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SECTION 5. GEOLOGY

5.1 INTRODUCTION

Ghana falls mostly within the West African Craton which stabilised in the early Proterozoic (2000

Ma) during the Eburnean Orogeny. This orogeny also stabilised the Zaire Craton and affected vast

parts of Western Africa and neighbouring regions in South America that were conterminous with

the Eburnean tectonothermal province. Outside South Africa, the West African Craton is the

second largest region in Africa where lower Proterozoic rocks are extensively preserved. These

early Proterozoic rocks comprise extensive belts of metamorphosed volcanic and sedimentary

rocks exposed in Ghana, Burkina Faso, Niger and Cote d'Ivoire. On the east and west, the

Craton is bounded by late Proterozoic mobile belts (700 - 500 Ma) referred to as the Pan African

mobile belts.

Recent reviews of the geology of Ghana by Kesse (1985), Wright (1985) and Leube et al. (1990)

provide useful summaries. These reviews are particularly relevant because of the attention paid to

mineral resource potential. Kesse presents a good treatise on specific mineral and rock resources

available in Ghana; Wright relates the geology of Ghana to the regional geology of West Africa,

and Leube et al. present a significantly different stratigraphic interpretation for the Birimian

System in Ghana, stressing lateral lithologic continuity and facies changes within the group.

Unlike many previous workers, Leube et al. believe that some of the granitoids possess significant

potential for gold mineralisation.

Geologically, Ghana can be divided into several distinct terranes (see Figure 5.1 and Map 15).

i) An early Proterozoic terrane (Birimian System) which hosts most of the country's

mineral deposits and occupies the western and northernmost part of the country;

ii) The Tarkwaian System, a distinctive sequence of clastic sediments within the

Ashanti, Bui, and Bole-Navrongo Belts;

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______________________________________________________________________________

Figure 5.1. Generalized Geologic Map of Ghana______________________________________________________________________________

iii) The Voltaian Basin, in which are preserved the late Precambrian to Paleozoic

sediments that mantle the craton;

iv) The Dahomeyan System, occupying the easternmost part of Ghana;

v) A pan-African mobile belt, the Togo and Buem Formations, separated from the

Birimian terrane by a prominent topographic feature known as the Akwapim -

Togo range;

vi) Phanerozoic sedimentary rocks; and

vii) Intrusive rocks.

5.2 THE BIRIMIAN

Rocks of the Birimian System underlie most of southern, western and northern Ghana. They host

most of the gold and diamond deposits in the country, hence they have been subjected to

considerable study. Ideas on the stratigraphy, structure and age of the Birimian rocks have

evolved over the years as a result of work by the Geological Survey Department (GSD), the

Soviet Geological Team and the Ghana-German Mineral Prospecting Project (GGMPP) in Ghana

and the work of French geologists in Francophone West Africa. Kesse (1985) gives an overview

of the ideas about the Birimian up through the early 1980's.

The Birimian consists of metamorphosed volcanic and sedimentary rocks which form five sub-

parallel belts of volcanic rock separated by broad “basins” of sedimentary rocks. Up to the early

1980's, except for Matthews and Milnes (1979) and Breakey and Breakey (1977), authors on the

Birimian adopted a chronostratigraphic nomenclature. They divided the rocks into an older

“Lower Birimian”, consisting of predominantly metasedimentary rocks, and a younger “Upper

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Birimian”, comprising chiefly metavolcanic rocks (Junner 1935, 1940; Bates, 1955). These ideas

were based largely on mapping by the GSD in southern Ghana and are widely supported in Ghana

(Tables 5.1 and 5.2).

The rock types present in the Lower Birimian sedimentary belt are greywackes with turbidite

features, phyllites, slates, schists, weakly metamorphosed tuffs and sandstones. Some of the

phyllites contain pyrite, and finely divided carbonaceous matter is present in most of them.

Silicification is common in the phyllites, particularly towards the boundary with the Upper

Birimian.

The Upper Birimian volcanic succession consists of lava flows and dyke rocks of basaltic and

andesitic composition. Most of these rocks have now been metamorphosed to hornblende

actinolite-schists, calcareous chlorite schists and amphibolites (the greenstones). Pillow structures

indicating subaqueous eruption of the original basaltic lavas are commonly observed. Available

major and trace element chemical data show that these Birimian metabasalts are tholeiitic.

