225 CHAPTER 10. DIAGENETIC FLUIDS IN PALEO- AND MESO-PROTEROZOIC SEDIMENTARY BASINS AND THEIR IMPLICATIONS FOR LONG PROTRACTED FLUID HISTORIES Kurt Kyser 1 , Eric Hiatt 1 , Christophe Renac 2 , Kyle Durocher 1 , Greg Holk 1 and Katja Deckart 3 1 Department of Geological Sciences and Geological Engineering Queen’s University Kingston, ON K7L 3N6 Canada 2 Universite de St. Etienne, Dept. Geologie 23, rue du Docteur Paul Michelon 42023 Saint Etienne Cedex, France 3 Dept. of Geology, University of Chile Casilla 13518 Correo 21 Santiago, Chile THE NATURE OF THE PROTEROZOIC Substantial portions of our continents are made of large and extensive Proterozoic basins (Fig. 10.1). Their spatial relationship to Proterozoic orogens is striking. The best described and studied are those on the North American and Australian continents because many of these are hosts to substantial U, Pb, Zn, Cu, Ag and Au deposits. Descriptions of basins on other continents, including their ages, sedimentology, mineral potential, geologic setting and evolution, or even their names, are lacking, confusing, and frightening. Nevertheless, the coverage of the continents by well-characterized Proterozoic basins is substantial (Fig. 10.1). Preservation of so many Proterozoic basins begs many questions. For example, why should these ancient basins be preserved, many of them nearly as flat-lying now as when they originally formed? What information do they harbor about the evolution of the earth? What circumstances fostered such a plethora of mountains in the first instance, depressions to receive the weathering products of these in the second and products that seem to have survived the ravages of time in the third? What was the nature of the fluid events that produced such vast quantities and concentrations of specific metals? The answer to these requires, first and foremost, an analysis of how the basins, and the fluids therein, changed with time. The Proterozoic era was a remarkable period in earth history (Fig. 10.2). The Paleo- proterozoic, which includes 2500-1600 Ma, is characterized by several substantial orogens associated with the assemblage of the mega- continents of Arctica (Canada, Siberia and parts of Greenland) and Atlantica (Africa and South America). Growth of Arctica occurred during the end of the Paleoproterozoic through the beginning of the Mesoproterozoic (1.7-1.5 Ga) with the accretion of Baltica, North America and East Antarctica into the larger continent of Nena (Rogers, 1996). Among these orogens, the coeval Barramundi and Trans-Hudson orogens (2.0-1.8 Ga) are particularly significant for the formation of the basins to be discussed in this chapter (Fig. 10.1). The end of the Paleo- proterozoic and beginning of the Meso- proterozoic is marked by the general termination of orogens, and a period of relative tectonic quiet for about 500 m.y., during which several large intracratonic basins formed and evolved. Atlantica and other continental blocks would be accreted to Nena during the Grenville event at ca. 1.0 Ga to form the supercontinent Rodinia. Near the termination of the Proterozoic, the earth becomes agitated once again as the mega- continent Rodinia is tectonically dissected into several fragments.
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CHAPTER 10. DIAGENETIC FLUIDS IN PALEO- AND MESO-PROTEROZOICSEDIMENTARY BASINS AND THEIR IMPLICATIONS FOR LONG PROTRACTED FLUIDHISTORIES
Kurt Kyser1, Eric Hiatt1, Christophe Renac2, Kyle Durocher1, Greg Holk1 and Katja Deckart3
1Department of Geological Sciences and Geological EngineeringQueen’s UniversityKingston, ON K7L 3N6 Canada
2Universite de St. Etienne, Dept. Geologie23, rue du Docteur Paul Michelon42023 Saint Etienne Cedex, France
3Dept. of Geology, University of ChileCasilla 13518Correo 21Santiago, Chile
THE NATURE OF THE PROTEROZOICSubstantial portions of our continents
are made of large and extensive Proterozoicbasins (Fig. 10.1). Their spatial relationship toProterozoic orogens is striking. The bestdescribed and studied are those on the NorthAmerican and Australian continents becausemany of these are hosts to substantial U, Pb, Zn,Cu, Ag and Au deposits. Descriptions of basinson other continents, including their ages,sedimentology, mineral potential, geologicsetting and evolution, or even their names, arelacking, confusing, and frightening.Nevertheless, the coverage of the continents bywell-characterized Proterozoic basins issubstantial (Fig. 10.1). Preservation of so manyProterozoic basins begs many questions. Forexample, why should these ancient basins bepreserved, many of them nearly as flat-lyingnow as when they originally formed? Whatinformation do they harbor about the evolutionof the earth? What circumstances fostered sucha plethora of mountains in the first instance,depressions to receive the weathering productsof these in the second and products that seem tohave survived the ravages of time in the third?What was the nature of the fluid events thatproduced such vast quantities and concentrationsof specific metals? The answer to theserequires, first and foremost, an analysis of how
the basins, and the fluids therein, changed withtime.
The Proterozoic era was a remarkableperiod in earth history (Fig. 10.2). The Paleo-proterozoic, which includes 2500-1600 Ma, ischaracterized by several substantial orogensassociated with the assemblage of the mega-continents of Arctica (Canada, Siberia and partsof Greenland) and Atlantica (Africa and SouthAmerica). Growth of Arctica occurred duringthe end of the Paleoproterozoic through thebeginning of the Mesoproterozoic (1.7-1.5 Ga)with the accretion of Baltica, North America andEast Antarctica into the larger continent of Nena(Rogers, 1996). Among these orogens, thecoeval Barramundi and Trans-Hudson orogens(2.0-1.8 Ga) are particularly significant for theformation of the basins to be discussed in thischapter (Fig. 10.1). The end of the Paleo-proterozoic and beginning of the Meso-proterozoic is marked by the general terminationof orogens, and a period of relative tectonicquiet for about 500 m.y., during which severallarge intracratonic basins formed and evolved.Atlantica and other continental blocks would beaccreted to Nena during the Grenville event atca. 1.0 Ga to form the supercontinent Rodinia.Near the termination of the Proterozoic, theearth becomes agitated once again as the mega-continent Rodinia is tectonically dissected intoseveral fragments.
Kyser, T.K., Hiatt, E.E., Renac, C., Durocher, K., Holk, G.J., Deckart, K., 2000, Diagenetic fluids in paleo- and meso-Proterozoic sedimentary basins and their implications for long protracted fluid histories: p. 225-262 in Kyser, K., (ed.) Fluids and Basin Evolution, Mineralogical Association of Canada, Ottawa, Canada, 262 p.
226
The Proterozoic was a critical time forthe evolution of life and global biogeochemical
cycles. Stromatolites were widespread,eukaryotic cells developed, and eventually so
Nabberu
Kimberley
McArthur
Waterburg
Gawler
Hammersley
Ngalia
Tennant Creek
Albany-Frazer (1.8-1.0)
Svecofennian
Lapland-Belmoran(1.3-1.0)
Brioverien
MPMB(1.6-1.5)
Vindyan
Chuddapah
Gourma
El Hank
Gascoyne (1.8-1.4)
Musgrave (1.8-1.0)
Bangemall
Middleback
Umkondo Mt. Isa (1.6-1.4)
Pine Creek (1.8-1.7)
Arunta (2.0-1.0)
Ubendian (2.0-1.8)
Georgetown(1.6-1.0)
Kibaran (1.4-1.2)
YanshanNorth China
Chartai-Bayan Obo
Kunlun
Karelia-Ladoga
TarkwaianFrancevillian
Limpopo (2.8-2.0)
Namaqua-Natal(1.0)
Barramundi(2.0-1.8)
Capricorn (2.1)
Hornby Bay
Elu
Thelon
Purcell& Belt
Athabasca
Trans-Hudson(2.0-1.8)
New Quebec(2.0-1.0 react.)
Mazatzal(1.8-1.6)
Churchil(2.0-1.0 react.)
Sioux
Keweenawan
Apache
Basins
Orogens (and ages)
Borden
Tocantins (1.7-0.5)
Roraima
Ketilidian
Penokean (2.0-1.8)
Grenville (1.2-1.0)
Corregoand
Rio do Ouvo
Figure 10.1. Present distribution of selected Proterozoic basins andorogens for which there are adequate descriptions in the literature.Sources of most data are discussed in the text; additional sources includeStewart (1976), Hunter (1981), Medaris et al. (1983), Condie (1992),Hofmann (1992), Rundqvist and Mitrofanov (1993), Rogers (1996) andXingyuan and Jin (1998).
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did multicelled organisms (Fig. 10.2). Blue-green algae, simple bacteria, and simpleeukaryotic green algae in the oceans producedsignificant amounts of organic matter. As aconsequence, there are deposits of Proterozoichydrocarbons and significant accumulations ofmicrobial matter (Fig. 10.3). Due to the absenceof land plants, soils and terrestrial sedimentationwere very different relative to the modern. As aresult, the very nature of weathering on the
continents would have been radically different.Soils would not have had the binding nor theenhanced chemical weathering actions that arefacilitated by plants. Thus, the alluvial plainenvironment would have been without cohesiveriverbanks preventing streams from developingmeandering systems. Instead, braided streamswould have dominated the terrestrial landscapeforming extensive sand and gravel-covered braidplains. The exposed plains would have been
Timebeforepresent
Evolution oflife and other
importantprocesses
Composition ofatmosphere
Propertiesof ocean
MaTrace
elementsMajorcomp.
Response ofsediments
pH8 9 10
Bioclastic sediments
First massive sulfate deposits?
Onset of abundantred beds
Banded iron formations(+FeCO3, Fe-silicates,
and chert)
Uranium andpyrite as placer
minerals
(Chemicallyprecipitated cherts
and carbonates)
0
Phan
er-
ozoi
c
1000
Prot
eroz
oic
2000
BiomineralizationMetazoa
(multicelled)
First eukaryoticcells,
developmentof aerobicrespiration
Widespreadstromatolites
Oxi
dizi
ngRe
duci
ng
3000
4000
FIirststromatolites
Onset ofphotosynthesis
Prokaryoticbateria
Outgassing ofearly atmos.and oceans
Accretion of earth,losss of H2 and He
to space Qualitativescale
Haliteocean
pH
CaContent
(Qualit.scale)
Soda-ocean
0.1 0.5 1.0
Halite and Soda
?
Arc
hean
CO2
O2
N2
CO2
Major Orogens
Soils become stabilizedby land plants
Possible snow-ball Earth
Rodinia begins to break-up
Grenville Orogeny
A
B
Figure 10.2. Possible evolution of life, atmosphere, ocean and otherproperties throughout earth history. Curves A and B represent rapidchange in O2 content of the atmosphere and gradual change,
respectively. Modified from Einsele (1992).
228
subject to the action of wind without theprotection of plants allowing wind to transportfine-grained sediment probably producing duststorms. As a result, very little mud is found inProterozoic sedimentary successions depositedin fluvial environments.
Minerals that can be used to inferpaleoenvironmental conditions, such as pyriteproduced by bacterially-mediated sulfatereduction, are preserved in sediments, as are“evaporites” in the form of bedded sulfates andcasts of halite crystals. Substantial deposits of
Figure 10.3. Generalized variation in the agedistribution of selected types of sedimentary-hostedmineral deposits (italics) and organic material.Modified from Lambert (1989).
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Mn occur and enormous deposits of banded irongive way to abundant red-bed Cu deposits (Fig.10.2). Due to low oxygen levels in the Archeanand early Paleoproterozoic, uranium wasdeposited in placer deposits, however, due torising oxygen levels, these become less abundantafter the Paleoproterozoic. In the Late Paleo-proterozoic to Mesoproterozoic, formation ofeconomic uranium deposits shifted to thesubsurface of intracratonic basins and resulted inunconformity-type deposits. The disappearanceof detrital uraninite and massive banded irondeposits in conjunction with the appearance ofabundant red beds has been cited by some (e.g.Holland, 1984; Karhu and Holland, 1996) asevidence of rapidly increasing oxygen in theatmosphere (Fig. 10.2).
