PII S0016-7037(01)00588-9

A detrital model for the origin of gold and sulfides in the Witwatersrand basin based onRe-Os isotopes

JASON KIRK,1,* JOAQUIN RUIZ,1 JOHN CHESLEY,1 SPENCERTITLEY,1 and JOHN WALSHE2

1University of Arizona, Department of Geosciences, Tucson, AZ 85721, USA2CSIRO Division of Exploration and Mining, 39 Fairway, Nedlands 6009, Western Australia, PO Box 437, Australia

(Received August10, 2000;accepted in revised form February15, 2001)

Abstract—The Re-Os systematics of gold and sulfides from the Witwatersrand basin were utilized todetermine whether the gold is detrital or was introduced by hydrothermal solutions from outside the basin.

Gold from a gravity concentrate from the Western Areas Gold Plant and gold from the Vaal Reef have veryhigh Os concentrations of approximately 73 to 10000 ppb and 3 to 32 ppb Re, resulting in187Re/188Os ratiosof 0.010 to 0.185. The gold has subchondritic187Os/188Os ratios between 0.1056 to 0.1099 and an averagevalue of 0.1067. Rhenium depletion ages (TRD) range from 3.5 Ga to 2.9 Ga, with a median age of 3.3 Ga.

Pyrite from the Vaal Reef have Os concentrations ranging from 0.26 to 0.68 ppb, Re concentrations of 1.7to 2.8 ppb and187Re/188Os ratios of approximately 14 to 87. The pyrite samples have measured187Os/188Osratios of 0.84 to 4.7 and define an isochron with an age of 2.996 0.11 Ga (MSWD5 0.77).

The Os isotopic data from the direct measurement of gold preclude introduction of gold to the Witwa-tersrand basin from crustally derived metamorphic or hydrothermal fluids between 2.7 to 2.0 Ga. Theunradiogenic187Os/188Os ratios, old TRD ages of the Western Areas and Vaal Reef gold samples, as well asthe contemporaneously old age of the Vaal Reef pyrite are consistent with detrital deposition of gold duringthe formation of the Witswatersrand basin. The Os data will allow for minor hydrothermal remobilizationand/or overprinting of hydrothermal gold on preexisting detrital gold grains but does not support theintroduction of gold solely by hydrothermal fluids.Copyright © 2001 Elsevier Science Ltd

1. INTRODUCTION

The Witwatersrand basin is located in the Kaapvaal craton ofsouthern Africa (Fig. 1) and is the largest known gold provincein the world. Three models have been used to explain the originof the greater than 40000 tons of gold contained in the basin(Fig. 2).

1. A placer model, which proposes that gold is detritus from anolder granite-greenstone source area and has been mechan-ically transported into the basin and concentrated by fluvial/deltaic processes (e.g., Mellor, 1916; Pretorius, 1974; Hall-bauer and Utter, 1977).

2. A modified placer model, which has the same assumptionsas the placer model, but emphasizes the hydrothermal mod-ification of much of the gold. In this model, detrital goldmay be mobilized by hydrothermal or metamorphic fluidsand locally re-precipitated with other associated phases(e.g., Frimmel and Gartz, 1997; Minter, 1999).

3. A metamorphic/hydrothermal model, which proposes thatgold is transported in solution from outside of the basin bymetamorphic or hydrothermal fluids between 2.7 and 2 Ga,after basin sedimentation ceased (e.g., Graton, 1930; Phil-lips and Law, 1994; Barnicoat et al., 1997).

The ultimate source of the gold has been a difficult issue toresolve as all of the information on the source and absolutetiming of gold deposits are largely indirect in nature, such asthe isotopic dating of paragenetically associated ore-sulfides or

silicate gangue minerals (reviewed in Kerrich and Cassidy,1994).