However, felsic volcanic rocks also occur in this succession as well as in the predominantly

sedimentary sections. The felsic units include dacitic pyroclastic rocks, minor andesitic and

rhyolite flows, and undifferentiated volcanogenic sediments. Minor intrusions of mafic and

ultramafic rocks cut the volcanics in some places. Mn-rich horizons also occur at stratigraphically

lower level in the Upper Birimian and have been found in the uppermost Lower Birimian as well.

In 1964-66, the Soviet Geological Team (SGT) mapped the Bole and Lawra Belts. The SGT

classified the Birimian into three sub series: Lower (sediments), Middle (pyroclastics) and Upper

(lavas).

The GSD developed a classification which incorporated and modified the SGT classification

(Asihene and Barning 1975). In the late 1970's the GSD developed an accepted stratigraphic

nomenclature for the Birimian (Kesse, 1985).

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TABLE 5.1THE PRESENT ACCEPTED VERSION OF THE DIVISIONS OF THE LOWER

BIRIMIAN SERIES BY GHANAIAN GEOLOGISTS

Subseries Composite Lithology

Upper Arenaceous subseries Yellowish brown to buff and in some places purple, massive

meta-sandstones, meta-greywacke and minor thin beds of

meta-siltstone.

Upper Argillaceous subseries Predominantly yellowish brown to ochre coloured

assemblage of phyllitic siltstone and their tuffaceous

equivalents.

Middle Arenaceous subseries Meta-greywacke, meta-siltstone - phyllite assemblage which

is characteristically rhythmically bedded in the lower parts

and is also typically tuffaceous and manganiferous in the

middle parts.

Lower Argillaceous subseries Predominantly black, grey and dark grey phyllite

interbedded with greenish grey and buff-coloured tuffaceous

phyllite.

Lower Arenaceous subseries Lithic assemblage of meta-greywacke, meta-sandstone,

meta-siltstone, phyllite and tuffaceous varieties of these

rock types.

Source: Kesse, 1985

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TABLE 5.2THE PRESENT ACCEPTED VERSION OF THE DIVISIONS OF THE UPPER

BIRIMIAN SERIES BY GHANAIAN GEOLOGISTS

Subseries Composite lithology

Basic Volcanic subseries Makes up the bulk of the Upper Birimian and is further

divided into normal greenstones (metabasalt and

metadolerite), amphibolite intrusions, and greenschists and

actinolite-chlorite-greenschists.

Acid Volcanic subseries Meta-rhyolites, quartz-feldspar porphyry, felsites, and

quartz-chlorite schists.

Sedimentary-volcanic

subseries

Meta-tuffaceous greywacke, quartzites and schistose

conglomerate, and grit.

Source: Kesse, 1985

As GSD work proceeded, notably in the Southwest Mapping Project, new ideas began to evolve,

including the suggestion that sediments and volcanics of the Birimian might be laterally

equivalent. Instead of a chronostratigraphic approach, Breakey and Breakey, (1977) introduced a

lithofacies approach for the sediments of their map area, and Matthews and Milnes (1979)

concluded that for the area in which they were working, the metasediments are either younger

than or coeval with the metavolcanics.

Geologists in Francophone countries developed a stratigraphic classification of the Birimian which

included the same lithologies but had the reverse stratigraphic order. These ideas are summarised

by Kesse (1985).

From 1983 to 1994 the GSD and the [German] Federal Institute for Geosciences and Natural

Resources (BGR) engaged in a co-operative project which focused on gold in Ghana (GGMPP).

As the Birimian hosts most of the gold mineralisation, it was the focus of considerable work. The

GGMPP included remapping of parts of western Ghana, plus a variety of topical studies on

structural geology, geochemistry, and stratigraphy, as well as radiometric dating. The results

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have been published in series of papers from 1986 to the present. (Leube, et al., 1990, Eisenlohr

and Hirdes, 1992, Taylor, et al. 1992, Hirdes, et al. 1992, 1993, Davis, et al. 1994, and papers in

Oberthur, 1994). The work has challenged many long held ideas on Ghanaian geology, and some

ideas are still not fully accepted in the Ghanaian geologic community.

The summary which follows presents a description of the Birimian which incorporates the above

ideas (Table 5.3).

The Birimian terrane of Ghana is part of the West African Craton. In western Ghana, the

Birimian consists of five northeast to north trending belts of volcanic rocks separated by broader

belts (basins) of sedimentary rocks. The volcanic belts are typically 15 to 40 km wide and spaced

60 to 90 km apart.

The Birimian volcanic rocks consist mainly of tholeiitic basalts of oceanic affinity. Pillow

structures indicate that the lavas were deposited in a submarine environment. There are lesser

amounts of andesite, dacite and rhyolite in the volcanic sections. The total thickness of the lava

sequences is unknown owing to folding. The geochemistry of the lavas is summarised by Leube

et al. (1990).