Reconstruction of the continents duringthe Neoproterozoic (Fig. 10.4) indicates that
many of the large intracratonic basins wereprobably associated with related orogensinitiated during the Paleoproterozoic and sub-sequently reactivated. Much of the reactivationoccurred near the end of the Mesoproterozoicand beginning of the Neoproterozoic during theGrenville event and the subsequent initial break-up of the megacontinent. Break-up finally wasachieved at ca. 900-700 Ma, although paleo-magnetic data suggest it was a long, drawn outprocess. Depending on which part of the“elephant” is researched, a slightly different ageor process for the demise of Rodinia is obtained.
In this chapter we will examine andcompare the fluid history of the most substantialaquifers in three Paleo- to Mesoproterozoicbasins to see what events are recorded by thediagenetic minerals they contain so that theymay be related to Proterozoic environmental and
Grenvillian sutures
Pre-Grenvilliancratons
0o 0o
AustraliaIndia
SiberiaAntarctica
LaurentiaKalahari
CongoAmazonia
BalticaAfricansubcontinent
N
Broken Hill1690
HYC1690
Mount Isa1670
Zawar1700
Athabasca
Thelon
KimberleyMcArthur
PurcellKiggavik
CigarMcArthur River
Figure 10.4. Assemblage of continents at ca. 900 Ma showinglocations of selected Proterozoic ore deposits, intracratonicbasins discussed in this chapter, general direction of movementof continents and Grenvillian sutures (ca. 1000 Ma).
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tectonic events. The three basins include theAthabasca and Thelon basins in Canada and theKombolgie Basin in Australia. These basinscontain unconformity-type uranium deposits andare part of a much larger system of intracratonicbasins. All are flat-lying, have had severalkilometers of sediments, and are comprisedpredominately of clastic sediments in their basalunits and succeeded by marine/terrestrialsediments. The clastic sediments they containpresumably were sourced from significantmountain ranges associated with the Trans-
Hudson Orogen in Canada and the BarramundiOrogen in Australia. How these sediments weredeposited, how they changed during burial, andwhat effects they had on the fluid evolution ofthe basins will be used to track basin evolutionand determine how these basins differ relative totheir more modern counterparts.
THE ATHABASCA BASIN, CANADAThe Athabasca Basin has been the
subject of several studies because of the world-class, unconformity-type uranium deposits it
Sandstone, conglomerated: intraclast-rich sandstonec: sandstoneb: conglomeratea: sandstone, minor conglomerate
Basement, undivided
Proterozoic-Archean
Proterozoic
William River Subgroup (W)
Manitou Falls Fm. (MF)
Uranium orebody (drillhole)
Cluff Lake
Figure 10.5. General geology and location (inset) of the Proterozoic AthabascaBasin, Canada. Also shown are the locations of major unconformity-type uraniumdeposits and the general stratigraphy of the basin.
231
hosts (Fig. 10.5). Indeed, the vast majority ofstudies have focussed on the uranium deposits,which is a myopic approach to take if the goal isto understand the mineralization in the scope ofbasin evolution. Studies centered on thedeposits themselves, by their nature, missimportant questions as to why mineralizationdoes not occur regionally, but is instead limitedto very small geographic areas. Broad-based
regional studies that have considered theevolution of the Athabasca Basin are relativelyrare. Some of these include studies of basin-wide lithostratigraphy by Ramaekers (1990), andthe studies concerning the mineralogical andgeochemical characteristics of rocks within, andaway from, uranium mineralization of Kotzerand Kyser (1992, 1995) and Fayek and Kyser(1997). Isotopic, chemical, microthermometric
N0 100
km
Diabase dikeMajor fault
Black BayFault
Grease R. Fault
Virgin R. - Black Lk.Shear Zone
Cable BayShear
Keefe Lk.Fault
CreeBasin
MirrorBasin
JackfishBasin
PattersonHigh
A
A'
Bsmt
OFOF
LLLL
WPb WPbWPb
WPa WPa
FPMFc+z
Lzl MFc+d
MFa+b
MFc+d
MFa+b
A A'
0 100
km
500
0 m
-500
-1000
OF
LL
WPb
WPa
Lzl
FP
MFd
MFc
MFb
MFa
Bsmt
Otherside
Locker Lk.
WolverinePoint
Lazenby Lk.
Fair Point
Manitou FallsSandstone
Aphebian andArchean metasedimentsand gneisses
mar
ine
sand
stone
s, ph
osph
atic
siltst
ones
and
mud
stone
s
Figure 10.6. Position of sub-basins, major faults and diabase dykes(Mackenzie) in the Athabasca Basin. Also indicated is a crosssection of the basin and the basic stratigraphic units (after Hoeveand Quirt, 1984).
232
and petrologic data indicate that the AthabascaBasin has had a protracted fluid history unlikethat documented for any Phanerozoic sedimen-tary basin. The data clearly show that the majorsandstone aquifers have been affected by wide-spread lateral flow of diagenetic fluids overdistances of hundreds of kilometers and,importantly for mineralization, these fluidmigration paths have been modified by cross-formational fluid flow near active fault zones.
Geologic settingThe Athabasca Basin formed as a series
of NE-SW-oriented sub-basins (Fig. 10.6) at ~1700-1750 Ma (Armstrong and Ramaekers,1985; Kotzer and Kyser, 1992, 1995). Sub-basin formation was controlled by major NE-SW Hudsonian-age faults rooted in underlyingAphebian metasediments and Archean gneisses.The Athabasca Group makes up the sedimentarybasin-fill, and consists of flat-lying, quartz-richsandstone and conglomerate that are interpretedto have been deposited in major river systemsand near shore to shallow-shelf environments(Ramaekers and Dunn, 1977, Ramaekers, 1990).There is a well-developed paleoregolith on thecrystalline basement rocks underlying theAthabasca Group that extends to a depth ofseveral meters. The Hudsonian-age faults aremajor crustal lineaments that cut the AthabascaGroup sandstones that have remainedintermittently active to recent times (Hoeve andQuirt, 1984). The Aphebian metasedimentaryrocks and Archean gneisses that comprise thebasement are members of the Wollaston Domainof the Trans-Hudson Orogen (Lewry andSibbald, 1979, 1980; MacDonald, 1985). TheAphebian metasedimentary rocks unconform-ably overlie the Archean granitoid gneisses andconsist typically of quartz, plagioclase, biotite,cordierite, garnet and tourmaline, with severalanatectic and graphitic layers (Ey et al., 1991;Marlatt et al., 1992). Individual quartzite unitsoccur locally and are generally separated byintervals of garnet-cordierite gneiss (Marlatt etal., 1992).
The Athabasca Group sedimentary rocksand the basement complex are also cut by aseries of northwest-trending mafic dykes, whichwere emplaced along reactivated fractures
during post-Athabasca tectonic activity at 1350-900 Ma (Ey et al., 1991). The dykes are believedto be related to the Mackenzie dyke swarm(Cumming and Kristic, 1992) and represent theonly igneous activity throughout the evolution ofthe basin. The sedimentary fill in the AthabascaBasin is presently 1 to 2 kilometers thick (Fig.10.6). Temperature estimates derived from fluidinclusions, however, indicate that thesedimentary sequence may have reached athickness of ca. 5 km during the peak diagenesis(Pagel et al., 1980).
Sedimentology and StratigraphyThe basal sequence of the Athabasca
Group (Manitou Falls and Fair Point formations)consists of permeable coarse- to fine-grained,and hematite-rich conglomerates and sandstones(Fig. 10.6) with paleocurrent directionspredominately from the east. In the ManitouFalls Formation, hematite is disseminated alongthin stratigraphic horizons, indicating localoxidation of heavy mineral suites. The basalportion of the Manitou Falls Formation iscomposed of coarse-grained conglomerates thathave attributes that suggest deposition in high-energy, braided streams, and, to a lesser degree,alluvial fan settings. This lower coarse-grainedinterval is overlain by finer-grained sandstoneswith minor conglomerate that is marked byabundant trough cross-bedding and relativelyrare siltstone layers that average three to fivecentimeters thick. This middle intervalrepresents deposition in more distal braidedstream environments, and due to the much bettersorting, has coarse-grained textures, the nearabsence of mud matrix, and very high hydraulicconductivities. The sandstones that make up theupper third to half of the Manitou FallsFormation consist of medium grained sandstonewith abundant ripple marks, rare thin mudstonelayers that average one to three centimetersthick, and mud rip-up clasts. These sandstonesare consistent with deposition in lower energydistal braided stream systems to possibly braiddeltas. The detrital material that makes up theManitou Falls sandstones is composed of almost100% of well-rounded quartz with very minormuscovite and rare heavy minerals, such aszircon and apatite. The absence of feldspar and
233
rarity of any other minerals further suggestsrelatively long transport and perhaps intenseweathering in the source area. Based onpaleocurrents, the source area for the sedimentsthat make up the Manitou Falls Formation is tothe east (Ramaekers, 1983). Trackingstratigraphic units in drill core reveals that thereare two unconformity-bound stratigraphicsequences in the eastern portion of theAthabasca Basin. These are marked first bycoarse-grained units that appear in associationwith topography on the basal unconformity. Assuggested by Ramaekers (1981), the AthabascaBasin can be divided into sub-basins (Fig. 10.6).These early coarse-grained basal units probablymark the time when subdivision of the overallAthabasca was at its greatest with alluvial fansforming adjacent to paleo-highs, and high-energy braided streams flowing in the deeperparts of the sub-basins. After this initial stage ofbasin formation, the source area shifted furtherto the east and the braided streams flowing intothe Athabasca sub-basins began filling themwith sediment. Sedimentation and basinsubsidence continued, resulting in a rise in baselevel, so that by the time the uppermost portionof the Manitou Falls Formation was deposited,the braided stream systems had become lowenergy and distal. This relationship is reflectedin stratigraphic cross sections that indicate thatthe upper portion of the Manitou Falls onlapsonto the basal unconformity in the easternmostparts of the basin.
The sandstones of the Manitou FallsFormation are overlain by a succession of lesspermeable marine sandstones, phosphaticsiltstones, and phosphatic mudstones (theLazenby Lake Formation and Wolverine PointFormation, respectively), which are in turnoverlain first by sandstones (Locker Lake,Otherside and Tuma Lake Formations, and theWilliam River Subgroup), then by shales(Douglas Formation), and finally bystromatolitic dolomite (Carswell Formation).Cumming and Kristic (1992) reported U-Pb agesof 1650-1700 Ma for fluorapatite from theAthabasca Basin, which they interpreted as aminimum age of deposition in the basin.
Paragenesis and fluid evolution in theAthabasca Basin
A detailed paragenesis of clay andsilicate minerals in the basal clastic units of theAthabasca Group was developed for the entirebasin by Kotzer and Kyser (1992, 1995), andrefined by Fayek and Kyser (1997). Theseparagenetic relationships are based on usingpetrographic information, stable and radiogenicisotope compositions, and analyses of fluidinclusions in altered sandstones and meta-sedimentary rocks proximal to, and distal from,the uranium deposits. A revised paragenesisbased mainly on these studies is shown in Figure10.7. This paragenesis is applicable to the basalunits throughout the basin.
The basin-filling sandstones presentlyconsist of ca. 95% detrital quartz (+ minor heavyminerals) and 5% secondary minerals byvolume. Fluid inclusions in the detrital quartzare aqueous or CO2+H2O and most have highhomogenization temperatures (ca 400oC),consistent with derivation from relatively high-grade metamorphic rocks not unlike those of thebasement and adjacent terrain in the Trans-Hudson Orogen. Trace amounts of detritalfluorapatite and zircon occur mainly within thedetrital quartz grains and rarely as detritalinterstitial minerals, indicating that these heavyminerals may have been removed by fluidsflowing through the sandstone. A few Pb-Pbages determined from analysis of detrital zirconspreserved primarily in the quartz cements ofwell-cemented sandstones range from between2400 and 1850 Ma.
