plusmn 3
514
0ndash2
55
165
plusmn 1
115
0ndash1
78
14
7
11V
1
21 plusmn
15
95ndash1
31
84 plusmn
40
14ndash1
39 1
22 plusmn
27
86ndash1
5098
plusmn 2
548
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60
Cr
106
plusmn 3
180
ndash162
146
plusmn 2
277ndash
550
134
plusmn 2
810
1ndash17
713
4 plusmn
4394
ndash194
244
35
Co
170
plusmn 9
44
3ndash31
2 1
24
plusmn 7
800
98ndash2
40
216
plusmn 4
316
5ndash3
07
232
plusmn 4
017
6ndash2
90
26
7
10N
i92
plusmn 8
323
ndash256
44
plusmn 4
99ndash
135
66 plusmn
18
41ndash9
584
plusmn 4
639
ndash173
157
20
Cu
50
plusmn 37
18ndash1
14
14 plusmn
5 lt
2ndash19
0 2
7 plusmn
107ndash
3334
plusmn 1
8lt2
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25
Zn10
0 plusmn
3466
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6
3 plusmn
2225
00ndash
960
92
plusmn 28
44ndash1
4179
plusmn 1
067
ndash93
23
0
71A
s13
6 plusmn
25
61
06ndash6
58
38
2 plusmn
395
093
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25
05 plusmn
34
82
38ndash1
24
388
plusmn 0
71
288
ndash48
6
045
1
5Se
27
plusmn 4
70
2ndash12
13
plusmn 0
60
4ndash2
31
2 plusmn
14
02ndash
22
18
plusmn 0
31
6ndash2
0
023
50
Rb
72 plusmn
29
22ndash9
5 4
87
plusmn 17
619
4ndash7
96
69
plusmn 29
34ndash1
2658
plusmn 7
46ndash6
5
660
112
Sr18
1 plusmn
8965
ndash320
430
plusmn 3
2015
7ndash12
05 2
63 plusmn
35
195ndash
308
362
plusmn 20
322
2ndash77
3 2
60 3
50Y
29 plusmn
22
5ndash64
1
2 plusmn
210
ndash18
16
plusmn 7
9ndash29
18 plusmn
312
ndash21
22
Zr13
2 plusmn
3493
ndash181
151
plusmn 5
878
ndash247
148
plusmn 1
513
1ndash16
916
5 plusmn
1614
5ndash19
2 1
34 1
90N
b9
5 plusmn
21
61ndash
12
10
plusmn 4
7ndash20
10 plusmn
19ndash
1110
plusmn 1
9ndash10
25
Sb1
02 plusmn
15
50
11ndash4
02
01
9 plusmn
011
002
ndash03
60
31 plusmn
00
50
25ndash0
37
029
plusmn 0
07
022
ndash04
1
023
0
2C
s2
52 plusmn
10
30
81ndash3
66
22
8 plusmn
104
077
ndash44
24
01 plusmn
12
92
24ndash6
08
326
plusmn 0
42
263
ndash37
2
367
3
7B
a67
9 plusmn
290
344ndash
1170
516
plusmn 3
8116
8ndash14
20 6
52 plusmn
152
506ndash
947
700
plusmn 23
153
0ndash11
58 3
27 5
50La
273
plusmn 4
15
203
ndash110
23
4 plusmn
190
761
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230
7 plusmn
13
420
7ndash6
27
320
plusmn 4
96
283
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5
207
30
Ce
576
plusmn 8
33
404
ndash223
45
7 plusmn
329
185
ndash127
521
plusmn 1
38
412
ndash81
557
9 plusmn
16
045
6ndash8
10
41
7
64N
d28
6 plusmn
43
52
15ndash1
1623
0 plusmn
16
56
17ndash6
17
261
plusmn 1
14
168
ndash52
924
6 plusmn
44
420
2ndash3
30
21
8
260
Sm6
01 plusmn
88
80
52ndash2
37
41
5 plusmn
270
115
ndash10
34
81 plusmn
19
63
34ndash9
57
448
plusmn 0
98
367
ndash64
3
395
450
Eu1
52 plusmn
20
70
17ndash5
63
11
9 plusmn
070
032
ndash27
71
31 plusmn
04
41
05ndash2
39
134
plusmn 0
21
109
ndash17
0
120
088
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
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3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
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11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
530 F Karikari et al
Gd
529
plusmn 7
01
080
ndash19
2 3
13
plusmn 1
361
50ndash6
13
390
plusmn 1
36
243
ndash69
63
73 plusmn
07
13
08ndash4
95
3
34
380
Tb0
82 plusmn
09
10
14ndash2
55
04
5 plusmn
014
025
ndash06
80
61 plusmn
02
10
39ndash1
08
056
plusmn 0
10
048
ndash07
6
056
0
64
Tm0
37 plusmn
02
20
16ndash0
74
01
9 plusmn
005
011
ndash02
70
28 plusmn
00
90
16ndash0
46
026
plusmn 0
04
021
ndash03
3
030
0
33Y
b2
51 plusmn
14
01
28ndash4
96
12
9 plusmn
047
065
ndash21
11
81 plusmn
05
91
03ndash2
80
166
plusmn 0
24
148
ndash21
3
179
2
20Lu
038
plusmn 0
19
020
ndash06
7 0
18
plusmn 0
080
06ndash0
33
027
plusmn 0
09
017
ndash04
50
23 plusmn
00
30
21ndash0
30
0
24
032
Hf
296
plusmn 0
74
236
ndash41
9 3
64
plusmn 1
662
28ndash6
72
344
plusmn 0
31
312
ndash40
43
36 plusmn
04
42
90ndash4
12
3
38
580
Ta0
41 plusmn
01
70
08ndash0
57
05
0 plusmn
040
020
ndash13
00
42 plusmn
00
60
34ndash0
53
045
plusmn 0
03
040
ndash04
8
034
2
20A
u(p
pb)
45
plusmn 5
90
2ndash15
0
9 plusmn
06
00ndash
19
16
plusmn 0
50
8ndash2
31
0 plusmn
05
07ndash
19
0
56
180
Th3
26 plusmn
08
32
44ndash4
64
36
1 plusmn
212
148
ndash83
73
64 plusmn
03
23
37ndash4
33
362
plusmn 0
24
336
ndash40
5
354
10
7U
259
plusmn 1
88
112
ndash62
0 1
23
plusmn 0
660
65ndash2
72
117
plusmn 0
26
078
ndash14
20
95 plusmn
02
30
70ndash1
29
0
94
28
CIA
7667
ndash91
62
48ndash7
871
63ndash7
5
6552
ndash73
76
46
KU
9855
plusmn 6
407
1842
ndash16
189
108
75 plusmn
356
676
19ndash1
878
514
344
plusmn 6
288
6626
ndash26
788
170
95 plusmn
730
988
80ndash3
004
517
287
100
76Th
U1
71 plusmn
08
40
39ndash2
67
30
2 plusmn
110
186
ndash54
53
25 plusmn
08
02
42ndash4
72
395
plusmn 0
78
286
ndash48
3
377
3
82La
Th
100
plusmn 1
73
054
ndash44
9 6
55
plusmn 2
381
74ndash1
03
826
plusmn 2
76
02ndash1
45
889
plusmn 1
42
700
ndash11
3
585
2
8Zr
Hf
459
plusmn 1
36
348
ndash72
3 4
33
plusmn 9
7325
7ndash5