The Birimian sediments comprise greywacke, turbidites, volcaniclastics and argillites. In general,

the proportion of argillite increases towards the centre of the basins. Mn-rich siliceous chemical

sediments are common near the volcanic-sediment transition. The contact between the sediments

and volcanics is poorly exposed, but in place, interlayering has been reported.

As noted above, the stratigraphic relationship of the two groups of Birimian rocks has been

interpreted differently over the years. Leube et al., (1990) concluded that the sediments and

volcanics represented lateral facies equivalents. This was supported by the interbedding of the

two units, their similar depositional environment and similar geochemistry.

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Radiometric dates of lavas from four of the volcanic belts give a Sm-Nd isochron age of 2,166±

66 Ma (Taylor, et al. 1992), with the main episode of volcanism in the period 2,155-2,185 Ma.

Dating of zircons in Birimian metasediments (Davis et al. 1994) and dating of granitoid intruding

sediments in the Kumasi basin (Hirdes et al. 1992) brackets Birimian deposition between 2,135

and 2,116 Ma, i.e., some 35 Ma younger than the lavas. These dates thus support the earliest

GSD interpretation of the relative ages of the two lithologic groups.

The Birimian rocks are generally tightly and isoclinally folded; they are also commonly sheared

and fractured. It is therefore not easy to establish stratigraphic succession and estimate thickness.

However the total thickness of the Birimian in Ghana may be 10,000 to 15,000 m. The rocks

have dips generally greater than 60°. Faulting tends to follow the trend of the folds.

Metamorphism in the Birimian is “low-grade” greenschist facies except near intrusive contacts

where amphibolite assemblages occur in the metasediments.

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TABLE 5.3

FACIES OF THE BIRIMIAN SYSTEM AND THEIR CHARACTERISTIC LITHOLOGIC ASSOCIATIONS

Facies Lithology Series or subseries of the oldclassification to which faciesgenerally corresponds (seeAsihene and Barning, 1975)

Depositional environment

Volcanic-volcaniclastic

Presence of lava essential;volcaniclastic rocks (pyroclastics orepiclastics) may be locallypredominant. Rare argillaceoussediments

Br2 (Upper Birimian), the

Br2b1 of Trashliev (1992)

could be placed in thiscategory.

Water/air interface volcanicislands or volcanic ridges

Wacke (turbiditerelated)

Reworked, allochthonous, notablyquartz enriched volcaniclastic rocksdisplaying graded bedding

Br11 of Trashliev (1992) partly

Br31, Br51

Turbidites at lower end ofslopes of volcanic ridges

Volcaniclastic argillite Interbedding of volcaniclastic(predominantly sand-to-silt-size non-or little transported pyroclastics andargillitic rock with the formerdominant in thickness andproportional abundance. Rare in-layers of lava)

partly Br31, some Br51Depository proximal tovolcanic island or ridges

Argillite-volcaniclastic As above, but a preponderance inthickness and proportional abundanceof argillites

Br21, Br41

More distal portionsof the depositionalbasin argillite

Argillites, commonly finely laminatedand graphitic

Br21, Br41Low energy environments inthe most distal, (i.e. central)portions of the basins

Chemical Rich in cherts, carbonates,manganese, sulphides, carbon.

In transitional zones betweenbelts and basins

Source: Leube et al. 1990

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5.3 THE TARKWAIAN

A distinctive sequence of clastic sedimentary rocks occurs in elongate troughs developed on top

of the Birimian System. These rocks host important paleoplacer gold deposits and are known as

the Tarkwa System. Most workers agree that the Tarkwaian sediments were deposited in

intermontane grabens formed by preferential rifting along the axes of the volcanic belts and that

there is no evidence that the depositional basins were ever linked.

Kesse (1985) and Leube and Hirdes (1986) summarised the literature on the Tarkwaian up

through the mid-1980's. More recent data is presented in Eisenlohr and Hirdes, (1992), and

Oberthur (1994) for the Ashanti Belt and by Zitzman et al. (1993 a, b) for the Bui Belt.

The rocks of the Tarkwaian System represent erosional products of the Birimian and are

dominated by coarse clastic sediments. They are widespread in the Ashanti and Bui volcanic belts

and, to a lesser extent, in all of the other belts.