Early diagenesisEarly diagenesis of the Athabasca Basin
produced quartz overgrowths (Q1), a crandallite-group phosphate mineral (P1) and hematite (H1)on detrital quartz in the Manitou Falls Formation(Kotzer and Kyser, 1992, 1995). The quartzovergrowths have fluid inclusions with homo-genization temperatures of 150-170oC, between10 and 25 wt% NaCl, δD values of -50 ‰, andcalculated δ18O values from -8 to -2 ‰ (Fig.10.8). The isotopic compositions arecharacteristic of low-latitude, near coastalmeteoric waters (Longstaffe, this volume), but
234
the high salinities suggest that these watersinteracted with evaporites and minerals in thebasin; the high temperatures indicate burialdepths of 3-4 km. The crandallite-groupdiagenetic mineral present has a compositionconsistent with solid solution between goyasiteand crandallite and is F-poor (Fayek and Kyser,1997). An abundance of the crandallite-groupmineral and authigenic xenotime overgrowths onaltered detrital zircon is associated with thehydrothermal clay and silicate minerals thatcomprise the alteration halo around uraniumdeposits in the basin. Kotzer et al. (1992)suggested that the hematite (H1) throughout theAthabasca corresponds to a paleomagneticdirection (A-type) coincident with an age of1600-1750 Ma.
Peak diagenesisA basin-wide mineral assemblage
consisting of variable proportions of 2M illite(I1) and dickite (K1) was produced by thealteration of detrital silicates during peakdiagenesis of the Athabasca Group sediments.Thus, in the Athabasca Basin, the predominantdiagenetic clay assemblage is illite + kaolin.Areas proximal to faults, fractures and the oredeposits in the sandstones are hydrothermallyaltered to illite (I1) and kaolinite (K2)intergrown with euhedral quartz (Q2) anddravite (T1), Al-Mg-bearing chlorite (C2) andhematite (H2), with varying amounts ofuraninite (U1, U2) (Fig. 10.7). This hydro-thermal alteration is largely contemporaneouswith the later stages of basin-wide, peak-diagenetic clay-mineral assemblage and
C1, C2
I1, K1
Chlorite basement
Mineral Stage Hydrothermalalteration
Late meteoricevents
Temp(°C) Fluid
Quartz overgrowth
Hematite (A-mag.)
Hematite (B-mag.)
Quartz+ Dravite alteration halo
Uranium
Cu-Ni-As-sulphides
Hematite (C-mag.)
Dravite fracturesKaolinite pervasive
Pyrite fractures
Uranium fractures
Siderite vugs
Kaolinite fractures
Fluorapatite
Crandallite P1
Xenotime
Rutile
Age (Ga)1.7 1.4 1.0 0.4
Athabasca Basin Mineral Paragenesis
150-170
180-240
<50
pore-fluids(10-25 wt% NaCl)
basementfluids (reducing)
mid-latitudebasinal brine
(oxidizing; 30-33 wt% NaCl)
high-latitudemeteroric waters(<5 wt% NaCl)
Q1
H1
H2
Q2,T1
U1,U2
A2X1
S1
R1H3
T2
K2
S2
U3S
K3
U1 U2
Illite + Dickite diagenetic
Figure 10.7. Paragenesis of minerals in the Athabasca Basin as a function ofage. Also shown are the temperatures and general characteristics of fluids inthe basin. Data from Kotzer and Kyser (1995), Fayek and Kyser (1997) andreferences discussed in the text.
235
postdates the early overgrowths of quartz (Q1).In metasedimentary rocks and in sandstonesproximal to the unconformity throughout thebasin, rosettes of trioctahedral chlorite(clinochlore, C1) infill pore spaces. However,varying amounts of dioctahedral chlorite(sudoite, C2) occur in hydrothermally alteredsandstones and metasedimentary rocks of thebasement near unconformity-type uraniumdeposits and fault zones that intersect theunconformity (Hoeve and Quirt, 1984; Wilsonand Kyser, 1987; Kotzer and Kyser, 1995).Sudoite also occurs in fault zones up to 300 mabove the basal unconformity, suggesting thatthese faults allowed fluid associated withuranium mineralization access to much higherstratigraphic levels (Kotzer and Kyser, 1995).
The isotopic compositions of peakdiagenetic quartz (Q2), hematite (H2), illite (I1),dravite (T1) and dickite (K1) and fluidinclusions, in conjunction with inclusioncharacteristics in diagenetic quartz (Q2), can beused to indicate the temperature, isotopic andchemical composition, and age of the fluid thatwould have equilibrated with these minerals(Fig. 10.8). The δD values of dickite, 2M illite,dravite and fluid from inclusions in the euhedralquartz indicate formation at ca. 240oC from abasinal fluid that had δD values near -60 ‰,similar to the fluid that formed the earlier quartzovergrowths. The δ18O values, phase relationsand fluid inclusion characteristics indicate thatthis fluid underwent salinity, temperature and18O increases relative to the fluid present whenthe quartz overgrowths formed from 15 to 30
-20 -10 0
Tem
pera
ture
(oC)
δD (o
/oo)
δ18O (o/oo)
Flui
d In
clus
ion
Salin
ity (w
t.% N
aCl)
Age
(Ma)
500
1500300
100
-40
-120 10
Athabasca Basin Fluids
1000
late
late
30
late
-20 -10 0
early
early
early
early
late
peakbasement
peakbasinal andbasement
peak
MW
L
seawater
peakbasinal
peakbasement
peakbasinal
Figure 10.8. Relation among ages, temperatures, salinities, δD and δ18Ovalues of diagenetic basinal and basement fluids recorded in fluidinclusions and diagenetic mineral assemblages in the Athabasca Basin.Minerals involved with each stage of diagenesis shown in Fig. 10.7.MWL refers to the meteoric water line, bsmt indicates fluids from thebasement and seawater indicates values of seawater. See text for detail.
236
wt% NaCl, 120 to 240oC, and -5 to +3 ‰,respectively. These changes resulted fromincreasing burial depth and sustained water-rockinteractions with the basinal sediments oversubstantial time periods. The lack of change inthe δD values indicates that the basin-wide fluidin the basal units was not diluted with, orreplaced by, other fluids. The 18O/16O and D/Hratios of sudoite and clinochlore inmetasedimentary basement rocks within thealteration haloes indicate formation from abasement-derived fluid with a similartemperature, but different δ18O and δD values of+ 4 and -30 ‰, respectively. Initial Sr isotopiccompositions of fluids in the basin and basementrocks, reflected by the 87Sr/86Sr ratios of chlorite,illite and dravite, are consistent with mixing oftwo isotopically-distinct fluids in the vicinity offaults as the process by which uranium wasprecipitated, similar to that suggested by oxygenand hydrogen isotope systematics in the claysand silicate minerals (Kotzer and Kyser, 1995).
Rb-Sr and Ar-Ar isotope systemsindicate ca. 1500 Ma for the oldest ages of the2M illites (Bray et al., 1987; Wilson and Kyser.,1987; Kotzer and Kyser, 1995), which iscoincident with the paleomagnetic age of 1450-1600 for peak diagenetic hematite (Kotzer et al.1992) and the oldest ages for primary uraninitein the deposits (e.g. Fayek and Kyser, 1997).Ages for illite formed during peak-diagenesisvary between 1500 and 900 Ma, and indicatefluids, similar in temperature and composition tothose at 1500 Ma, were active in the basin atleast until 900 Ma (Fig. 10.7). Ages for some2M illites associated with later kaolinite anddravite (K2, T2) are 900-1000 Ma, which is alsopartly coincident with another distinctpaleomagnetic age for later hematite (H3; Fig.10.7) and recrystallization ages of uraninite(U2). This age is found in the U-Pb systematicsin uraninites as either a lower intercept or asyounger concordant uraninites (e.g. Kotzer andKyser, 1993) and marks the end of the Grenvilleorogeny (Fig. 10.4) and the beginning of thedemise of Rodinia.
Late fluid eventsLate kaolinites (K3) in reactivated fault
zones have isotopic compositions indicative oflow-temperature (25-50oC) fluids having δ18Oand δD values of -16 and -130 ‰ (Fig. 10.8),respectively, and are similar to recent meteoricwaters in the area (Fig. 10.7). Indeed, manyuraninites are highly discordant and have lowerintercept ages of less than 300 Ma (e.g. Kotzerand Kyser, 1993). Illite associated with theuraninites have low δD values (i.e. <-100 ‰),and also have K-Ar ages of ca..300 Ma. Latesiderite (S) precipitated in vugs typically hasfluid inclusions with homogenizationtemperatures <50oC and low salinity fluids(Fig.10.7). During the late Phanerozoic andagain in the Tertiary, the Athabasca Basin wasuplifted in response to distant orogens. Thisperiod, and later continental glaciation events,would have provided a change in the hydrologicregime of the basin and facilitated flow of fluidsalong highly permeable fractures.
Some illites, dickites, dravites andkaolinites have “normal” δ18O values, but lowδD values that suggest they formed frommeteoric waters at low-latitudes, but haveexperienced varying degrees of hydrogenisotope exchange with relatively modernmeteoric waters. In the vicinity of re-activatedfaults (some of these are associated with thestructures that host the uranium deposits), illite,sudoite, and dravite have δD values that vary by100 ‰ without concurrent changes to their δ18Ovalues, indicating preferential hydrogen isotopeexchange with relatively modern meteoricwaters at low temperatures and under conditionsof high integrated water/rock ratios. Illites thathave some of the lowest δD values are as youngas 300 Ma and indicate late-stage resetting oftheir radiogenic isotope systems. These low δDvalues in illites were attributed to radiolysisreactions on water from uranium by Halter et al.(1989). Such a process is unlikely, however,because (1) the expected changes in δ18O valuesfrom such a process are absent in the clays, (2)those minerals with aberrantly low δD valuesalso occur far from mineralization and are infractures where the integrated water/rock ratioshave been high (Kotzer and Kyser, 1991), (3)
237
this effect is absent in all other uranium depositsregardless of their location and (4) thepreferential exchange of hydrogen isotopes hasbeen documented in other hydrous mineralsfrom a variety of geologic settings (Bird andChivas, 1988; Longstaffe and Ayalon, 1990;Kyser et al., 1999; Longstaffe, this volume).The significance of the late kaolinites (K3) andminerals that have undergone preferentialhydrogen isotope exchange with relativelymodern waters is that late influx of meteoricwaters occurred, but primarily along faultswhere permeabilities are enhanced.
Relation between orogeny and formation ofthe Athabasca Basin
The Athabasca is an intracratonic basinwith no direct evidence of rift-related igneousactivity. Volcanic units occur in the MartinGroup below the Athabasca Group, but there isan erosional unconformity that represents asignificant break in time between them. By theinferences discussed above, an upper age for thebasin is 1780 Ma and a minimum age is 1600Ma. Given that the likely source for the basin-fill came from adjacent rocks to the east anduplifted during the Trans-Hudson Orogen, thetemperature-time-pressure history of this orogenmay help constrain the age of the basin. Peakmetamorphism throughout the orogen is 1800-1820 Ma, so this age represents a minimum fordeepest burial of the roots of the orogen (andmaximum uplift). Ar-Ar ages of metamorphicand plutonic rocks throughout the orogen andpresently exposed at the surface are consistentwith a relatively rapid uplift beginning at ca.1750 Ma (Fig. 10.9). Present exposuresrepresent the core of the orogen and the roots ofthe mountain ranges that must have formed.Uplift was most likely coincident with rapidexhumation of the overlying rocks that werebeing transported to the basin. Therefore, majorsedimentation in the basin was ca.1750 Mabased on the thermal and temporal evolution ofthe most likely source terrains.