58
431
plusmn 5
06
372
ndash51
649
8 plusmn
70
439
6ndash5
90
39
6
328
HfT
a10
1 plusmn
89
95
62ndash2
84
89
3 plusmn
307
430
ndash12
08
38 plusmn
13
86
20ndash1
06
752
plusmn 0
86
633
ndash88
6
994
2
64La
N
Yb N
507
plusmn 5
43
107
ndash14
915
0 plusmn
15
73
50ndash5
34
121
plusmn 4
29
627
ndash17
313
1 plusmn
09
712
0ndash1
48
7
81
921
Gd N
Y
b N1
28 plusmn
09
80
51ndash3
14
23
0 plusmn
157
097
ndash55
21
78 plusmn
03
41
29ndash2
23
183
plusmn 0
27
156
ndash22
3
151
14
EuE
u 0
85 plusmn
00
6 0
80ndash
095
09
9 plusmn
013
070
ndash11
90
94 plusmn
00
90
82ndash1
12
100
plusmn 0
05
092
ndash10
8
101
065
a Dat
a fr
om K
oebe
rl et
al
(199
8)
b Dat
a fr
om T
aylo
r and
McL
enna
n (1
985)
M
ajor
ele
men
ts in
wt
tra
ce e
lem
ents
in p
pm e
xcep
t as
note
d a
ll Fe
as
Fe2O
3n
= nu
mbe
r of s
ampl
es b
lank
spa
ces
= no
t det
erm
ined
N =
cho
ndrit
e-no
rmal
ized
(Tay
lor a
nd M
cLen
nan
1985
) ch
emic
alin
dex
of a
ltera
tion
(CIA
) = (A
l 2O3[
Al 2O
3 + C
aO +
Na 2
O +
K2O
]) times
100
in m
olec
ular
pro
porti
ons
Eu
Eu =
Eu N
(Sm
N times
Gd N
)05
Tabl
e 4
Con
tinue
d A
vera
ge c
ompo
sitio
ns (p
lus
1 s
tand
ard
devi
atio
ns) o
f ana
lyze
d co
untry
rock
s s
uevi
tes
and
mel
t fra
gmen
ts c
ompa
red
to a
vera
ge
com
posi
tion
of Iv
ory
Coa
st te
ktite
s an
d up
per c
ontin
enta
l cru
st
Shal
e-ph
yllit
e (n
= 6
)G
rani
te (n
= 9
)Su
evite
(n =
8)
Mel
tgla
ss fr
agm
ents
(n
= 6
)
Ivor
y C
oast
te
ktite
aver
agea
Upp
erco
ntin
cr
ustb
Ave
rage
Ran
geA
vera
geR
ange
Ave
rage
Ran
geA
vera
geR
ange
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 531
152 ppm respectively These values are also comparable to aBelt-type granite sample reported in John et al (1999) with359 wt CaO 458 wt Na2O 189 wt K2O and 50 ppmRb The published CaO content of this Belt-type sample isthus far higher than the CaO contents of the samples analyzedhere
The total alkali element abundance (Na2O and K2O)ndashsilica diagram TAS (Fig 11) after Cox et al (1979) showsthat most of the Bosumtwi granites have a dioritic to quartz-dioritic composition Pearce et al (1984) classified granitesinto ocean-ridge granites (ORG) volcanic-arc granites
(VAG) within-plate granites (WPG) and syn-collisionalgranites (syn-COLG) using discrimination diagrams based onthe trace elements Nb Y Ta and Yb The Nb-Ydiscrimination diagram in Fig 12a indicates that theBosumtwi granites belong to a VAG or syn-COLG tectonicsetting However this discrimination diagram is ambiguousas VAG cannot be distinguished from syn-COLG In contrastthe Ta-Yb diagram of Fig 12b shows the fields of VAG andsyn-COLG It is clear that the Bosumtwi granites relate to aVAG setting and therefore might have a genetic associationwith the metavolcanic package in the Ashanti belt This
Fig 9 Harker variation diagrams for metasedimentary lithologies granite suevite and meltglass fragments in suevites compared to theaverage values for Ivory Coast tektites (with data from Koeberl et al 1997 1998 Boamah and Koeberl 2003)
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
532 F Karikari et al
conclusion is however preliminary as a more representativesample suite needs to be studied
Metasediment Classification and Provenance The use of detrital modes of sandstone grains to study
provenance was described by Dickinson and Suczek (1979)This method is based on the fact that sandstone compositionsare directly influenced by the character of the sedimentaryprovenance and that the key relations between provenanceand basin are governed by plate tectonics The relativeproportions of terrigenous sandstone grains are therefore
guides to the nature of the source rocks in the provenanceterrane from which sandy detritus was derived The methodhas been used by many authors for sandstone provenancestudies (eg Dickinson et al 1983 Mader and Neubauer2004 Osae et al 2006) and focuses mainly on proportions ofdetrital framework grains
For this study thin-section point counting of fourmeta-graywacke samples was carried out to establish theirquantitative modal composition The modal analysis wasdone by counting more than 1000 points per thin section Theresults are presented in Table 6 Mineral grains less than
Fig 10 Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite granite meta-graywackeand suevite) as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997 1998 and Boamahand Koeberl 2003) Normalization factors from Taylor and McLennan (1985)
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 533
0064 mm apparent diameter were counted as matrix Thematrix of the meta-graywackes is generally composed ofsecondary minerals such as sericite and chlorite Modalcompositions of the samples were recalculated as volumetricproportions of the framework categories described by theGazzi-Dickinson method (eg Dickinson and Suczek 1979)The framework categories are monocrystalline quartz (Qm)polycrystalline quartz (quartzose grains) (Qp) feldspar (F)including plagioclase (P) and K-feldspar (K) lithic grainsother than quartzose grains (L) and total lithic fragments (Lt)which comprises both L and Qp the results of therecalculation in vol are presented in Table 7