In the Ashanti Belt, the Tarkwa is made up of four units. The lowest unit, the Kawere Group,

consists of immature, polymictic, matrix supported, large pebble conglomerate dominated by

mafic (Birimian) pebble lithologies. The Kawere is overlain by, and is in marked contrast with,

the Banket Series which consists of mature, clean, quartzite, grit, breccia and conglomerate

composed in part of well sorted quartz pebble conglomerate beds known as “reefs” that host the

gold mineralisation. The Banket is overlain by the Tarkwa Phyllite which consists of a transition

sequence from sandstone to chloritic and sericitic phyllite. The uppermost Tarkwa unit is the

Huni Sandstone; sandstone and quartzite with interbeds of phyllite (Table 5.4).

The Banket Series in the Ashanti Belt has been the subject of a number of sedimentological

studies owing to its association with gold mineralisation. These studies indicate that the source

area for the Banket has to be to the southeast of the present outcrop (Sestini, 1973, Strogen,

1991, Hirdes and Nunoo, 1994) and that the Tarkwaian sediments were derived largely from the

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erosion of Birimian rocks, as shown by the similarity of detrital zircon populations in Tarkwaian

and Birimian sediments (Davis, et al., 1994).

The series includes quartz-pebble conglomerates. The overall thickness of the series ranges from

120 to 600 m. Junner et al., (1942) named four “reefs” or conglomerate bands in the following

succession: Breccia Reef - Middle Reef - Basal or Main Reef and Sub-basal Reef. The Basal or

Main Reef is the most persistent conglomerate bed in the Tarkwa goldfield area and is by far the

richest in gold.

The structure of the Tarkwaian rocks differs from belt to belt. In the Ashanti Belt, the Tarkwa

System is folded into a series of northeast trending, northeast plunging antiforms and synforms.

Along the northwest margin of the belt, the Tarkwaian is overturned and locally overthrust by

Birimian rocks (Eisenlohr and Hirdes, 1992). The sediments have a moderate primary foliation

(S1) and a strong secondary foliation (S2) near the margins of the belt.

In the Bui Belt, Tarkwaian rocks are folded into a regional syncline with a steep northeast

trending normal fault parallel to the fold axis. Along the northwest margin of the belt the

Tarkwaian rocks are strongly tectonised and overturned. The Tarkwaian in the Kibi-Winneba

Belt has been less well studied but appears also to be a northeast trending overturned syncline.

The age of the Tarkwa System has been the subject of recent study by Davis et al., (1994) and

Hirdes and Nunoo, (1994). Davis et al., (1994) dated zircons from the Kawere conglomerate and

several of the reef horizons in the Banket Series. They also dated an authigenic rutile from the

main reef of the Banket Series using U-Pb methods. The age of deposition of the Tarkwa group

can be bracketed by the youngest zircon grain from the lowermost Kawere series and age of the

authigenic rutile which formed after deposition. These dates give a time range of 2,132 ± 3 Ma to

2,096 Ma (Hirdes and Nunoo, 1994). An additional upper time limit on the age of the Tarkwa is

the age of the granitoid from the Cape Coast area. Granitoid pebbles of this type are not found in

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the Tarkwaian, thus the granite intruded after the deposition of the Tarkwa System. This

granitoid complex gives a U-Pb date of 2,090 ± 1 Ma (Davis, et al., 1994).

TABLE 5.4GENERALIZED GEOLOGICAL COLUMN, TARKWA GOLDFIELD, GHANA

System Group, etc. Lithology

Post Tarkwaian Intrusives Epidorite, norite, gabbro, amphibolite,porphyry and diabase.

Intrusive Contact

Tarkwaian Huni Sandstone Sandstone, grit, quartzite and phyllite.

Tarkwa Phyllite Phyllite and chloritoid-bearing phyllitewith subsidiary arenaceous beds.

Banket Series Quartzites, grits, breccias and banketconglomerates. Four reefs arerecognized: a sub-basal reef, main orbasal reef, a middle reef and an upperbreccia reef.

Kawere Group Sandstones, quartzites, grits, brecciasand conglomerates.

Great Unconformity

Birimian Upper Birimian Volcanics (greenstones), pyroclastics,phyllite, greywacke and manganiferousphyllite, intruded by and granitised inplaces to granites and porphyries.Also intruded by epidiorites in places.

Source: Junner et al. (1942)

The Tarkwa Phyllite ranges in thickness between 120 and 400 m. The phyllites are divisible into

those with and those without chloritoid. The phyllites without chloritoid range from sandy to

fine-grained lustrous types and in some cases contain abundant hematite or magnetite.