Multiple fluid events, involvingisotopically and chemically distinct fluids thatmigrated laterally for considerable distances andalong fault zones, produced a parageneticallyidentifiable assemblage of clay, silicate and
oxide minerals in the basin and basement rocks.Isotopic, chemical, microthermometric andpetrologic data indicate that the major sandstoneaquifers in the Athabasca Basin, which weremost pronounced in the poorly-sorted basalunits, have been affected by widespread lateralflow of diagenetic fluids over distances ofhundreds of kilometers. These fluids appear tohave been resident in the basinal units attemperatures in excess of 200oC for at least 600my, but reacted with minerals in the basin duringflow in response to orogenic events such asintrusion of the Mackenzie dykes and the break-up of Rodinia. These fluid migration pathsfollowed stratigraphic units and flowed up dipfrom the basin center. The stratigraphic path-ways, however, were modified by crossform-ational fluid flow near active fault zones untilfault zones became the exclusive pathways.
A standard ancient basin?Although the fluid history of the
Athabasca Basin is not complete because otherlithologies in the basin have yet to be analyzedin the same detail as the basal units, the fluidevents recorded in the basin have a duration thatis longer than most other basins are old! Aswith most basins, the first major diageneticevents recorded in the clastic sediments of theAthabasca Basin are early hematite and quartzovergrowths. However, the apparent character ofthe basin fill is different than most modernbasins where detrital smectites and plagioclasegenerally result in authigenic illite, feldspar andcarbonate as the main diagenetic phases ratherthan the illite-kaolin (without carbonate-seeChapter 4) assemblage that characterizes theAthabasca Basin.
Basin straddles the border between Nunavut andthe Northwest Territories of Canada (Fig.10.10). It is a potential analog of the AthabascaBasin in Canada and the Kombolgie sub-basin ofthe McArthur Basin in Australia, both in termsof its probable age of formation and itssedimentological history. Also, the ThelonBasin is host to two areas of uraniummineralization (Fig. 10.10). Until recently
238
(Renac and Kyser, 2000), the timing andcharacter of fluid events in the Thelon werepoorly constrained and the similarities between
it and the aforementioned Proterozoic basinswere drawn primarily on sedimentological ratherthan diagenetic features.
8
6
4
2
1840 164016801800 1760 1720
800
600
400
200
1840 164016801800 1760 1720
peak
met
amor
phism
peak
met
amor
phism
Tem
pera
ture
(°C)
Time (Ma)
Pres
sure
(kba
r)Ar-Ar hornblende
Ar-Ar biotiteand muscovite
Figure 10.9. Temperature-time and Pressure-time evolution of rocks fromthe major lithostructural domains of the Trans-Hudson Orogen (THO)proximal to the Athabasca Basin. Sediments for the basal units in thebasin were derived from the east, most likely from the uplifts rooted bythese domains. Ages are deduced from Ar-Ar ages of hydrous mineralshaving different closure temperatures and temperatures and pressures arefrom mineral assemblages. The relatively rapid uplift at ca. 1750 Ma mostlikely represents the upper limit of the age of sedimentation in theAthabasca Basin.
239
Geologic settingThe Thelon Basin (Fig. 10.10) is located
between the Slave and Churchill geologicalprovinces. Along with the Athabasca Basin, it ispart of a series of spatially and temporallyrelated Paleo- to Mesoproterozoic intracratonicbasins floored by Archean and Paleoproterozoicbasement rocks. The Baker and Hornby Baybasins are also in Nunavut, both of which areremnants of larger basins. All of these basinsformed following the Trans-Hudson Orogen andconsist of thick sequences of mature quartzsandstone, conglomerate, siltstone, and shale(e.g. Athabasca and Barrensland Groups) that lie
unconformably on paleoregoliths on Archeanand Paleoproterozoic crystalline basement rocks.The eastern portion of the Thelon Basin, whichwill be referred to as the Thelon sub-basin (Fig.10.11), is the focus of this study. Descriptionsof the Thelon Formation in the eastern andwestern sub-basins are similar (e.g. Gall et al.,1992), implying similar diagenetic histories.
Basement rocks in the Thelon sub-basininclude a series of Archean metapelites andgneisses of the Woodburn Lake Group,Aphebian metapsammites to metapelites of theAmer Group, and Late Aphebian sediments andvolcanics (including the Pitz Formationvolcanics) of the Wharton Group (Fig. 10.11).
Figure 10.10. Various lithostructural and selected geologic features around the ThelonBasin. The figure also shows the location of the Thelon relative to the Athabasca Basinand location of unconformity-type uranium deposits and prospects in the area.
240
The overlying Helikian Thelon Formationbelongs to the Barrensland Group, is composedof conglomerate and coarse-grained sandstonesthat are up to 2 km thick and was deposited inbraided fluvial to near-shore marineenvironments (Donaldson, 1973; Gall et al.,1992). As with other Paleoproterozoic basins, apaleoregolith, consisting of a paleosoil horizonvariably preserved on the sub-Thelon Formationunconformity, marks the boundary between theThelon Formation and basement rocks (Gall,1994). The regolith and basal units of theThelon Formation contain cements of possiblediagenetic fluorapatite minerals with U-Pb agesof 1720 to 1760 Ma; these are consideredminimum ages for the onset of ThelonFormation sedimentation (Miller, 1995). Themaximum age for formation of the basin isconstrained by the age of emplacement offluorite-bearing granites into the Amer Group atca. 1753 Ma (Miller, 1995). Later eventspreserved in the eastern portion of the basininclude Mackenzie diabase dikes at ca. 1270 Ma
(LeCheminant and Heaman, 1989), deposition ofOrdovician limestones, and Quaternary glacialdeposits.
Sedimentology and StratigraphyThe unmetamorphosed and flat-lying
Thelon Formation reaches a thickness of 1 km inthe eastern sub-basin, and is composed of thick(meters to tens of meters), poorly sorted, troughcross-bedded conglomerate and coarse-grainedsandstone units, and to a lesser extent, wellsorted, medium to coarse-grained sandstoneunits. Thin, clay-rich, fine-grained sandstonesand siltstones punctuate the coarser-grainedlithofacies. The basal conglomerates containabundant clasts of the underlying Pitz Formationvolcanic rocks and clasts of the older Pitz andAmarook Formation sandstone (Rainbird andHadlari, 2000).
The Thelon Formation consists of fourbroad lithofacies types. The first is a coarse-grained trough cross-bedded, sublithic areniteand conglomerate lithofacies. This is composed
0 100
km
Thelon R.
Baker L.Kiggavik
ProterozoicBarrensland Group
Thelon Formation
Wharton Gp. and Baker Lk. Gp.
Amer Gp.
Metasedimentary rocks
Plutonic, volcanic andsedimentary rocks
Archean-ProterozoicUndivided metamorphosedrocks
Uranium orebody
66o
62o104o 99o
Boomerang
Back R.
Duba
wnt
Lake
Figure 10.11. Location of the Thelon Basin in Canada (inset) and general geologyand stratigraphy around the basin. The locations of the Kiggavik and Boomeranguranium deposits are shown. Modified from Gall et al. (1992).
241
of pebbly and cobbly coarse-grained sandstoneand conglomerate that is very poorly-sorted,white to medium gray in color, and containsabundant well rounded quartzite pebbles andcobbles that sometimes make up >50% of therock. The gravel fraction of this lithofacies alsocontains minor volcanics from the underlyingPitz Formation, sandstone also from the PitzFormation and the Amarook Formation, andmetapelite clasts from the underlying AmerGroup. The coarse-grained units are marked byabundant trough cross-bedding, scour surfaces,and asymmetrical ripple marks. This coarse-grained lithofacies suggests deposition in high-energy braided fluvial systems, and is locatedboth at the base of the basal unit and at the basalsequences within the Thelon over intra-formational unconformities. Fluvial paleocurrentdirections are predominantly west to northwest-directed. Locally, this lithofacies contains lithicpebble-cobble conglomerate with angular clastsof metapelite and a muddy to sandy matrixsuggestive of alluvial fan deposition.
The second lithofacies is composed ofpoorly-sorted subarkosic arenite that is coarse tofine-grained, contains 6-10% white clay grainsthat are assumed to represent replacement oforiginal feldspar grains, and minor lithicfragments (primarily volcanic and quartzite).This lithofacies is composed of beds that aregenerally massive and thickly bedded. Bedsfine-upward stratigraphically and are pebbly attheir base but grade into fine-grained sand nearthe top; these units are often capped by clay-rich, red iron-stained fine-grained sandstone tosiltstone horizons. This lithofacies is interpretedto represent low-energy braided stream bar tosheet channel and braid plain deposition.
The third lithofacies is composed ofthinly-bedded, medium to coarse-grained, wellsorted quartz arenite with large-scale, low-anglecross-bedding and abundant wave and currentripple marks. This lithofacies is most commonin the western and northern parts of the sub-basin and is interpreted to represent depositionin upper shoreface settings.
The fourth, and most distinctivelithofacies in the Thelon is made up of thinly-bedded, medium-grained, well sorted quartzarenite with large-scale, high-angle wedge cross-
bedding (foresets 2 to 3 meters long that dip 20-25°). This lithofacies is spatially limited andinterpreted to represent aeolian deposition.
Three third-order sequences areidentified in the Thelon Formation (Hiatt et al.,1999). Systems tracts can be defined usinglithofacies analysis and distinct breaks in fining-upward cycle thickness. Fining-upward cyclethickness in the Thelon sandstone varies in asystematic way that reflects changes inaccommodation space through time. Intervalsthat record accommodation space minimums arecomposed of conglomerate and coarse-grainedlithic arenite with abundant large-scale troughcross-bedding and relatively thin fining-upwardcycles. These intervals mark periods in whichmost sand is transported into the basin centerand are interpreted as lowstand system tracts.Transgressive surfaces are defined by abruptincreases in fining-upward cycle thickness.Above these surfaces rapid creation ofaccommodation space produces cycles thataverage 9 meters thick, are laterally continuousand punctuated by weakly developed paleosoilhorizons, and are interpreted as transgressivesystem tract deposits. Intervals with fining-upward cycles of intermediate thickness andlaterally continuous beds of sandstone markedby sedimentary structures and fabricscharacteristic of upper shoreface and aeoliandeposition are interpreted to represent highstandsystem tracts. Recognition of accommodationspace relationships and multiple intervals ofbraided fluvial, shoreface, and aeoliansedimentation reveal a complex and dynamicdepositional history for the Thelon Basin-ahistory much more dynamic than that of theAthabasca Basin.
The Thelon Formation in the westernportion of the basin is capped by thin basalticflows of the Kuungmi (or Sanctuary) Formation.Dolomites of the Helikian Lookout PointFormation stratigraphically overlie thesevolcanic rocks and contain evidence of marginalmarine and evaporative conditions (Gall et al.,1992). The Kuungmi Formation flows attest tolate igneous activity during basin evolution, andthe Lookout Point Formation to evaporitic brinesthat may have charged the early Thelon aquifers,
242
and later circulated and diagenetically alteredthe rocks of the Thelon Formation.
Paragenesis and fluid evolution in the ThelonBasin
Petrographic examination reveals acomplex history of post-sedimentation eventsrecorded in the Thelon Formation (Fig. 10.12).Detrital phases preserved include rare clasts ofmetamorphic rocks, occasional zircon andmuscovite grains, and fluorapatite clasts (P0-thisphase is interpreted by some as authigenic, butits fluorine-rich chemistry and petrographicrelations with truly detrital phases suggestotherwise). The detrital quartz, which normallycomprises >95% of the Thelon Formation,occasionally contains inclusions of apatite,rutile, and zircon. In the southern portion of thesub-basin, the basal-most unit contains detritalmicrocline (F0) that is still preserved as a resultof early extensive quartz cementation (Fig.10.12). The regolith still retains kaolinite (K1)from original weathering.