According to Okada (1971) triangular diagrams ofdetrital modes could also be used in the classification ofsandstones using quartz feldspar and lithic fragments TheQm-F-Lt classification diagram in Fig 13a shows the studiedsamples are of lithic meta-graywacke composition Theresulting Q-F-L and Qm-F-Lt ternary provenance diagrams(eg Dickinson et al 1983 Mader and Neubauer 2004 Osaeet al 2006) are shown in Figs 13b and 13c The Qm-F-Ltternary provenance plot (Fig 13b) shows that the studiedBirimian meta-graywackes fall between the magmatic arcfield and the recycled orogen field within the undissected arcthe transitional arc and the lithic recycled subfields on theother hand the Q-F-L ternary provenance shows them tobelong only to the recycled orogen field Dickinson andSuczek (1979) described sediments from an undissected arcprovenance to be largely of volcaniclastic debris shed fromvolcanogenic highlands along active island arcs and that sites
for deposition include trenches and fore arc basins Sedimentsof recycled orogen provenance on the other hand aredescribed as recycled detritus derived from uplifted terranesof folded and faulted strata
In order to resolve this ambiguity in the interpretation ofthe two detrital mode diagrams we used geochemicaldiscrimination diagrams to classify and characterize thetectonic setting of the metasediments For this we used the
Fig 11 Provenance classification of Bosumtwi granites using a totalalkali-silica (TAS) plot (after Cox et al 1979) This indicates that thegranites have tonalitic to dioritic composition Granitoid averagesdata from Leube et al (1990) are plotted for comparison (Cape Coastand Winneba types are sedimentary basin granitoids the Dixcovetype represents volcanic belt granitoids)
Fig 12 a) Composition of Bosumtwi granites plotted in a Nb-Ydiscrimination diagram (after Pearce et al 1984) showing the fieldsof volcanic-arc granites (VAG) syn-collisional granites (syn-COLG) within-plate granites (WPG) and ocean-ridge granites(ORG) The Bosumtwi granites fall into the VAG or syn-COLGfields b) Ta-Yb discrimination diagram for the Bosumtwi granitesseparating the fields for VAG and syn-COLG granites (Pearce et al1984) The Bosumtwi granites seem to relate mostly to a volcanic-arcgranite (VAG) provenance
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
534 F Karikari et al
geochemical data of all the metasedimentary rocks ie6 shale-phyllite 3 meta-graywacke 1 siltstone and 1 arkosesamples The chemical classification diagrams of themetasedimentary rocks are shown in Figs 14a and 14b Thehigh abundance of alteration minerals (eg sericites andchlorites) causes a few of the samples to plot in the arkose fieldThe La-Th-Sc ternary plot with fields defined by Girty andBarber (1993) is shown in Fig 15a It shows most of themetasedimentary samples belong to or are close to themagmatic arc-related field Two out of the 3 meta-graywacke
samples fall into the magmatic-arc field According to Bhatiaand Crook (1986) four fields of principal tectonic settings canbe distinguished using the trace elements Th Co and Zr Theseare the passive margin (PM recycled sedimentary andmetamorphic source rocks) active continental margin (ACMgranites gneisses siliceous volcanics) continental island arc(CIA felsic volcanic source rocks) and oceanic island arc(OIA calc-alkaline or tholeiitic rocks) fields In Fig 15b theBirimian metasediment samples plot into or close to the fieldsfor the OIA and CIA tectonic settings
Table 5 Average Fe2O3 V Cr and Co contents of analyzed metasediments compared to average compositions of other Birimian metasediment samples (about 50 km southeast of Bosumtwi crater) published in Asiedu et al (2004)
This work Asiedu et al 2004Shale-phyllite (n = 6) Meta-graywacke (n = 3) Meta-graywacke (n = 19) Metapelites (n = 5)Average Range Average Range Average Range Average Range
Fe2O3 722 plusmn 173 552ndash105 456 plusmn 119 337ndash575 701 plusmn 100 550ndash856 106 plusmn 192 879ndash137V 121 plusmn 148 952ndash131 942 plusmn 238 774ndash111 156 plusmn 408 102ndash226 169 plusmn 518 126ndash254Cr 106 plusmn 31 800ndash162 570 plusmn 191 455ndash790 173 plusmn 106 540ndash529 165 plusmn 281 127ndash193Co 170 plusmn 940 429ndash312 151 plusmn 565 107ndash214 646 plusmn 264 408ndash166 576 plusmn 137 484ndash819 Major elements in wt trace elements in ppm Total Fe as Fe2O3 Blank spaces = not determined n = number of samples analyzed
Table 6 Results of point counting of thin sections of meta-graywackes (data in vol)Meta-graywacke samples
Qp F = K+P Lt = total polycrystalline lithic fragments
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 535