The Huni Sandstone, the uppermost Tarkwaian unit, contains cross bedding and channel scours,

indicative of shallow water conditions. The sandstone is the weathered representation of

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feldspathic quartzites which are in general finer grained than the Banket Series quartzites. They

contain variable amount of feldspar, sericite, chlorite, ferruginous carbonate, magnetite and, in

weathered outcrops, epidote.

The Tarkwaian sediments are generally weakly metamorphosed.

5.4 THE VOLTAIAN

Almost one third of Ghana is covered by sediments of the inland Voltaian Basin which covers an

area of about 103,600 km2. The Voltaian strata are nearly horizontal beds of sandstones, shales,

mudstones and conglomerates thought to be of Late Precambrian to Paleozoic age. In most

places, the flat lying Voltaian strata overlie the Birimian rocks with a marked angular

unconformity. Junner and Hirst (1946) subdivided the Voltaian sediments on the base of lithology

and field relationships into Lower, Middle and Upper units.

Many other authors have also discussed the age and stratigraphy of the Voltaian, such as the

Soviet Geological Team (1964), Jones (1978) and Anan-Yorke, (1980) (Table 5.5).

The Lower Voltaian sediments represent a marine transgression-regression cycle on the craton,

whereas the Middle Voltaian records a glacial event followed by prolonged marine incursion and

subsidence of the basin. In the Eastern part of the basin, the adjacent Togo Belt crops out.

The Upper Voltaian, otherwise known as the Obosum Formation, is thickest and coarsest in the

southeast. The conglomerates contain pebbles of granite and other igneous rocks, as well as

quartzite fragments. Sedimentary structures show the direction of transport to have been from

the southeast. The Obosum beds are molasse deposits formed by the erosion of the Togo Series

following its uplift in the Pan African event.

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TABLE 5.5

VOLTAIAN BASIN STRATIGRAPHY

Anan-Yorke (1980) SGT (1964) Junner& Hirst (1946)

Upper Voltaian Massive sandstone Massive cross beddedsandstone

Upper sandstone

Lower Carboniferous(450-320 Ma)

Thin beddedsandstone & Tamalered beds

Thin beddedsandstone

Thin beddedsandstone

Middle Voltaian;Lower Ordovician -Lower Vendian(480-675 Ma)

Upper green beds Tamale red beds Obosum beds

Afram shaleAkroso conglomerateLower green beds

Green-gray lowerseries

Oti beds

Lower VoltaianUpper to MiddleRiphean(700-1000 Ma)

Basal sandstone Basal sandstone Basal sandstone

Angular unconformity

Birimian

Source: Compiled from Jones (1978), Anan-Yorke, (1980), Kesse, (1985) and Junner and Hirst,

(1946)

The structure of the Voltaian Basin has been discussed by Ako and Wellman (1985), based on

their reviews of gravity and magnetic data for the basin. According to them, the basin overlies

magnetic rocks, probably Birimian, with west and northeast trending structures. Based on the

magnetic data, the basin deepens to the southeast, the maximum depth being almost 6 km. The

depth of the basin and dip of the strata suggests that the basin formed by lithospheric flexure.

Ako and Wellman (1985) interpreted the basin as a foreland basin developed by flexure due to

obduction of lower crust from the east and southeast during the Pan-African Orogeny. Jones

(1990) believed that the gravity highs in the basin are due to mafic subvolcanic intrusives below

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the Voltaian which were the feeders for the Buem Volcanics. This conclusion is supported by the

presence of abundant volcanic rocks in the lower sandstones cut by the Premuase well (Watt,

1977, Anan-Yorke, 1978).

5.5 THE DAHOMEYAN

The Dahomeyan System is a part of the second major tectono-stratigraphic terrane in Ghana; it

underlies eastern and southeastern Ghana. The Dahomeyan is the easternmost rock group in

Ghana and differs significantly from other rocks in Ghana in that it is composed of high grade

metamorphic rocks.

The system consists of four lithologic belts of granitic and mafic gneiss. The mafic gneisses are

relatively uniform oligoclase, andesine, hornblende, salite and garnet gneisses of igneous

parentage and generally tholeiitic composition (Holm, 1974). The granite gneisses interlayer with

the mafic gneiss and are believed to be metamorphosed volcaniclastic and sedimentary rocks.

Persistent bands of nepheline gneiss in the system appear to be metamorphosed calc-alkaline

igneous rocks (Holm, 1974).

A distinctive, but normal, lithology in the Dahomeyan is the “Kpong Conglomerate”, a calcareous

rock which has been interpreted to be a carbonatite (Mani, 1978).