Early DiagenesisIn units that had very high initial
hydraulic conductivities, such as aeolian sands,the earliest diagenesis is expressed by a phase ofquartz cement that forms isopachous rims of finecrystals. Cements of this morphology arecharacteristic of phreatic zone diageneticenvironments. Although not wide-spread, thiscement suggests that porosity and permeabilitynetworks in some units were being modifiedsoon after deposition. In most lithologies,however, early diagenesis is commonly manifestby fine veneers of reddish-brown or black ironoxides (H1) and syntaxial, syn- and post-compaction quartz overgrowths (Q1) on detritalquartz grains. UV optical microscopy reveals thepresence of detrital feldspar ghosts in the presentpore space in the sandstones, although thesespaces are now filled with quartz cement andillite. Local variations include areas that lackquartz overgrowths but display specularhematite (100 µm diameter, H2) and associatedneoformed hydroxy-phosphate cement of ca. 50
Dolomite (D1)
1450 1250 105016501720
–100 °C
Detrital Quartz (Q0)
Detrital Feldspar (F0)
Detrital Mica
Hematite (H1)
Quartz cement (Q1-Q2)
Fluorophosphates (P0)
Illite (I1-I2)
Rhombic K-Feldspar (F)
Al-Mg chlorite (C)
Hydroxy-phosphate (P1)Kaolinite regolith (K1)
–100 °C
EarlyDiagenesis
PeakDiagenesis
LateDiagenesis
PetrologicStages
Paragenesis of Minerals in the Thelon Basin
50 to 100 °C
130 °C
< 200 °Cca. 200 °C
Time (Ma)
–100 °C
Figure 10.12. Paragenetic sequence of minerals for the Thelon Formation forearly, peak, and late diagenesis. Temperatures of various events are based onfluid inclusion or crystallinity measurements and ages on U-Pb, Ar-Ar, orinterpolation (see text for discussion). Modified from Renac and Kyser (2000).
243
µm diameter (P1). Small, millimeter-widequartz veins containing euhedral quartz (Q2)along the margins and later minerals in thecenter occur locally in the basal ThelonFormation. Early diagenesis resulted insyntaxial quartz (Q1) on detrital quartz (Q0) anddetrital feldspar, and later quartz veining (Q2), at100-160oC from NaCl brines having ca. 17 wt%NaCl (Fig. 10.13). The δ18O value of this brinewas ca. 0 ‰, and it was distributed throughoutthe Thelon sub-basin. Quartz cementation of thistype is most pronounced in basal and fine-grained sequences throughout the Thelon Basin,and these may have acted as regional aquitardsduring later fluid circulation.
Sedimentation in the basin started priorto 1720 Ma, which is the age of early authigenicphosphate cement (Miller et al., 1989), and after1753 Ma, the youngest age for the last magmaticevent in the basement. Basin sedimentation,therefore, began during the Paleoproterozoic,
although, as discussed below, peak diagenesisoccurred dominantly during the Mesopro-terozoic at 1400-1650 Ma (Fig. 10.12).
Peak Diagenesis A period of silica dissolution occurred
prior to precipitation of illite (I1 and I2) in porespaces during peak diagenesis (Fig. 10.12).Dissolution most likely occurred in response toan increase in temperature of the pore fluids asburial proceeded. Although dissolution of over-growths is present in other Proterozoic basins, itis more wide-spread and substantial in theThelon Basin.
Peak diagenesis is manifest as illitecrystals (I1 and I2) distributed radially aroundquartz grains or remnant quartz overgrowths.Throughout the Thelon Formation, the basalunits have only 2M1 illite, consistent with fluidtemperatures near 200oC, whereas some of thestratigraphically higher units of the Thelon
-20 -10 0
Tem
pera
ture
(oC)
δD (o
/oo )
δ18O (o/oo)
Flui
d In
clus
ion
Salin
ity (w
t.% N
aCl)
Age
(Ma)
500
1500300
100
-40
-120 10
Thelon Basin Fluids
1000
latedolo
30
-20 -10 0
early
peak
fsp/chl
late
peak
peak
earlyMW
L
late(?)
early
early(?)
late(?)fsp/chl
fsp/chl
peak(?)
seawater
fsp/chl(?)
Figure 10.13. Relation among measured and inferred (question marks) ages,temperatures, salinities, δD and δ18O values of diagenetic fluids recorded influid inclusions and diagenetic mineral assemblages in the Thelon Basin.Data from Renac and Kyser (2000).
244
Formation have mixtures of 2M1 and 1Mpolytypes of illite. Ar-Ar total fusion ages ofillite produced during peak diagenesis from thebasal units (I1) range from 1516 to 1690 Ma(Figs. 10.12 and 10.13). In contrast, those fromstratigraphically higher areas from the centralportion (I2) have younger ages of ca. 1300 Ma.The separation in Ar-Ar ages and relativestratigraphic position of these illites suggeststhat they formed at different times. In addition,there was either an evolution in the δ18O valuesof these fluids from ca. 9 ‰ for illite (I1) in thebasal Thelon Formation to ca. 3.9 ‰ for illite(I2) in the more permeable upper stratigraphicunits, or a decrease in temperature of ca. 50oC(Fig. 10.13). However, δD values of the fluidswere relatively constant at ca. -50 +/- 15 ‰,which are values not unlike those of the low-latitude meteoric waters in the Athabasca Basin.The substantially higher δ18O values of the peakdiagenetic fluids relative to those for the quartzovergrowths imply that the peak diageneticfluids represent 18O-enrichment as a result ofmore extensive evaporation or water-rockinteraction, the latter during evolution of thebasin.
Late DiagenesisL a t e diagenetic f ea tu res a re
characterized by widespread rhombic K-feldsparcement (adularia) (F) and paragenetically laterAl-Mg chlorite (C) in the uppermost sandstonesof the central Thelon sub-basin (Fig. 10.12).The K-feldspar is poorly-ordered, consistentwith temperatures of precipitation of <150oC(Kastner and Siever, 1979). The K-feldsparshave total fusion Ar-Ar ages of ca. 1000 Ma(Figs. 10.12 and 10.13), substantially youngerthan the illite. Fluids related to the K-feldsparhad high salinities of 21 wt% NaCl and δ18Ovalues of -3 to 1 ‰, whereas those for thechlorite had much higher δ18O values ca. 5 ‰and δD values of -30 ‰ (Fig 10.13). Themineralogy, high salinities and heavy isotopiccompositions of these authigenic minerals areconsistent with an evaporitic source for thefluids, most likely related to evaporiticconditions suggested by the sedimentologic
evidence from the overlying Lookout PointFormation in the central part of the basin.
The youngest fluid event preserved inthe Thelon Basin is that associated with theprecipitation of minor dolomite cement (D) inreactivated quartz veins (Q2) in the basal units.Although the exact age is uncertain, the fluidwas ca. 100oC, with low salinities between 1 and5 wt% NaCl equiv., and low δ18O values of -8 to-4.5 ‰ (Fig. 10.13). The distinct occurrence,salinity, and composition of this fluid imply thatenough time had passed to totally expel thebrines that had formed the chlorite, even alongfractures. The dolomite represents the last ofseven distinct fluid events preserved in theauthigenic minerals of the Thelon Basin.
How different were the fluids in the Thelonfrom those in the Athabasca Basin?
The Athabasca Basin hosts world-classunconformity-type uranium deposits that formedas a result of extended diagenetic andhydrothermal fluid histories. It is spatially andtemporally related to the Thelon Basin, but largeuranium deposits in the Thelon Basin have yet tobe discovered. Despite similarities in thesedimentological and evolutionary record ofthese two basins (e.g. Miller, 1995), they havesome differences in their diagenetic historiesthat may have implications for their economicmineral potential.
Although the timing of sedimentation inthe Athabasca Basin is less certain than in theThelon, the early diagenetic history in bothbasins is virtually the same (Figs. 10.7 and10.13). Both begin with detrital quartz coatedwith early diagenetic hematite. The δ18O valuesof ca. -3 ‰ for the early hematite in theAthabasca Basin (Kotzer et al., 1992) areidentical to those for the Thelon Basin, implyingthat both the timing and isotopic composition ofthese fluids were similar. Both basins havelimited remnants of basal units with isopachousquartz cements, although aeolian units in theThelon Basin also have these. The hematite inboth basins is followed by quartz overgrowths(Q1) on detrital quartz that started early in theburial history of these sediments. In the ThelonBasin, there is evidence for detrital feldspar
245
grains at this time, but there is no such evidencefor their presence in Athabasca sediments. Thefluids associated with the quartz overgrowths inboth basins had similar salinities, temperatures,and δ18O values.
It is during peak diagenesis that theevolution of the Thelon and Athabasca basinsstart to become distinct. Although illite is thepeak diagenetic mineral in both basins, peakdiagenetic assemblages are more complex in theAthabasca Basin. The peak diageneticassemblage in the Athabasca Group is dickiteand illite, rather than simply illite. In addition,peak diagenesis in the Athabasca Basin issynchronous with formation of uranium depositsand extensive quartz veins (Q2) that formedfrom fluids having high temperatures (ca.240oC) and salinities (ca. 30 wt% NaCl equiv.).Evidence for these types of fluids in the Thelonis either not preserved, not yet discovered, or didnot exist. The peak-diagenetic fluids weredifferent in composition for this period of timein these two basins.
K-feldspar followed by chlorite pre-cipitation represent late events at ca. 1000 Ma inthe late diagenetic evolution of the ThelonBasin, but these are conspicuously absent in theAthabasca Basin. The fluids responsible forthese precipitates imply large-scale circulationof silica- and illite-undersaturated fluids thatinitially produced secondary porosity in thecentral portion of the sub-basin. Moreover,these fluids are chemically and isotopicallydistinct from any of the late diagenetic fluidsthat affected the Athabasca Basin.Contemporary late diagenesis in the AthabascaBasin is manifest primarily as uraniummobilization and hematite recrystallization at ca.900 Ma (U2; Fig. 10.7). The fluid associatedwith this event had similar salinities,temperatures, and isotopic compositions as thoseinvolved with peak diagenesis (Kotzer andKyser, 1995). Fluid movement in both basinsoccurred around ca. 900-1000 Ma and may havebeen facilitated by the Grenville event andbreak-up of Rodinia.
Later influx along reactivated structuresin both basins resulted in precipitation of vein-filling carbonates from fluids having relativelylow salinities and temperatures. These fluids
were probably meteoric in origin based on theirisotopic compositions. Incursion of meteoricwaters into the Athabasca Basin primarily alongreactivated structures resulted in alteration andprecipitation of low-temperature kaolinite. Theisotopic compositions of the fluids that producedthese kaolinites are representative of high-latitude, meteoric waters (Kotzer and Kyser,1995). Finally, recent (i.e. < 60 Ma) incursionof meteoric waters having low δD and δ18Ovalues along reactivated structures in theAthabasca Basin has resulted in the exchange ofhydrogen isotopes between this fluid andhydrous minerals near fractures throughout thebasin (Wilson and Kyser, 1987; Kotzer andKyser, 1995), and total exchange of oxygenbetween the groundwaters and uraninite (Fayekand Kyser, 1999). No evidence of these latterprocesses has been found in the sediments of theThelon Formation. Although temporally andspatially related, these two Proterozoic basinshave diagenetic and fluid evolutions that beganto diverge during an interval of time that wascritical to the development of large-scaleeconomic mineral deposits.
THE KOMBOLGIE BASIN, AUSTRALIAThe Kombolgie Basin is located on the
Arnhem Land Plateau area and is the northernpart of the larger McArthur Basin in theNorthern Territory of Australia (Fig. 10.14).Like the Athabasca Basin in Canada, theKombolgie is also host to large world-classuranium deposits and evolved from a thick (1-2km) sequence of flat-lying clastic basin-fillingsediments that were deposited in fluvial, aeolian,and marine paleoenvironments.