The above classification and discrimination diagramsindicate therefore that the Bosumtwi metasediments relate toa magmatic arc setting The volcaniclastic debris was perhapsderived from volcanogenic highlands along an active islandarc or from a continental margin This interpretation issupported by data presented in Leube et al (1990) Tayloret al (1992) Asiedu et al (2004) and Feybesse et al (2006)Leube et al (1990) suggested the magmatism andsedimentation in the Birimian to be coeval On the basis ofgeochronological studies (Rb-Sr PbPb and Sm-Ndanalyses) of the Birimian rock units Taylor et al (1992)concluded that the Birimian sediments were derived from theadjacent penecontemporaneous volcanic belts with nodetectable input from any significantly older terrane On thebasis of trace element data from the Birimianmetasedimentary rocks Asiedu et al (2004) also suggestedthat the Birimian metasedimentary rocks were mainly derived
from a juvenile arc source of mixed felsic and maficcomposition Feybesse et al (2006) in their geodynamicmodel of the Paleoproterozoic Birimian province alsofavored the emplacement of a juvenile basic volcanic-plutonic rocks accompanied by the deposition of thegraywacke sediments
DISCUSSION
Alteration in the Country Rocks
Alteration is the compositional change that occurs afterthe formation of a rock and may be due to metamorphically ormagmatically triggered activity In the context of impactstructures it may also be induced by hydrothermal activitytriggered by impact (Koeberl and Reimold 2004) TheBosumtwi country rocks have undergone various alteration
Fig 13 a) Qm-F-Lt (monocrystalline quartz feldspar lithic fragments) classification diagram for meta-graywackes (after Okada 1971)showing the fields of the various types of wackes Here the Bosumtwi meta-graywacke samples belong to the field of the lithic graywackeb) Qm-F-Lt diagram for provenance discrimination (after Dickinson et al 1983) showing the various provenance fields and subfields In thisdiagram the samples are classified into the undissected arc subfield of a magmatic arc c) Q-F-L (both monocrystalline and polycrystallinequartz feldspar lithic fragments) provenance discrimination diagram (after Dickinson et al 1983) for the Bosumtwi samples The samples inthis diagram fall into the field for the recycled orogen provenance
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
536 F Karikari et al
processes and were subject to various elevated temperatureand pressure conditions associated with a number ofmetamorphic events The Eburnean event (~19ndash21 Gyr ago)which is the last tectonothermal event to have stabilized theWest African craton (Wright et al 1985 Leube et al 1990)caused the Birimian supracrustal sequences to become foldedand metamorphosed to greenschist facies Feybesse et al(2006) considered the Eburnean event to have involvedseveral phases a D1 thrust tectonic phase (213 to 2105 Gyr)involved crustal thickening through unit stacking and wasrelated to horizontal crustal shortening this was followed byD2ndash3 events from 2095 to 198 Gyr which involved a periodof strike-slip movement
Pre-Impact Hydrothermal AlterationThe pre-impact hydrothermal activity and associated
gold mineralization in the Birimian according to theevolution model for the Birimian Supergroup by Eisenlohrand Hirdes (1992) is associated with late Eburnean-stagelateral strike slip and dextral shearing along the boundaries
between the metavolcanic and metasedimentary Birimianunits The hydrothermal process resulted in the greenschistmineral assemblages of the Birimian rocks forming newminerals under different conditions of temperature pressureand fluid composition Some minerals were also replaced bymetasomatism (eg addition of H2O and CO2) According toWoodfield (1966) the widespread presence of hydrousminerals such as chlorite epidote and sericite the consistentpresence of carbonates (ankerite and calcite) and thepresence of these minerals in veins give evidence ofinvolvement of carbon dioxide and water metasomatismCarbonate thermometry is based on the use of actualcarbonate solvi (Rosenberg 1967) On the basis of the moleFeCO3 content in siderite Manu (1993) suggested 350 degC asthe temperature of the hydrothermal alteration event thataffected the Ashanti belt in which the Bosumtwi structureoccurs On the basis of fluid inclusion studies Dzigbodi-Adjimah (1993) also suggested that the hydrothermal activitytook place at about 350 degC and involved dilute aqueoussolutions According to Yao and Robb (2000) hydrothermalalteration minerals at the Obuasi mine (south of the crater inthe Ashanti belt) are dominated by quartz sericite(muscovite) sulphides (mainly pyrite and arsenopyrite) andcarbonates From thermodynamic calculations Yao andRobb (2000) derived that the initial homogeneous H2O-CO2-rich fluid contained 50ndash80 mole H2O at 300ndash350 degC and2 kbar