U-Pb zircon ages of volcanic rocks that bracket the Witwa-tersrand basin indicate that it formed between approximately3.07 Ga and 2.71 Ga (summarized in: Robb and Meyer, 1995).Isotopic ages from ore minerals, osmiridium grains and sili-cates from the Witwatersrand basin are both older and youngerthan the end of basin sedimentation at 2.71 Ga (Fig. 3). U-Pband Pb-Pb isotopic ages from rounded uraninite (Rundle andSnelling, 1977) and pyrite grains (Saager, 1981; Giusti et al.,1986) as well as Os isotopic ages from osmiridium grains (Hartand Kinloch, 1989), are between approximately 3.5 to 2.9 Gaand have been used to conclude that the gold is detrital.However, younger ages between 2.7 and 2 Ga, which includeU-Pb and Pb-Pb ages on pyrite, uraninite, hydrothermal zircon,rutile and K-Ar ages on clays and micas have been used todefine five possible hydrothermal or metamorphic events whichmay have introduced or mobilized gold within the basin (Robband Meyer, 1995; Frimmel, 1997; Zartman and Frimmel,1999). Extrapolating these mineral ages to the timing of goldmineralization is problematic, as these ages are from spatiallyassociated minerals and may not directly date gold formation.Here we use Re-Os isotopes to directly determine ages on goldand pyrite. Rhenium and Os are concentrated in sulfides andmetals (e.g., Martin et al., 1993). Geochronological and prov-enance information can therefore be directly obtained on sul-fide and metal phases such as: pyrite (Freydier et al., 1997;Stein et al., 1998; Mathur et al., 1999), molybdenite (e.g., Luckand Allegre, 1982; Ishihara et al., 1989; McCandless et al.,1993), iron meteorites (e.g., Luck and Allegre, 1980; Shen etal., 1996), osmiridium (Hart and Kinloch, 1989; Hirata et al.,1998) and reported here, native gold.

*Author to whom correspondence should be addressed (jkirk@geo.Arizona.edu).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 65, No. 13, pp. 2149–2159, 2001Copyright © 2001 Elsevier Science LtdPrinted in the USA. All rights reserved

0016-7037/01 $20.001 .00

2149

2. ANALYTICAL TECHNIQUES

Approximately 0.1 to 0.4 g of an Aestar™ gold powder standard(stock # 39817), 0.001 to 0.3 g of Witwatersrand gold and 0.1 to 0.25 gof pyrite were spiked with185Re and190Os and dissolved in Cariustubes using 8 mL of 1:3, HCl:HNO3, with techniques similar to thoseof Shirey and Walker, (1995). Two mL of H2O2 were added to Cariustubes as an additional oxidizer for the pyrite samples. The Carius tubeswere frozen in an ethanol-liquid nitrogen slush, sealed and then heatedat ;200°C overnight. Osmium was separated from the dissolved sam-ple using a distillation technique similar to that of Na¨gler and Frei,(1997), or using a CCl4 solvent extraction method modified fromCohen and Waters, (1996). Osmium was further purified using micro-distillation techniques (Birck et al., 1997), and loaded on high purityplatinum filaments with Ba(OH)2 as an emission enhancer. After theinitial CCl4 extraction of Os, the remaining aqua regia solution wastaken to dryness and re-dissolved in either 0.1N HCl, 0.8N HNO3 or 5NH2SO4, to determine the best eluent for separation of Re from Au (seediscussion below). Rhenium was then separated on 1 mL anion ex-change columns with AG1-X8 (100–200 mesh) resin and furtherpurified on 0.1 mL anion exchange columns. Rhenium was dried downand loaded on platinum filaments with Ba(SO)4 to aid ionization.Rhenium and Os isotopic compositions were determined by negativethermal ionization mass spectrometry (NTIMS) (Heumann, 1988;Creaser et al., 1991; Volkening et al., 1991) on a VG 54 mass spec-trometer. Total measured Os procedural blanks were approximately 1to 4 pg and 10 to 20 pg Re, which are negligible for the concentrationsmeasured in the Witwatersrand gold and are corrected for in the pyritesamples.

Rhenium ionization tended to be poor when separated from goldusing either HNO3 or HCl as the eluent in the anion exchange columns.We achieved a more complete separation of Re from gold whensamples were loaded with H2SO4, using a procedure modified fromMorgan and Walker, (1989). The resin was cleaned with MQ water and8N HNO3 and then converted to sulfate form with 5 mL of 2.5N H2SO4

and conditioned with 2.5 mL of 5N H2SO4. Samples were loaded in 1to 3 mL of 5N H2SO4 and washed with 2.5 mL of 5N H2SO4 and twicewith 2.5 mL of 2.5N H2SO4. The resin was then rinsed with 10 mL of1N HCl and 2.5 mL of 0.8N HNO3. Rhenium was collected in 10 mLof 8N HNO3 and further purified using 0.1 mL resin columns.