Geologic settingThe Paleo- to Mesoproterozoic
McArthur Basin (Fig. 10.14) is filled with athick (5-15 km) sequence of nearly flat-lyingsedimentary rocks interpreted to have formed interrestrial and marine environments. Volcanicrocks deposited on the North Australian craton(Fig. 10.14) periodically punctuate thesesedimentary successions. Deposition of themixed siliciclastic-carbonate successions andminor volcanic units occurred in a variety ofintracratonic settings so that the McArthur Basin
246
actually consists of several adjoining sub-basins(e.g. Rawlings, 1999; Rawlings and Page, 1999).The McArthur Basin is host to Precambrianpetroleum deposits, the largest Pb-Zn-Ag districtin the world, and its northern-most sub-basincontains world-class unconformity-type uraniumdeposits. The basin is bounded by Paleo-proterozoic crystalline basement rocks of thePine Creek Inlier to the northwest and by otherinliers to the southeast and north. The NorthernMcArthur Basin begins east of the Pine CreekInlier and is represented by the Kombolgie sub-basin (Fig. 10.15). The results presented herehave been obtained from studies of theKombolgie sub-basin (hereafter referred to asthe Kombolgie Basin), although on-goingstudies of the McArthur and Mt. Isa basins(Figure 10.14) indicate a similar paragenesis intheir clastic lithologies. A detailed chrono-stratigraphic framework for the McArthur Basinhas been elegantly elucidated by integratingsequence stratigraphy (Southgate et al., 2000),U-Pb geochronology on the volcanic rocks(Page et al., 2000) and paleomagnetic directions
in the sedimentary units (Idnurm and Giddings,1995; Idnurm et al., 1985).
The Kombolgie Basin is floored by theequivalent rocks that comprise the AlligatorRiver Uranium Field (ARUF) of the Pine CreekInlier exposed to the west, and is equivalent tothe basal units of the McArthur Basin to thesoutheast (Fig. 10.15). The dominant tectonicextensional and compressional structures and themain metamorphic event in this area are due tothe Barramundi Orogen, which occurredbetween 1890 and 1870 Ma (Riley et al., 1988;Page and Williams, 1988) and continued withthe Top End Orogeny from 1870-1690 Ma(Needham et al., 1980). During the BarramundiOrogen metamorphic event, Paleoproterozoicsediments, volcanic and plutonic rocks weremetamorphosed in the western portion of theARUF to amphibolite facies (Snelling, 1990)and to granulite facies in the eastern portion(Ferguson, 1980). The metasedimentary cover isdomed by Archaean to Paleoproterozoic pre-orogenic granitic intrusions (Dodson et al.,1974) and the Zamu Dolerite, which was
GeorginaBasin
MurphyInlier
VictoriaRiverBasin
KimberleyBasin
Paleoproterozoiccrystalline basementMajor fault
0 300
km
126o 132o
138o
20o
16o
12oPine Creek
Inlier
BirrinduduBasin
McArthurBasin
HYC
Century
LadyLoretta
Hilton/GeorgeFischer
Mt. Isa
Mount IsaBasin
DugaldRiver
Figure 10.14. Location of the McArthur, Mount Isa, other major Paleoproterozoicand Mesoproterozoic basins and major Pb-Zn-Ag deposits in northern Australia.Modified from Lindsay and Braiser (2000).
247
emplaced ca. 1884 Ma. The Mesoproterozoic ischaracterized by postorogenic intrusions, such asthe Jimbu microgranite, which is restricted to thesoutheast portion of the basin (Rawlings andPage, 1999), and the Oenpelli Dolerite, whichoccurs throughout the Kombolgie Basin (e.g.Deckart et al., 2000). Both of these intrude theKombolgie Subgroup and have ages of ca. 1720Ma, which represents the minimum age for theformation of the Kombolgie Basin. Subsequentmagmatic episodes are characterized by minorintrusions of phonolitic and doleritic dykes thatoccurred between 1370 and 1200 Ma (Page,1988).
Sedimentology and StratigraphyOverlying the top of the steeply dipping
Paleoproterozoic basement metasediments is therelatively undeformed and flat-lying Kombolgie
Subgroup which consists of sandstone andconglomerate, and interlayered volcanic units ofthe Nungbalgarri Formation and GilruthMember (Fig. 10.16; Page and Williams, 1988).Economic deposits of uranium have primarymineralization ages near 1640 Ma (Maas, 1989)and are hosted in the Paleoproterozoic basementrocks, but near the unconformity between thebasement and overlying Kombolgie Subgroupsandstones. The Kombolgie Subgroup (formerlythe Kombolgie Formation was part of theKatherine River Group) consists predominantlyof clastic rocks made of sandstones and arkosesthat were deposited in fluvial and aeolianenvironments with occasional marine incursionsthat deposited marine sandstones and evaporites.Marine conditions dominated as suggested bythe presence of glauconite, halite crystal casts,and wave ripple marks in the sandstones of the
12°00'
13°00'
133°30'132°30'
0 50
km
Kombolgie Subgroup
Nungbalgarri andGilruth Volcanics
Oenpelli Dolerite
Volcanic and Intrusive Rocks
Uranium orebody
Pine Creek InlierPine Creek succession, igneousintrusives, Archean basement
Figure 10.15. General geologic relations and stratigraphy of the western portionof the Kombolgie sub-basin of the Northern McArthur Basin. Also shown are thelocations of uranium deposits in the area. The Alligator River Uranium Field iswithin the Pine Creek Inlier.
248
McKay Formation (uppermost KombolgieSubgroup; Fig. 10.16). The sandstones of theMcKay show minimal diagenesis relative to therest of the formations in the Subgroup.
The Kombolgie Subgroup is composedof at least three stratigraphic sequences. Thelowermost, like that of the Thelon andAthabasca basins, is coarse-grained andrepresents the early stages of basin formation.This lower sequence is composed of coarse-grained sandstones and conglomerates withabundant trough cross-bedding. These lowerstratigraphic units are interpreted to primarilyrepresent deposition in high-energy braided riversystems that transported sediment to the southand east from a proximal source region. Afterthis initial stage of basin evolution, coarse to
medium-grained quartz arenites were depositedand represent deposition in much lower-energy,more distal braided streams. These sandstoneswere deposited by sand bar migration and sheetflow across braid plains and exhibit much bettersorting than the sediments laid down by thehigher energy streams that preceded thisinterval. This evolution from proximal to moredistal fluvial environments occurs in thesandstones of the Gumarrirnbang Formation(Fig. 10.16). This package of facies representingfluvial deposition is overlain by a unit with verywell sorted, medium-grained quartz arenite thatexhibits large-scale wedge to trough crossbedding. Individual foresets in this cross-bedded unit are up to 3 meters in length and dip18-20°, and exhibit reverse grading. The unit is
NungbalgarriVolcanics
GumarrirnbangSandstone
MarlgowaSandstone
2-15 mGilruthVolcanics
McKaySandstone
Cottee Formation
NimbuwahComplex
Depositional Contact
Oenpellidolerite
(3-300 m)
MamadawerreSandstone
Unconformity30
0-60
0 m
100-
300
m10
0-30
0 m
130-
400
m
Figure 10.16. Stratigraphic relations of the KombolgieSubgroup in the western portion of the Kombolgie Basinshowing range in thickness of each unit.
249
interpreted as representing aeolian dunedeposition. Overlying the dune field is aninterval marked by mud-rich, fine-grainedsandstones and mud-cracked siltstones withwave ripple marks and minor flaser bedding,which are interpreted to represent tidal flatdeposition. The overlying Marlgowa Formationmarks a return to distal fluvial deposition. Basinsubsidence or sea level rise late in thedepositional history of the Kombolgie result inmarine conditions returning to the basin. Theseconditions are indicated by halite crystal casts,glauconite, stromatolites, and abundant waveripple marks found in the McKay Formation.
Paragenesis and fluid evolution in theKombolgie Basin
Sandstones in the Kombolgie Basin aretypically mature quartz arenite with detritalphases consisting of quartz (90-100%),occasional lithic fragments (0-10%), and tracesof hematite, zircon, and apatite. Moderate towell sorted detrital quartz sand grains rangefrom 0.1-2.5 mm in diameter and well-roundedlithic fragments are generally 0.2-1.0 mm indiameter. Detrital quartz separated from the
sandstones of the Gumarrirnbang andMamadawerre formations (Figs. 10.15 and10.16) have δ18O values from 11 ‰ to 13 ‰,consistent with a provenance similar in isotopiccomposition to the basement rocks.
Early DiagenesisThe paragenetic sequence recorded in
the Kombolgie sandstones (Fig. 10.17) ischaracterized by formation of early-stage quartzovergrowths (Q1) that formed at 80-130°C fromlow salinity NaCl fluids (i.e. <10 wt% NaCl)having δ18O from –4 to 2‰ (Fig. 10.18). Thefirst stage of hematite (H1) occurs along theinterface between the detrital and overgrowthquartz. Initial alteration of lithic fragments anddetrital feldspar to clay (probably smectite,precursor to illite)+quartz+hematite alsooccurred during this stage of diagenesis, andserved as the most dominant source of silica(Q1) in the Kombolgie Subgroup. This earlyquartz cement event resulted in the mostpronounced porosity reduction, particularly inthe well-sorted lithologies. The commonpresence of unaltered detrital zircon and apatitein quartz overgrowth-rich rocks suggests that
1700 1600 1500 1400 800 <50Time (Ma)
1800
Sediment depositionUranium mineralization
Q1 (overgrowths)
Illite
H1
Chlorite + Phosphates
Q2/Q3 (Euhedral)
Kaolinite (weathering)U-min U-min
H3
H2
Paragenesis-Kombolgie Basin
Oenpelli
Figure 10.17. Paragenesis of minerals in the Kombolgie Basin as afunction of age.
250
early fluids related to Q1 cementation did notsignificantly mobilize components hosted indetrital zircon and apatite. The principal impactof these silicified zones on basin hydrology is inthe creation of basin-wide impermeable zonescapable of focussing later fluid flow within andtoward zones of higher permeability (e.g. faultzones and zones without quartz cements).
Peak DiagenesisThe next stage recorded in the fluid
evolution in the sandstones of the KombolgieSubgroup is associated with filling of remainingpore space with diagenetic illite or lesscommonly, chlorite, coincident with peakdiagenesis (Fig. 10.17). Hematite and rareoccurrences of apatite are also formed at thisstage. The illite has a 2M1 crystal structure,thereby requiring temperatures of formationgreater than 200°C. The Ar-Ar ages of the illiteare 1650 +/- 80 Ma and fluids in equilibrium
with peak-diagenetic illite were basinal brineswith δ18O = +6‰ and δD = –30‰ (Fig. 10.18).
Chlorite follows illite in the lowermostparts of the Kombolgie, indicating a change offluid properties in some parts of the basin to onethat contained Mg. Euhedral quartz (Q2) thatpostdates the quartz overgrowths occurs with,and after, illite as massive veins and brecciapipes and as cement in some sandstones, butalways proximal to the Oenpelli Dolerite. Thisquartz has fluid inclusions with distinctcharacteristics that depend on their locationrelative to the Oenpelli Dolerite. Those farthest(up to several tens of meters) from the dolerite(Q2a; Fig. 10.18) have fluids with 20 wt% NaCl,δ 1 8O = +5‰ and δD = -30‰ and homo-genization temperatures from 100 to 350°C. Incontrast, those proximal to the Oenpelli (Q2b)formed from fluids with the same isotopiccompositions, but they are saline (20-30%), Na-Mg-Ca-Cl basinal brines having temperatures
-20 -10 0
Tem
pera
ture
(oC)
δD (o
/oo )
δ18O (o/oo)
Flui
d In
clus
ion
Salin
ity (w
t.% N
aCl)
Age
(Ma)
500
1500300
100
-40
-120 10
Kombolgie Basin Fluids
1000
30
-20 -10 0
MW
L
seawater
early(?)
early
early
early
peak
peak
peak
peakQ2a & Q2b
Q2a
Q2b
Q2a
Q2b
Q2a & Q2b
veins
veins
veins
veins
late
late
late
late
Figure 10.18. Relations among ages, temperatures, salinities, δD and δ18Ovalues of diagenetic fluids recorded in fluid inclusions and diageneticmineral assemblages in the Kombolgie Basin.