In this study we observed that some of the country rocksamples (eg granites LB-18 and 34 meta-graywacke LB-719B and 33 and shale LB-11 and 32) display the mineralassemblage plagioclase-quartz-biotite-chloriteplusmnepidoteplusmnmuscovite which is a typical metamorphic mineralassemblage for greenschist facies Birimian rocks (John et al1999) Other samples (eg granites LB-25 26 and 38meta-graywacke LB-8 and shale LB-5) display the mineralassemblage plagioclase-quartz-chlorite-sericiteplusmnsulfidesplusmnsphene In these altered samples most plagioclase is replacedby sericite and biotite is replaced by chlorite Also there isthe presence of disseminated secondary minerals such assulfides (pyrites) and of quartz veinlets in hand specimensThe latter mineral assemblage is similar in composition butnot in intensity to the hydrothermal alteration mineralassemblage at the Obuasi mines which are located about 30km south of the crater structure (Appiah 1991 and Yao andRobb 2000)
Further evidence of hydrothermal alteration is based ona) visual description of samples (presence of disseminatedsulphides bleached samples and quartz veinlets) and b) thin-section petrography (plagioclase strongly sericitized andbiotite replaced by chlorite) Some of the alteration occursalong foliation planes in fractures and in pore spaces causedby brittle-ductile deformation The presence of some of thesealteration minerals (eg sulfides and sericite) in some of thecountry rock fragments in the suevite suggests that thealteration is pre-impact
Fig 14 a) Classification diagram (after Pettijohn et al 1972)discriminating the metasedimentary rock samples by theirlogarithmic ratios of SiO2Al2O3 and Na2OK2O b) Classificationdiagram (after Herron 1988) discriminating metasedimentary rocksamples by their logarithmic ratios of SiO2Al2O3 and Fe2O3K2O
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 537
Post-Impact AlterationImpact cratering is a high-energy dynamic event that
results in shock-metamorphic effects due to very highpressure and temperature This leads to the irreversibledeformation of the crystal structure of minerals as well as tothe formation of high-pressure polymorphs On the basis ofthe presence of meltglass diaplectic quartz glass multiplesets of PDFs and lechatelierite clasts in Bosumtwi falloutsuevite Boamah and Koeberl (2006) estimated the maximumshock pressure experienced by the country rocks to be about60 GPa According to for example Koeberl and Reimold(2004 and references therein) the transient high-energyhigh-temperature impact event can also activate hydrothermalsystems within and even outside of the crater
There is evidence of post-impact alteration in the suevitesamples of the Bosumtwi structure as indicated by argillicalteration of melt particles and fine-grained clasts in thematrix to phyllosilicates Alteration in the form of fracturesfilled with iron oxides has also been observed in suevite egsample LB-39A This alteration of suevite unlike the pre-impact alteration of country rocks that is associated with shearzones and gold mineralization is not related to shearing
Geochemistry of Bosumtwi Country Rocks in Comparisonwith Published Data for Birimian Rocks
The country rocks melt fragments from suevites andbulk suevites have elevated values of Fe2O3 Co Cr and Vcompared to average upper continental crust (Table 4) Thesuevites have average Co Cr and V contents of 216 (plusmn43)134 (plusmn28) and 122 (plusmn27) ppm respectively and the meltfragments from suevites have average Co Cr and V contentsof 232 (plusmn40) 134 (plusmn43) and 98 (plusmn25) ppm respectively The
granites also have average Co Cr and V contents of 124(plusmn78) 146 (plusmn227) and 84 (plusmn40) ppm respectively Theshale-phyllites on the other hand have average Co Cr and Vcontents of 17 (plusmn9) 106 (plusmn31) and 121 (plusmn15) ppmrespectively These average Co Cr and V contents of thetarget rocks melt fragments from suevite and bulk suevitesare similar to and in some cases lower than the backgroundvalues reported for Birimian meta-graywackes andmetapelites by Asiedu et al (2004)
Asiedu et al (2004) analyzed 24 relatively fresh samplescomprising 19 meta-graywacke and 5 metapelites (gray andblack phyllites and schist) for their major and trace elementscontents The location of these samples is about 50 kmsoutheast of the crater and located within the Cape CoastBasin with coordinates defined by longitudes 0deg46 W to0deg54 W and latitudes 5deg54 N to 6deg04 N The BirimianSupergroup in the area is mainly composed ofmetasedimentary rocks comprising meta-graywackes withsubordinate quartzites and interbedded metapelites (gray andblack phyllites and schist) (Asiedu et al 2004)
Averages and ranges of siderophile and chalcophileelement (Fe2O3 Cr Co and V) contents of these meta-graywacke and metapelite samples were calculated from thereported data (see Table 5)