2.1. Gold Standard

The gold standard was used to develop the chemical techniques andto demonstrate reproducibility. Two lots of 99.96% pure, 0.8 to 1.5micron, spherical gold powder of the same stock number, but ofdifferent lot numbers were obtained from Aestar™. The fine size andprocessing of the powder was expected to have a homogenizing effectand would likely minimize intersample osmium variation, i.e., the“nugget effect” (e.g., Walker et al., 1989b; Freydier et al., 1997). The187Os/188Os ratios of lot A, were within 2-sigma error of each other andhad a mean value of 0.177, with a standard deviation of 0.14%.Replicate analysis of sample Au-powderA-1 were virtually identical(std dev of,0.1%) (Table 1 and Fig. 4). Analyses of lot B displayedmore187Os/188Os variability and were not within 2-sigma error of eachother. The mean187Os/188Os value was also 0.177 and had a standarddeviation of 0.42%. Osmium concentration of the gold standards re-vealed slightly more variation, ranging from 43 to 91 ppt, with anaverage of 70 ppt Os. Osmium standards of similar concentrationsreproduce to better than 0.1% and it is likely that the higher variabilityof 187Os/188Os and Os concentrations in the gold powder is due to realheterogeneity and not measurement error. Rhenium was separated fromthe gold powder using either HCl or HNO3 as the eluent and as discussedabove, led to poor ionization of Re, however the concentration of Reseparated from gold powder was typically at or below detection limits.

2.2. Samples

Samples analyzed in this study represent a) a gravity concentratefrom the Western Areas Gold Plant and b) gold and pyrite from

Fig. 1. Simplified geologic map of the Witwatersrand basin modified from Barnicoat et al., 1997, showing samplelocations, stratigraphic column and ages (summarized in: Robb and Meyer, 1995) of major groups.

2150 J. Kirk et al.

Fig. 2. Schematic diagram illustrating the three models proposed for the origin of gold within the Witwatersrand basin.Model 1 illustrates an unmodified placer model in which the TRD ages of the gold in the basin are equivalent with the sourcegold. Model 2 illustrates a modified placer model in which the TRD ages of the gold in the basin could be as old as the ageof source gold or younger and equivalent to the age of introduction of fluids. Model 3 illustrates a metamorphic/hydrothermal model in which TRD ages of the gold in the basin would reflect the younger ages of the hydrothermal fluidsintroduced into the basin.

2151Origin of gold in the Witwatersrand basin

representative rock sample of Vaal Reef ore from the KlerksdorpGoldfield (Fig. 1 and Table 2).

The gravity concentrate was obtained through the Placer Domecorporation from the Western Areas Gold Plant (located approximatelyfive kilometers east of the Carletonville Goldfield) and represents anapproximately 70:30 mixture of ore from the Upper Elsberg andVentersdorp Contact Reefs, respectively (A. Cheatle, written commu-nication 2000). It comprises 75 volume percent gold, with the remain-ing 25 volume percent a mixture of pyrite, arsenopyrite, alumino-

silicates, Fe-Oxide, galena and uraninite. Eight separate splits of thegravity concentrate were analyzed (WA-Au-1 through WA-Au-8) aswell as five splits of hand-picked gold from the concentrate (WA-Au-9though WA-Au-13, noted with asterisk in Table 2).

The whole rock sample from the Vaal Reef was obtained throughAngloGold from their Vaal River operations 10 km east of Orkney and2.5 km southwest of Great Noligwa (Vaal Reefs 8 shaft). The sampleis a fine-grained quartzitic conglomerate with angular to subroundedhorizontally fractured quartz clasts 5 to 10 mm in size. The conglom-erate is clast supported and is approximately 10% matrix and 90%clasts. The matrix consists of sericite, fine-grain quartz and carbonseams and patches. The majority of the pyrite/arsenopyrite, and;10%of the visible gold are within the quartz and sericite matrix, while;90% of the gold, the uraninite and some of the pyrite are confined tothe carbonaceous material. The sample was crushed, sieved to;250

Fig. 3. Geochronology of the Witwatersrand basin. Limits on the age of basin formation are shown in shaded gray area.Maximum and minimum ages of the basin are constrained by igneous zircon ages from volcanic rocks that bracket the basin.Division between West- and Central Rand groups is from an igneous zircon age of the Crown lava just below theWest/Central Rand transition. All geochronology besides osmiridium model ages (Hart and Kinloch, 1989) and Re-Os data(this study) are from various sources, summarized in Robb and Meyer, 1995 and Zartman and Frimmel, 1999. U-Pb/Pb-Pbages are from pyrite, sphalerite, rutile, uraninite, and carbonaceous seams, while K-Ar ages are from clays, shales and micas.Osmiridium model ages of Hart and Kinloch, 1989, were recalculated using the same Re decay constant and present daymantle 187Os/188Os ratio as our age calculations. Error bars associated with TRD ages of the Witwatersrand gold arecalculated with error associated with present day mantle187Os/188Os value of Meisel et al., 2001 (0.12966 0.01).