251
from 150 to 400°C (Fig. 10.18). In rareinstances, euhedral quartz can contain zones ofprimary Na-rich inclusions with earlier, or later,zones of Na-Mg-Ca-Cl inclusions andsubstantial variations in homogenizationtemperatures between the zones. In detail,silicified zones (200-250°C) occur proximal tothe dolerite just beyond an inner zone ofdesilicification (250-400°C). This associationformed in response to changes in SiO2 solubilityas convective circulation of H2O occurredduring crystallization and cooling of sills. Thefluid properties are consistent with shallowconvective circulation of evolved formationfluids during the emplacement of the maficintrusives. Intrusion of the Oenpelli Doleritewas early-peak diagenesis as evidenced by theage of the intrusion, older Ar-Ar ages of theillites adjacent to the Oenpelli, and spilitizationof the Oenpelli and volcanic units.
The third stage in the fluid evolution ofthe Kombolgie Subgroup is related to fracturing,faulting, and quartz vein formation. Veins fillingheavily fractured and slightly desilicifiedsandstone have primary aqueous fluid inclusionswith δD values near -30‰, homogenizationtemperatures of ca. 200-400°C and salinities of22% Na-Mg-Ca-Cl brines. Fluids in isotopicequilibrium with this vein quartz have δ18O ofca. 10‰ and δD = –30‰. These high-18O andhigh-D fluids are similar to those related to thehydraulic breccias (Q2) in the basin and likelyreflect overpressured fluids that formed from thesame extensive fluid-rock interaction withvolcanic rocks and sedimentary rocks of thebasin during emplacement of the Oenpelli.
The McKay Formation (Fig. 10.16) ispredominantly composed of submature tomature quartz arenite, but feldspathic arenitesare common. Like the underlying sandstones ofthe rest of the Kombolgie Subgroup, quartz isthe primary detrital phase (70-100%), butfeldspar (0-30%) is much more common. Otherdetrital phases include lithic fragments (up to10%), hematite, zircon, and apatite. Early-stageauthigenic quartz overgrowths are not aspronounced as in the underlying Kombolgieunits. Stable isotope data for the quartzovergrowths and peak-diagenetic illite from the
McKay Formation are similar to those for theunderlying formations described above,indicating a common fluid between theKombolgie and McKay at early and peakdiagenesis. The difference is that the McKayFormation was much less permeable and sawmuch lower fluid/rock ratios than the lowerKombolgie Formation.
Late DiagenesisThe final stage of alteration is pervasive
kaolinite that permeates the KombolgieSubgroup at the surface, and down to severalhundreds of meters depth. This kaolinite ispervasive along fractures and in the center ofpores where it replaces illite (I1). This late-stagekaolinite has isotopic compositions typical ofthose expected from modern weathering (last 50m.y.) associated with the development of thevast Australian regolith.
Thermal evolution of fluids in the KombolgieBasin
The thermal evolution of the KombolgieBasin can be estimated using Ar-Ar mineralformation and closure temperatures in combin-ation with fission track data on apatite andzircon and U-Pb and Sm-Nd isotope data fromthe U-mineralization in the area (Fig. 10.19).The oldest recognized thermal event whichaffected the Kombolgie sub-basin basementrocks was peak metamorphism at 700°C(Ferguson, 1980) and ca. 1870 Ma (Page, 1988;Riley et al., 1988), which cooled to ca. 300oC by1780 Ma (Deckart et al., 2000). The exposedbasement was eroded and weathered producing athick regolith horizon. Formation of theKombolgie Basin and other sub-basins of thelarger McArthur Basin began between 1780 and1760 Ma and the temperature in the deeperportion of the basin increased as burialproceeded. In contrast, rapid uplift of the rootsof the Trans-Hudson Orogen in Canada occurredat ca. 1750 Ma based on concordant Ar-Ar agesfor biotite, muscovite, and amphibole inbasement rocks of the Glennie, La Ronge, andWollaston domains, thereby limiting initiation ofthe Athabasca Basin to <1750 Ma. This is muchyounger than the Kombolgie Basin, but virtuallyidentical to the Thelon.
252
Fluid inclusions in quartz near theOenpelli Dolerite have homogenizationtemperatures of 200-400oC in fluids havingeither NaCl or Na-Mg-Ca-Cl compositions,indicating that the intrusion did produce athermal “spike” in the basin, but this could nothave been long-lived given consistent biotite andamphibole Ar-Ar ages of 1720 Ma fromspatially diverse samples of the Oenpelli.
The best Rb/Sr, Sm/Nd and U/Pbisotope data on the uranium deposits in theARUF indicate that primary U-mineralization atKoongarra, Jabiluka and Nabarlek took place atca. 1640 Ma (Ludwig et al., 1987; Maas, 1989).Palaeotemperatures for uranium mineralizationindicate a formation temperature near 200°C(Gustafson and Curtis, 1983; Wilde, 1988;Wilde et al., 1989). Paleomagnetic studies in theadjacent McArthur Basin show a significantchange in the apparent polar wander path(APWP) at 1640 Ma (Idnurm and Giddings,
1995) which is related to major tectonic eventsin the overall McArthur Basin region. Thesetectonic events may have stimulated fluid flowpossibly related to primary U-mineralization inthe Kombolgie sub-basin (Loutit et al., 1984).
After 1640 Ma (primary U-mineral-isation), the overall cooling path of the basalKombolgie Basin shows a slow decrease intemperature from ca. 230°C to 100°C until 400Ma (latest U-remobilization between 600 and400 Ma; Ludwig et al., 1987). This slightdecrease of the temperature in the Kombolgiesub-basin is indicated by Ar/Ar ages of K-feldspar of 1100-1260 Ma, which is when thetemperature of the basal portion of the basin fellbelow 200°C. In addition, fission track zirconand apatite ages in samples from the basalKombolgie Subgroup are near 1420 and 490 Ma,respectively (Koul et al., 1988a). Zircon andapatite anneal above 200oC and 100oC,respectively (Koul et al., 1988b). Except for the
distal
?
?
?
0
100
200
300
400
500
600
700
800
400600800100012001400160018002000
Age (Ma)
Inferred Cooling History ofthe Kombolgie sub-basin
Tem
pera
ture
(°C)
Figure 10.19. Thermal and temporal evolution of the Kombolgie Basinbased on data discussed in the text.
regional peakmetamorphism
initiation of basinsedimentation ca.1750 Ma
Oenpelli Ar-Ar1720 Ma
Polar wander changes1650 Ma
Primary U-mineralizationRb/Sr, Sm/Nd, U/Pb ca.1640 Ma
EVENTS:
Fission trackzircon (Naberlek)
K-feldspar Oenpelli Ar-Ar
ProximalU-remobilization
Fission trackapatite
253
intrusion of the massive Oenpelli Dolerite, thelower sedimentary units of the Kombolgie Basinshow a very slow cooling history with tem-peratures between 230 and 100 °C from 1640Ma until 400 Ma (Fig. 10.19). The Oenpelli did,however, provide both heat and a source of Mgthat facilitated exchange between basinal fluidsand the dolerite at 1720 Ma, resulting in Na-Mg-Ca-Cl brines in addition to the normal Na-K-Clbrines in the basin. This is in marked contrast tothe Athabasca and Thelon basins where NaClbrines dominate.
FLUID HISTORY COMPARISON FORTHE ATHABASCA, THELON, ANDKOMBOLGIE BASINS
The underlying initial difference in thepaleohydrologic and diagenetic evolutionpathways between these three basins is in thecomposition and distribution of the basin-fillingsediments. In the Thelon and Kombolgie basinsthere is abundant evidence that detrital feldsparwas present in the sediments as they moved intothe burial setting. In stratigraphic units that hadextremely high hydraulic conductivities initially,early and pervasive quartz cementation some-times resulted in excellent preservation of thisdetrital feldspar. This is not the case for thesediments of the Athabasca Basin. Furthermore,the presence of multiple unconformity-boundedstratigraphic sequences within the ThelonFormation and the Kombolgie Subgroup indicatea much more dynamic and proximal sourceregion for these basins. In fact, there are clastsof volcanic rock and older sandstone not onlynear the base, but also near the top of the ThelonFormation indicating that these basementlithologies were exposed to erosion, weathering,and reworking multiple times through theevolution of the basin. In the Athabasca, on theother hand, the Manitou Falls Formationexhibits a much simpler sedimentologic evo-lution. After initial basin formation, strati-graphic units in the upper half of the formationaccumulated as the basin underwent simplesubsidence, filling the basin. There is no goodevidence of preservation of feldspar survivinginto the deep burial setting in the Athabasca.There is, however, abundant evidence suggest-ing that kaolin minerals were present, perhaps as
detrital clay matrix. This suggests thatweathering may have been much more intense inthe Athabasca source region. This difference inbasin-fill composition sets the stage for markeddifferences in the later diagenetic evolution ofthese basins.
Paleomagnetic resultsKotzer et al. (1992) established that
sandstones from the Athabasca Basin have acomplex paleomagnetic history that can be tieddirectly to the diagenetic fluid history of thebasin. They showed that these rocks normallyhave distinct, one-component directions ofpaleomagnetism corresponding to earlydiagenesis (A-magnetization at 1600-1750 Ma),peak diagenesis coincident with major Umineralization and mobi l i za t ion (B-magnetization at ca. 1500 Ma), later Umobilization and deposition (C-magnetization at900-1000 Ma), and incoherent, recent diagenesis(D-magnetization at ca. < 300 Ma). Thedirections recorded in basement samples nearthe unconformity are identical to the A-magnetization in the Athabasca and correspondto an age of 1600-1750 Ma. Consequently, thehematization that is so pervasive in regolith ofthe Athabasca Basin has been over-printed byearly diagenetic fluids that affected the entirebasin.
The majority of samples from drill corein the Thelon Basin have much weakermagnetizations than those from the AthabascaBasin, and most record multi-componentmagnetic directions rather than the single-component directions in many of the Athabascasamples. These multi-component magnetizationsin the Thelon Basin consist predominately of ahigh-temperature primary magnetization of lowto high angle normal polarity in the illite-richareas of the basin (corresponding to the A-magnetization direction in the Athabasca), alater B’-magnetization, and a low-temperaturesecondary magnetization of high angle normalpolarity or recent magnetic overprints in areasnear the edge or in chlorite-rich areas of thebasin (i.e. C- and D-magnetization). Samples ofsandstones from drill core at the edge of theThelon Basin and close to major faults recordweak multi-component and few, strong single-
254
component directions throughout the drill corebecause the hematite has been degraded duringrecent weathering.
In the Thelon Basin, the A-magnetization is more prominent than in theAthabasca, which may reflect early closure ofpermeability in some strata, particularly thebasal units of the Thelon Formation. This A-magnetization corresponds to the oldest Ar-Arages of 1550-1690 Ma for peak diagenetic illitein the Thelon. This illite is well-crystallized andis 2M1 polytype. The B-magnetization mostlikely corresponds to illite that has retrogressedto 1M polytype during a later fluid event.Although not widespread, this illite has Ar-Arages of 1150-1300 Ma and is found near thecentral portion of the sub-basin. The C-magnetization that was mainly restricted tofaults in the Athabasca is more widespread,though rare, in the Thelon. This direction inboth corresponds to an event at ca. 900-1000Ma, but is manifest in the Thelon Basin byreplacement of illite by feldspar, then chlorite.