The elevated values obtained in this study for thesiderophile elements in country rocks and suevites comparedto average upper continental crust values therefore do notindicate the presence of an extraterrestrial component in thesuevite because the Birimian rocks already had elevatedcontents of these elements Pyrite chalcopyrite andpyrrhotite are just some of the sulfides occurring in significantamounts in the country rocks The only indication for apossible meteoritic component could be the small excess in Ni
Fig 15 a) La-Th-Sc ternary plots with fields defined by Girty and Barber (1993) Source rock compositions are for Early Proterozoic volcanicrocks (basalts [BAS] andesites [AND] and felsic volcanic rocks [FVO]) Proterozoic tonalite-trondhjemite-granodiorite (TTG) EarlyProterozoic crust (EPC) Early Proterozoic graywackes (EPG) Proterozoic sandstones (PSS) Proterozoic granites (GRA) (Condie 1993) b)Co-Th-Zr diagram for tectonic setting discrimination (after Bhatia and Crook 1986) Oceanic island arc (OIA) continental island arc (CIA)active continental margin (ACM) passive margin (PM)
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
538 F Karikari et al
and Co in melt fragments from suevites as in similar glassyfragments Koeberl and Shirey (1993) detected a very smallmeteoritical component from Os isotope ratios
The meta-graywacke samples of this study and those ofDai et al (2005) also have low SiO2Al2O3 ratios which isindicative of their immaturity (Asiedu et al 2004) and that thesediments of the meta-graywacke might not have beentransported far from their source
The analyzed granite samples from Bosumtwi generallybelong to the Belt-type (Dixcove) granitoids This is based onthe classification parameters of Leube et al (1990) whichindicate that the Belt-type rocks have higher Na2O and CaOcontents and lower K2O and Rb contents than the Basin-typerocks The lower CaO contents of the analyzed samplescompared to those obtained by Leube et al (1990) may beattributed to alteration in the analyzed samples The totalalkali element abundance (Na2O + K2O versus silica) diagram(Fig 11) after Cox et al (1979) shows that most of theBosumtwi granites are clearly Belt-type granitoids furthertheir dioritic to quartz-dioritic composition comparesfavorably with the Belt-type granites described by Hirdeset al (1992)
SUMMARY AND CONCLUSIONS
We describe some petrographic and geochemicalcharacteristics of the country rocks and suevites at Bosumtwicrater and try to constrain the various phases of alteration thatare believed to have affected the country rocks namely a)pre-impact hydrothermal alteration associated with goldmineralization and the formation of hydrothermal alterationhalos along the contacts between metasediments andmetavolcanics (Leube et al 1990) and b) the much morelocalized post-impact alteration associated with the impactcratering The following conclusions can be drawn from ourstudies
1 The Birimian country rocks in the area around theBosumtwi impact structure are characterized by pre-impact alteration associated with shearing This isindicated by the abundance of hydrous minerals such aschlorite sericite sphene quartz and sulfides whichtend to fill the pore space between primary mineralsSome of these secondary minerals also replace the pre-existing metamorphic minerals for example sericiteafter plagioclase There is also post-impact alterationwhich is characterized by argillic alteration of glass andmelt particles to phyllosilicate minerals this post-impactalteration is not associated with shearing
2 Optical microscopy revealed shock metamorphic effectsin mineral and rock clasts of suevite samples such as thepresence of melt clasts diaplectic quartz glass ballenquartz and a few quartz grains with PDFs There is noevidence of shock metamorphism in the studied countryrocks
3 The elevated siderophile element contents of suevitesand country rocks are attributed to the sulfide mineralsassociated with the Birimian hydrothermal alteration anddo not indicate the presence of an extraterrestrialcomponent in the suevite samples
4 The granites of the country rocks have tonalitic to quartz-dioritic composition and on the basis of trace elementdiscrimination plots are of volcanic-arc tectonicprovenance The provenance studies have indicated thatthe Bosumtwi metasediments are volcanic-arc relatedThis supports the view of Leube et al (1990) indicatingthat the metasedimentary and the metavolcanic unitswere formed contemporaneously
AcknowledgmentsndashThe authors thank the Austrian ExchangeService (OumlAD) for providing a PhD scholarship toF Karikari This work was supported by the Austrian FWF(grant P17194-N10 to C K) and by a grant of the AustrianAcademy of Sciences (to C K) We are grateful to theGeological Survey of Ghana for logistical support and toK Atta-Ntim (GSD Kumasi Ghana) and D Brandt (thenUniversity of the Witwatersrand Johannesburg) for help inthe field in 1997 We appreciate the help of H Boumlck M VillaM Bichler and G Steinhauser (Atominstitut Vienna) with theirradiations We are grateful to J Morrow (San Diego StateUniversity) and an anonymous reviewer for very helpfulreviews and extensive comments on this manuscript