Structurally, the granite gneiss is the lowest unit in the system. All of the gneisses have

undergone at least two stages of penetrative deformation. The latest deformation is believed to be

of Pan-African age (500-600 Ma) and is referred to as a “reactivation” of Birimian crust by

Kennedy, (1964). Fitches (1970), however, suggests that the metamorphism may be Eburnian

(Proterozoic) in age.

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The gross structure of the Dahomeyan is that of alternating northeast trending lithologic belts

with moderate dips to the southeast. Along the western boundary of the belt the gneisses are in

fault contact and overthrust onto rocks of the Buem-Togo Belt.

The original age of the Dahomeyan protolith is unknown. Rb-Sr and K-Ar dates by Agyei, et al.

(1987) show Pan African ages for the last metamorphism. Blay (1991) postulates that the

Dahomeyan are Birimian rocks. This conclusion is supported by Grant (1969) and Affaton, et al.

(1980).

5.6 THE TOGO BELT

The second major lithologic group which makes up the eastern Ghana terrane is the Togo Belt

comprising the Buem and Togo Series. This group of rocks comprises three distinctive lithologic

assemblages.

Togo Series

The rocks which comprise the north to northeast trending Togo Range consist of strongly

tectonised phyllite, quartzite and serpentinite. These rocks are variously known as the Togo

Series (Kesse, 1985) or the Togo Tectonic Unit (Blay, 1991). The contacts between the Togo

and Dahomeyan to the east and the Buem to the west are thrust faults. The unit grades from east

to west from phyllite and chlorite schist upwards into quartzite, micaceous quartzite and

sandstone. Serpentinites occur along the western contact and appear to be emplaced along thrust

faults (Grant, 1969).

Buem Series

West of the Togo Range is a belt of volcanic and sedimentary rocks known as the Buem Series

(Kesse, 1985, Jones, 1990) or the Buem Tectonic Unit (Blay, 1991). The Buem consists of two

lithologic assemblages, volcanic and sedimentary. The volcanic assemblage is made up of pillow

basalt, agglomerate, hawaiite and trachyte. The sedimentary assemblage, which encloses the

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volcanics, consists of red shales, feldspathic to quartz arenite, conglomerate, tillite, jasper and

minor limestone. Whereas the volcanics were deposited in a submarine environment, the

sediments appear to be shallow water to subaerial in origin (Jones, 1990). Jones (1990) describes

the Buem as an eastward dipping homoclinal sequence, whereas Kesse (1985) states that the

rocks are strongly folded. Blay (1991) cites the presence of a basal conglomerate at the Buem-

Togo contact as an indication that the Buem is younger than the Togo unit.

Sandy mudstone unit

Lashmanov (1991) mapped the northern part of the Dahomeyan belt and noted the presence of a

sandy mudstone unit between the Buem and the Togo. Lashmanov's mapping supports Blay

(1991) in concluding that the Buem is younger than the Togo.

The age of the Dahomeyan rocks is problematic. Cahen et al. (1984) give K-Ar ages of 528 to

492 Ma for samples of the Buem volcanics. Jones (1990) concludes that these dates represent a

metasomatic event. Lashmanov's sandy mudstone unit is laterally equivalent to the “Oti Beds” in

the Voltaian Basin, a conclusion reached earlier by Grant (1969). Radiometric dating of

glauconite in the Middle Voltaian Obosum Beds, which overlie the Oti Beds, gives an Upper

Precambrian age of 620 Ma (Bozhko, et al., 1971). This would indicate an Upper Proterozoic

age for the Togo and a Proterozoic to Lower Cambrian age for the Buem.

5.7 PHANEROZOIC SEDIMENTARY ROCKS

Relatively minor outcrops of sedimentary rocks along the coast from Keta and Accra in the east

to Half Assini in the west constitute remnants of rocks of the Phanerozoic coastal basins. From

east to west, these rocks occur in the Keta, Accraian, Sekondian and Tano basins.

Rocks of the Keta basin are of Cretaceous age and consist of sandstones, siltstone, shales,

claystone and fossiliferous limestone beds (Kesse, 1985). The Accraian is considered mid-

Devonian in age and consists predominantly of sandstones and shales. The Sekondian strata are

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made up of sandstones, shales, silts and beds of chalcedony, sands and pebbly beds and range in

age from Devonian to Cretaceous. The Tano basin is located in the extreme southwestern corner

of Ghana. It is made up of Cretaceous-Tertiary sediments consisting of limestones, shales and

sands which have a large off-shore extension.