In contrast to what was found in samplesof the Athabasca Group, most of thepaleomagnetic directions in sandstones from theMcArthur and Kombolgie basins are multi-component. Paleomagnetic directions recordedby the Nungbalgarri volcanic unit were multi-component, but the highest temperature stepscorrespond to an age of ca. 1760 Ma. Thedirections in quartz-cemented samples of theMamadawerre Sandstone correspond to ages of1640 Ma, which are coincident with the bestages for uranium mineralization in the basin.Radical changes in the apparent paleomagneticwandering path (APWP) in samples fromthroughout the McArthur Basin correspond withmajor tectonic events (Idnurm et al., 1995), andthe consequent fluid events, which are related toboth U and Pb-Zn-Ag deposition at ca. 1640 Ma.Paleomagnetism may be an indicator of fluidtiming as well as chemical and thermal intensityof fluid-rock interactions.
Chemical composition of peak diagenetic illiteIllite is a mineral phase indigenous to all
three basins. It is a diagenetic fluid product andtherefore has potential to reflect some of thesubtleties of fluid-rock interactions. Illites from
the Athabasca Basin incorporate the leastamount of Mg and Fe in their crystal structurerelative to those from the Thelon and Kombolgiebasins (Fig. 10.20). There is little or nocorrelation between the chemical composition ofthe illites and stratigraphy, although sandstoneshaving higher permeability (i.e. high claycontents) have the lowest potassium contents,consistent with alteration of these illites todickite. Illites in the Athabasca (and the otherbasins) formed during peak diagenesis and theirconstant chemical and isotopic compositionswould imply that the composition of the fluid inthe basin during peak diagenesis did not changemuch during the interval from ca. 1600 to 1200Ma, the interval of illite formation (Fig. 10.7).
Illites from the Thelon Basin have thewidest range in compositions of the three basins(Fig. 10.20). This may be a function of thegeneral lack of preserved quartz cementation(there was an extensive desilicification event-Fig. 10.12), paucity of post-peak quartzprecipitation, and interaction with a high-Mgfluid source. Chlorite is a common authigenicclay constituent in the basin, and theseobservations suggest that fluids rich in Mg andFe were in circulation after peak diagenesis. Thelikely source would be fluids associated with thelate chlorite event (see Fig 10.12).
Illites from the Kombolgie Basin areintermediate to those from the Athabasca and theThelon in composition. Samples with moderateto high porosity, however, show a relativelywide range of illite compositions (Fig. 10.20).Mg and Fe substitution is most prevalent in theMamadawerre sandstone. Quartz cementationassociated with portions of the Gumarrirnbangsandstone or proximal to the Oenpelli havepreserved early diagenetic illite compositions.
Differences in chemical compositions ofbasinal fluids are imparted on the composition ofillite, but in subtle ways. Illites from theAthabasca Basin have the highest muscovitecomponent whereas illites from the Kombolgieand Thelon basins have higher Mg and Fecontents due to the influence of Mg-rich brines,though at very different times. Illite chemicalcompositions within each basin normally do notmirror changes in stratigraphy, but rather thedegree of quartz cementation (Fig. 10.20). With
255
an increasing degree of de-silicification, up to50% of the 6-fold coordinated Al is replaced byMg, Fe, and to a lesser extent Mn. Conversely,zones of early quartz cementation preserve illite.Lithologies that were cemented early during theprecipitation of quartz overgrowths served asbarriers to fluid mixing and fluid-rockinteractions (i.e. aquitards), thereby preventinglater alteration of peak diagenetic illite.
Fluid compositionsThe paragenesis of the minerals, and the
compositions of the fluids that formed them, inall three basins is similar in some respects, butdistinctly different in detail (Fig. 10.21). Forexample, the Kombolgie is predominantly anillite basin with several generations of quartz inthe sandstone and basement, and chlorite andphosphate minerals in the basement or near theunconformity. The uranium mineralizationappears to be ca. 1640 and 900 Ma. All of thekaolinite to date is poorly-crystalline kaolinite
high moderate low
Degree of Quartz Cementation
0.5
1.0
1.5
2.0
0.6 0.8 1.0
Al per total Y cations
16
n=79
71Mg, Fesubstitution
K p
er 2
2 O
's
loss
of K
0.5
1.0
1.5
2.0
0.6 0.8 1.0
Al per total Y cations
n=34
6996
Mg, Fesubstitution
K p
er 2
2 O
's
loss
of K
0.6 0.8 1.0
Al per total Y cations
19
69
n=96Mg, Fe
substitution
0.5
1.0
1.5
2.0
K p
er 2
2 O
's
loss
of KKombolgie
Athabasca Thelon
Figure 10.20. K vs. Al plots from calculated mineral formulae. Totalnumber of analyses is shown in each field (i.e. n=). The greater the spreadof data, the greater the degree of alteration/intergrowths and fluid-rockinteraction. Note tight clusters of data from samples with a high degree ofquartz cementation.
256
and is a relatively recent weathering effect onthe sandstone.
As with the other two basins, the earliestevent (ca. 100 million years earlier than theother basins) in the Kombolgie is the formationof quartz overgrowths from NaCl fluids. Theseevolved into NaCl fluids of higher salinity andhigher temperature as the basin evolved (Fig.10.22), and later became Na-Mg-Ca-Cl brines.The source of the higher temperatures, Mg andCa appears to result from the interaction of theoriginal NaCl fluids with the Oenpelli and othervolcanic units such as the Nungbalgarri For-mation. The igneous units also supplied silicafor local quartz cementation of the surroundingsandstone units during alteration, which affectedthe permeability of the basin-filling sedimentaryrocks.
The Thelon contains a significantamount of phosphate as early phases. Theseinclude cements deposited along fractures in the
basement rocks (Miller et al., 1989), andsedimentary phosphate deposited in shallowmarine or lacustrine settings. The latter of thesetwo was eroded and reworked by streams, andduring burial these intraclasts became localsources of phosphate that was mobilized andreprecipitated as an authigenic cement phase.Two generations of illite with distinct agescorrespond to peak diagenesis. Long after peakdiagenesis, authigenic feldspar and chloriteformed as the result of evaporative brinesflowing through the sandstone at ca. 1000 Ma.Late hydrothermal fluids moved up alongstructures late in basin development andprecipitated small quantities of drusy quartz(Q3). As with the Athabasca Basin, there is aminor late carbonate cement-forming event. Ofparticular note is that there have been nosignificant quartz veins found in the Thelon, norare there significant sulphides or oxides as latephases that is so characteristic of the Athabasca
Figure 10.21. Comparison of the mineral paragenesis of the ProterozoicAthabasca, Thelon and Kombolgie basins showing initial similarities and thendivergence. All basins show major fluid events at ca. 1600-1700 Ma, 900-1000 Ma and <300Ma that correspond to major, but distal, tectonic events.
257
Basin. The fluids in both the Athabasca andThelon basins are NaCl, in marked contrast tothe Kombolgie where there are coexisting NaCland Na-Mg-Ca-Cl fluids.
The most complex fluid evolutionhistory is recorded in the Athabasca Basin (Figs.10.21 and 10.22). For example, peak diagenesisin the Athabasca Basin was associated with illiteand dickite rather than simply illite, indicatingthat either there was an effective higherwater/rock ratio in the basin, or more likely, thatthe composition of the detrital material wasdistinctly different, i.e. already substantiallyweathered. However, it is significant that theillite in all three basins is isotopically similar,although their distinct chemical compositions ineach basin reflect differences in the peakdiagenetic fluids and in permeability of thesandstones in each basin (Fig. 10.22). This
indicates that the fluids in each basin originatedfrom a similar paleoenvironment (and this musthave been marginal marine) and that eachdeveloped as NaCl fluids, requiring evaporites inthe sedimentary succession that contributed saltsas these fluids migrated to the bottom of thebasins. These peak-diagenetic fluids remained inall three basins for at least 600 million years(several hundred million more in the KombolgieBasin)—a fluid history that is longer than mostbasins are old. The fluid events must haveresulted from tectonic processes (such as distalorogens at ca. 1000 and 1600 Ma and initialbreakup of Rodinia at ca. 900 Ma) that allowedhydraulic gradients to change slightly. TheAthabasca Basin has a complex history as aresult of later influx of meteoric waters duringthe past 100 million years along well-developedstructural zones (Fig. 10.21). The Kombolgie
6.0
8.0
10.0
12.0
K-feldspar
Muscovite
Kaolinite
AthabascaBasementKombolgie
Basement
ChloriteMg-biotite
AthabascaBasin
KombolgieBasin
4.0-3 -1 1 3 5 7
K-feldsparMuscoviteKaolinite
Chlorite
Mg-biotiteChl 900 Ma
Ksp1000 Ma
I1&I21600 &1250 Ma
AlbiteParagonite
Kaolinite
PyrophylliteDiaspore
Athabasca
Thelon
Kombolgie
1.0
2.0
3.0
4.0
5.0
-5 -4 -3 -2 -1 0
K-feldspar
Muscovite
KaolinitePyrophyllite
GibbsiteAthabasca
Thelon
Kombolgie
200°C1kbar
200°C1kbar
quar
tz sa
tura
tion
2.0
3.0
4.0
5.0
log (aSiO2aq)lo
g (a
Na+
/aH
+ )
200°C1kbar
200°C1kbar
6.0
8.0
10.0
log (aK+/aH+)
log
(aM
g2+ /
a(H
+ )2 )
Figure 10.22. Chemical compositions of fluids deduced from mineralassemblages at peak diagenesis for the Proterozoic Athabasca, Thelon andKombolgie basins. Fluid compositions from the basin and basement areshown for both the Athabasca and Kombolgie basins (upper left) andcompositions of various diagenetic fluids are shown for the Thelon(bottom left).
258
has a complex early history because of theOenpelli Dolerite, which affected thetemperatures and salinities of the fluids, but nottheir isotopic compositions.
Recent paleomagnetic results indicatethat the basins evolved together from ca. 1400-1750 Ma and seem to have split apart around1000 Ma, only to come back together and thenfinally separate at ca. 700 Ma with the break-upof Rodinia. Significant mountain ranges resultedas a result of Paleoproterozoic orogens on boththe Australian and North American continents.This uplift supplied detrital phases, predom-inately quartz and feldspar in the Thelon andKombolgie basins, but quartz and kaolin in theAthabasca Basin. The only evidence forvolcanism in the Athabasca and Thelon basinsare the Martin Group and Pitz Formation, whichoccurred before the basins began to fill. Incontrast, there was continued volcanic activity inthe McArthur Basin to the southeast of theKombolgie and in the Kombolgie Basin itself,which produced numerous flows, ashes andsubsidence for this basin. The igneous activityin the Kombolgie affected the chemicalcomposition of fluids in the basin and, alongwith the original sedimentology, had a verypronounced affect on the location of aquifersand aquitards throughout the evolution of thebasin.
IMPLICATIONS FOR INTERPRETINGTHE EVOLUTION OF PROTEROZOICBASINS
Understanding paleohydrologic systemsin terms of basin evolution requires theintegration of information derived from thesedimentology, stratigraphy, diagenesis, andgeology of ancient basin-filling successions.Combination of these is pre-requisite for valid“basin analysis” and to guide any hydrologic orgeochemical modeling. Proterozoic basins, inparticular, represent systems that record “nearequilibrium” conditions, thereby constrainingthe composition of specific fluid events thatnormally affected vast areas. They also reflect aperiod of earth history when there were massiveorogens as the continents came together,followed by relative tectonic quiescence thatallowed many intracontinental basins (and the
ore deposits therein) to form and evolverelatively undisturbed until the breakup of themegacontinent. Such a series of events over along time period would never again be realizedin earth history.
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