Editorial HandlingmdashDr Bernd Milkereit
REFERENCES
Appiah H 1991 Geology and mine exploration trends of PresteaGoldfields Ghana Journal of African Earth Science 13235ndash241
Asiedu D K Dampare S B Asamoah-Sakyi P Banoueng-Yakubo B Osae S Nyarko B J B and Manu J 2004Geochemistry of Paleoproterozoic metasedimentary rocks fromthe Birim diamondiferous field southern Ghana Implications forprovenance and crustal evolution at the Archean-Proterozoicboundary Geochemical Journal 38215ndash228
Bhatia M R and Crook K A W 1986 Trace element characteristicsof greywackes and tectonic setting discrimination of sedimentarybasins Contributions to Mineralogy and Petrology 92181ndash193
Boamah D 2001 Bosumtwi impact structure Ghana Petrographyand geochemistry of target rocks and impactites with emphasison shallow drilling project around the crater PhD thesisUniversity of Vienna Vienna Austria
Boamah D and Koeberl C 2002 Geochemistry of soils from theBosumtwi impact structure Ghana and relationship toradiometric airborne geophysical data In Meteorite impacts inPrecambrian shields edited by Plado J and Pesonen L ImpactStudies vol 2 Heidelberg Springer pp 211ndash255
Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159
Boamah D and Koeberl C 2006 Petrographic studies of falloutsuevite from outside the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 411761ndash1774
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
Petrography geochemistry and alteration of country rocks from Bosumtwi 539
Chao E C T 1968 Pressure and temperature histories of impactmetamorphosed rocks-based on petrographic observations InShock metamorphism of natural materials edited by FrenchB M and Short N M Baltimore Maryland Mono Book Corppp 135ndash158
Condie K C 1993 Chemical composition and evolution of the uppercontinental crust Contrasting results from surface samples andshales Chemical Geology 1041ndash37
Cox K G Bell J D and Pankhurst R J 1979 The interpretation ofigneous rocks London Allen amp Unwin 450 p
Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511
Dickinson W R and Suczek C A 1979 Plate tectonics andsandstone compositions The American Association of PetroleumGeologists Bulletin 632164ndash2182
Dickinson W R Beard L S Brakenridge G R Erjavec J LFerguson R C Inman K F Knepp R Lindberg F A andRyberg P T 1983 Provenance of North American Phanerozoicsandstones in relation to tectonic setting Geological Society ofAmerica Bulletin 94222ndash235
Dzigbodi-Adjimah K 1993 Geology and geochemical patterns ofthe Birimian gold deposits Ghana West Africa Journal ofGeochemical Exploration 47305ndash320
Earth Impact Database 2006 httpwwwunbcapasscImpactDatabase Accessed 08 October 2006
El Goresy A 1966 Metallic spherules in Bosumtwi crater glassesEarth and Planetary Science Letters 123ndash24
El Goresy A Fechtig H and Ottemann T 1968 The opaqueminerals in impactite glasses In Shock metamorphism of naturalmaterials edited by French B M and Short N M BaltimoreMaryland Mono Book Corp pp 531ndash554
Eisenlohr B N and Hirdes W 1992 The structural development ofthe early Proterozoic Birimian and Tarkwaian rocks of southwestGhana West Africa Journal of African Earth Sciences 14313ndash325
Feybesse J Billa M Guerrot C Duguey E Lescuyer J Milesi Jand Bouchot V 2006 The paleoproterozoic Ghanaian provinceGeodynamic model and ore controls including regional stressmodeling Precambrian Research 149149ndash196
Gentner W Lippolt H J and Muumlller O 1964 The potassium-argonage of the Bosumtwi crater in Ghana and the chemicalcomposition of its glasses Zeitschrift fuumlr Naturforschung 19A150ndash153
Girty G H and Barber R W 1993 REE Th and Sc evidence for thedepositional setting and source rock characteristics of the QuartzHill chert Sierra Nevada California In Processes controlling thecomposition of clastic sediments edited by Johnsson M J andBasu A GSA Special Paper 284 Boulder Colorado GeologicalSociety of America pp109ndash119
Herron M M 1988 Geochemical classification of terrigenous sandsand shales from core or log data Journal of SedimentaryPetrology 58820ndash829
Hirdes W Davis D W and Eisenlohr B N 1992 Reassessment ofProterozoic granitoid ages in Ghana on the basis UPb zircon andmonazite dating Precambrian Research 5689ndash96
Hirdes W Davis D W Luumldtke G and Konan G 1996 Twogenerations of Birimian (Paleoproterozoic) volcanic belts innortheastern Cocircte drsquoIvoire (West Africa) Consequences for theldquoBirimian controversyrdquo Precambrian Research 80173ndash191
John T Klemd R Hirdes W and Loh G 1999 The metamorphicevolution of the Paleoproterozoic (Birimian) volcanic Ashantibelt (Ghana West Africa) Precambrian Research 9811ndash30
Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576
Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349