5.8 INTRUSIVE ROCKS

5.8.1 BIRIMIAN GRANITOIDS AND ASSOCIATED INTRUSIVES

Four main types of granitoids are recognised in the Birimian of Ghana. They include Winneba,

Cape Coast, Dixcove and Bongo granitoids (Junner 1940; Kesse, 1985). The latter three have

been recently termed “Basin”, “Belt” and “K-rich” granitoids. (Leube et al., 1990; Mauer, 1990;

Hirdes et al., 1993). The Cape Coast and Dixcove type granitoids are widespread in Ghana, the

Winneba type is limited to small areas near Winneba, and the Bongo type crops out in the Bole-

Navrongo Belt and in the Banso area. The features of these granitoids are listed in Table 5.6.

Cape Coast type Granitoids (G1)

The Cape Coast granites occur only within the Birimian sedimentary basins. Some of them are

two mica granites. This group also includes gneisses, and these are especially well developed in

the metasedimentary belts. They are typically biotite-bearing. It has been suggested that the Cape

Coast granitoids, which appear migmatitic in some localities, might represent an older continental

basement on which the Birimian supracrustals were deposited. However, there is no

geochronologic support for this theory. Contacts between these granitoids and the metasediments

are irregular; rafts of metasediments and relict structures from metasediments rise into the

granitoids, and tendrils of granite vein the metasediments (Taylor et al.,1992). Based on the

degree of foliation, early workers assumed that the Cape Coast granitoids intruded during

regional deformation and that Dixcove granites were emplaced after deformation. (Junner, 1940;

Kesse, 1985). However, later work by Hirdes et al. (1992) demonstrated, in contrast to long held

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views, that Dixcove granitoids formed at about 2,175 Ma and are about 60 and 90 Ma older than

the Cape Coast granitoids. Taylor et al. (1988, 1992) suggest that the Cape Coast and Dixcove

granitoids are coeval.

Dixcove type Granitoids (G2)

Dixcove-type granitoids are metaluminous and typically dioritic to granodioritic in composition.

They intrude Birimian volcanic rocks. They are typically hornblende-bearing and are commonly

associated with gold mineralisation where they occur as small plutons within the volcanic belts.

The granitoids are massive in outcrop, do not have a compositional banding or foliation, and are

thus generally considered post-deformation. However, Dixcove-type granitoids have never been

shown to intrude or crosscut Cape Coast granitoids, and some workers (Murray, 1960) have

recognised Dixcove granitoid clasts in Cape Coast granitoids. The presence or absence of a

foliation is not a sufficient criterion to establish timing relationships in granitoids (Paterson et al.,

1989). In particular, amphibole bearing granitoids have been demonstrated to be less likely to

develop a foliation during deformation than biotite-rich granitoids (Vernon and Flood, 1988).

Locally, sheared granitoids were observed in the Sefwi Belt, and these may have been deformed

during regional deformation (Eisenlohr and Hirdes, 1992). Dixcove granitoids have a porphyritic

texture defined by plagioclase set in a quartz-hornblende-actinolite matrix. The plagioclase is

always saussuritised or sericitised and actinolite appears to crosscut the fabric. Such features are

typical of granitoids that have undergone metamorphism (Vernon and Flood 1988). The

granitoids commonly contain basalt xenoliths, and there appears to be a gradational boundary

between finer and coarser grained Dixcove granitoids and basalts (e.g., Hirst, 1946). These

observations indicate a close association between Birimian basalts and Dixcove granitoids and

suggest they may be part of the same igneous event. The above features do not necessarily prove

pre-deformation Dixcove granitoid emplacement, but the data contradict the established view that

the granitoids intruded after deformation.

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Winneba Granitoid

The Winneba granitoid occurs at a single locality near the town of Winneba. It is the only rock

suite so far encountered in Ghana which shows evidence for an Archean sialic precursor (Sm/Nd

model age of about 2.6 Ga (Taylor et al., 1988, 1992).

Bongo-type Granitoid

The type locality for this granitoid is located in northern Ghana where the granites intrude

Tarkwaian sediments that overlie the Bole-Navrongo Volcanic Belt. This granitoid is

peraluminous and lacks a foliation (Leube et al., 1990). The granitoid’s Rb-Sr whole-rock

isochron age is 1,968 ± 49 Ma (Lenz in Hirdes et al., 1992).