Junner N R 1937 The geology of the Bosumtwi caldera andsurrounding country Gold Coast Geological Survey Bulletin 81ndash38
Karp T Milkereit B Janle J Danour S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743
Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60
Koeberl C and Reimold W U 2004 Post-impact hydrothermalactivity in meteorite impact craters and potential opportunitiesfor life In Bioastronomy 2002 Life among the stars edited byNorris R P and Stootman F H San Francisco AstronomicalSociety of the Pacific pp 299ndash304
Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)
Koeberl C and Shirey S B 1993 Detection of a meteoriticcomponent in Ivory Coast tektites with rhenium-osmiumisotopes Science 261595ndash598
Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772
Koeberl C Reimold W U Blum J D and Chamberlain C P1998 Petrology and geochemistry of target rocks from theBosumtwi impact structure Ghana and comparison with IvoryCoast tektites Geochimica et Cosmochimica Acta 622179ndash2196
Leube A Hirdes W Maur R and Kesse G O 1990 The earlyProterozoic Birimian Supergroup of Ghana and some aspects ofits associated gold mineralization Precambrian Research 46139ndash165
Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana (abstract) GSASpecial Paper 68 Boulder Colorado Geological Society ofAmerica 218 p
Mader D and Neubauer F 2004 Provenance of Palaeozoicsandstones from the Carnic Alps (Austria) Petrographic andgeochemical indicators International Journal of Earth Science93262ndash281
Manu J 1993 Gold deposits of Birimian greenstone belt in GhanaHydrothermal alteration and thermodynamics PhD thesisBraunschweiger GeologischndashPalaumlontologische Dissertationenvol 17 Braunschweig Germany
Melcher F and Stumpfl E F 1994 Palaeoproterozoic exhaliteformation in northern Ghana Source of epigenetic gold-quartzvein mineralization Geologisches Jahrbuch 100201ndash246
Milesi J P Ledru P Feybesse J L Dommanget A and Macoux E1992 Early Proterozoic ore deposits and tectonics of theBirimian orogenic belt West Africa Precambrian Research 58305ndash344
Moon P A and Mason D 1967 The geology of 14deg field sheets 129and 131 Bompata SW and NW Ghana Geological SurveyBulletin 311ndash51
Oberthuumlr T Vetter U Schmit-Mumm A Weiser T Amanor J A
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
540 F Karikari et al
Gyapong J A Kumi R and Blenkinsop T G 1994 TheAshanti gold mine at Obuasi Ghana Mineralogicalgeochemical stable isotope and fluid inclusion studies on themetallogenesis of the deposit Geologisches Jahrbuch D10031ndash129
Okada H 1971 Classification of sandstones Analysis and proposalsJournal of Geology 79509ndash525
Osae S Asiedu D K Banoeng-Yakubo B Koeberl C andDampare S B 2006 Provenance and tectonic setting of lateProterozoic Buem Sandstones of southern Ghana Evidence fromgeochemistry and detrital modes Journal of African EarthSciences 4485ndash96
Pearce J A Harris N B W and Tindle A G 1984 Trace elementdiscrimination diagrams for the tectonic interpretation of graniticrocks Journal of Petrology 25956ndash983
Pelig-Ba K B Parker A and Price M 2004 Trace elementgeochemistry from the Birimian metasediments of the northernregion of Ghana Water Air and Soil Pollution 15369ndash93
Pesonen L J Koeberl C and Hautaniemi H 1998 Aerogeophysicalstudies of the Bosumtwi impact structure GSA Abstracts withPrograms 30A190
Pettijohn F J Potter P E and Siever R 1972 Sand and sandstoneBerlin-Heidelberg-New York Springer 618 p
Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546
Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710
Rollinson R H 1993 Using geochemical data Evaluation
presentation interpretation Harlow Essex Longman GroupUK Limited 347 p
Rosenberg P E 1967 Subsolidus relations in the system CaCO3-MgCO3-FeCO3 between 350 and 550 degC American Mineralogist52787ndash796
Scholz A C Karp T Brooks M K Milkereit B Amoako P Y Oand Arko A J 2002 Pronounce central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942
Sylvester P J and Attoh K 1992 Lithostratigraphy and compositionof 21 Ga greenstone belts of the West African Craton and theirbearing on crustal evolution and the Archean-Proterozoicboundary Journal of Geology 100377ndash393
Taylor P N Moorbath S Leube A and Hirdes W 1992 EarlyProterozoic crustal evolution in the Birimian of GhanaConstraints from geochronology and isotope geochemistryPrecambrian Research 5697ndash111
Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p
Woodfield P D 1966 The geology of the 14deg field sheet 91 FumsoNW Ghana Ghana Geological Survey Bulletin 301ndash66
Wright J B Hastings D A Jones W B and Williams H R 1985Geology and mineral resources of West Africa London Allen ampUnwin 165p
Yao Y and Robb L J 2000 Gold mineralization inPalaeoproterozoic granitoids at Obuasi Ashanti region GhanaOre geology geochemistry and fluid characteristics SouthAfrican Journal of Geology 103255ndash278