A granitoid similar in composition to the Bongo type, the Banso granitoid, crops out within the

Ashanti Belt south of Kumasi. Formerly, this granitoid was thought to be unconformably overlain

by Tarkwaian rocks (Woodfield, 1966), but recent mapping indicates that the granite crosscuts

the tectonised Birimian/Tarkwaian boundary. Contact metamorphic minerals have been observed

in Tarkwaian rocks close to the granitoid (Mauer, 1986). The latter observations plus

petrographic and geochemical similarities described by Mauer (1986) suggest that the Banso

granitoid intrudes the Tarkwaian and thus occupies a similar tectonic position to that of the

Bongo type granitoid.

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TABLE 5.6

CHARACTERISTICS OF GRANITOID ROCKS IN GHANA

Name Size/Geologic setting Contact Aureole Foliation Mafic Minerals Aluminouscharacter

Belt(Dixcove)

Small to mediumsized plutonsrestricted to Birimianvolcanic belts

Contact aureoles of a fewtens of metres maximum

Seldom foliated (exceptfor local intenseshearing). Nocompositional banding

Hornblendedominant

Metaluminous

Basin (CapeCoast)

Large batholithsrestricted to Birimiansedimentary basins

Extensive contactmetamorphic aureoles

Foliated, compositionalbanding ubiquitous andpronounced

Biotite dominant Peraluminous

K-Rich(Bongo)

Intrudes Tarkwaian inNavrongo Belt,Banso area in AshantiBelt

Thin contactmetamorphic zone

Unfoliated Hornblende, biotiteat Banso

Metaluminous

Winneba Single known localewithin basinsediments in theWinneba-Kibi Belt

Not Determined Foliation common Biotite Peraluminous

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TABLE 5.6 (CONTINUED)

Composition(Typical)

CompositionRange

Geochemistry Alteration Association Age (Ma)

Belt (Dixcove) Typicallydioritic togranodioritic

True granite todiorite

Na2O, CaObelt >Na2O, CaObasin

Pronouncedretrogradealteration

Similar geochemicalcharacteristics astholeiitic basalts inbelts for someelements

2,170-2,181

Basin (CapeCoast)

Typicallygranodioritic

True granite totonalite

Rb, K2Obasin >Rb, K2Obelt

Littlealteration

No evidence forgeochemical similarityto tholeiitic basalts inbelts

2,083-2,118

K-Rich (Bongo) Granite Granite togranodiorite

High K (>4%K2O), Sr>>Beltand Basin

Littlealteration

Post tectonic 1,968±49

Winneba Granodiorite Granite togranodiorite

Similar to BasinK2O> Belt andBasinK2O <Bongo

Littlealteration

Sm-Nd evidence ofArchean sialicprecursor

Rb-Sr2,024±159Pb-Pb2,172±110

Source: Leube and Hirdes (1986), Eisenlohr and Hirdes (1992), Hirdes et al. (1992), Taylor, et al. (1988, 1992), Kesse, (1985).

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5.8.2 SERPENTINITES

Ultramafic bodies are associated with major faults. The largest is a lensoid body 2.5 km long and

100 m thick and deep to southeast in the Anum area (Jones, 1990). These bodies were probably

emplaced during thrusting at around 500 Ma. Grant (1969) interprets them as alpine serpentinite

emplaced at the base of the Togo Series.

5.8.3 CARBONATITE

An unusual rock interpreted to be a carbonatite occurs at Kpong on the Volta River (Mani, 1978,

Akiti et al., 1972, Bondesen, 1972). The rock is composed of rounded 1 cm diameter plagioclase

clasts set in a matrix of carbonate and biotite. Agyei et al., (1987) dated samples of this rock by

both Rb-Sr and K-Ar methods. The rounded plagioclase clasts gave K-Ar and Rb-Sr dates of 665

± 20 and 975 ± 167 Ma respectively. These are too old for the geologic setting of the Kpong

conglomerate, and the plagioclase clasts must be lower crust or upper mantle xenocrysts (Agyei et

al., 1987). Biotite from the groundmass gave K-Ar and Rb-Sr dates of 545±11 and 572±15 Ma,

respectively. Agyei et al. (1987) concluded that this may be a more meaningful age for the

carbonatite. They interpret the age data as indicating that the carbonatite was emplaced late in the

Pan-African orogeny.

5.8.4 MAFIC DIKES AND SILLS

These represent the youngest volcanic rocks in Ghana. They are mainly gabbro, dolerite,

epidiorite and norite. They cut both the Birimian and Tarkwaian rocks. The dolerites are not

metamorphosed and commonly have intruded parallel to bedding. They are porphyritic containing

plagioclase phenocrysts in a carbonatised groundmass.

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