Table 1. Os concentrations and isotopic compositions of gold pow-ders.

Sample name Re ppb Os ppt 187Os/188Os

Au-powderA-1 — 78 0.177(1)*Au-powderA-1B — 79 0.175(2)Au-powderA-2 — 72 0.1760(8)Au-powderA-3 — 70 0.1783(8)Lot A mean — 75 0.177std dev — — 0.14%Au-powderB-4 — 70 0.1741(8)Au-powderB-5 — 81 0.1716(7)Au-powderB-6 — 58 0.1783(6)Au-powderB-7 — 60 0.183(2)Au-powderB-8 — 91 0.180(1)Au-powderB-9 — 43 0.1746(9)Lot B mean — 67 0.177std dev — — 0.42%

* Numbers in parenthesis are 2-sigma error in last digit.

Fig. 4. Plot of187Os/188Os for two separate gold powders. Samplesof powder A (diamond symbols) show high reproducibility within thesame sample (1 and 1B) and overall high reproducibility (0.13%standard deviation). Powder B (squares) has more scatter and a stan-dard deviation of 0.4%. Error bars shown are fixed at the largestmeasured 2-sigma error of 0.002.

2152 J. Kirk et al.

mm size and then separated using heavy liquids to preferentially sep-arate gold and other heavy minerals (VR-Au-1 and VR-Au-3). Gold(VR-Au-2 and VR-Au-4) and pyrite were then picked by hand (seeTable 2).

Osmiridium (OsIr alloy) or other high concentration Os-bearingphases were not found by back-scattered and qualitative energy dis-persive system (EDS) analysis in either the Western Areas gravityconcentrate or in a polished section of Vaal Reef rock. Gold samplesranging in weight from 0.002 to 0.009 g and consisting of approxi-mately 10 to 100 gold grains were hand-picked under a binocularmicroscope to ensure pure separates and the absence of other opaquephases that may contain Re and Os.

3. RESULTS

The Re-Os data for all Witwatersrand samples are listed inTable 2. Osmium concentrations of gold from Western Areasrange from 224 ppb to 10350 ppb. Rhenium concentrations arebetween 3 ppb and 31 ppb. Measured187Re/188Os ratios rangefrom 0.010 to 0.185. Measured187Os/188Os values are between0.10562 to 0.10993. Multiple runs of sample WA-Au-1 arenearly identical, with a mean187Os/188Os value of 0.10621, anda standard deviation of 0.0045%.

Although the data on Gold from the Vaal Reef are morelimited, Os concentrations are 73 ppb and 4162 ppb. Rheniumconcentrations range between 2.5 ppb and 32 ppb.187Re/188Osratios are 0.151 and 0.011. Measured187Os/188Os ratios are0.105336 0.00002, 0.10626 0.0004 and 0.108486 0.00004.

Four pyrite separates from the Vaal Reef have Os concen-trations of 0.26 to 0.68 ppb, Re concentrations of 1.7 to 2.8 ppband 187Re/188Os ratios between approximately 14 to 87. Themeasured187Os/188Os ratios of the pyrite samples range from0.840 to 4.68.

3.1. Geochronolgy

The Re-Os isotopic data were used to determine rheniumdepletion ages (TRD), and model ages (TMA) of the Witwa-tersrand gold and direct ages on the pyrite (Table 2). TRD ages(Walker et al., 1989a), may represent the age at which thesample was removed from a convecting mantle. Rhenium in thesample is assumed to be secondary and accordingly, is not usedto correct for the in-growth of radiogenic187Os from the decayof 187Re (line A-B, in Fig. 5a).TRD ages therefore represent aminimum age of separation from the mantle. Rhenium deple-tion ages (TRD) have yielded useful geochronologic informa-tion on samples that have subchondritic187Os/188Os ratios,such as depleted mantle xenoliths (e.g., Walker et al., 1989a;Pearson et al., 1995; Chesley et al., 1999) and minerals such asosmiridium (Hart and Kinloch, 1989; Hirata et al., 1998) thatincorporate very little Re into their crystal structure.

Model ages (TMA) (Luck and Allegre, 1984), assume that theRe in the samples is primary and use the measured187Re/188Os

Table 2. Re and Os isotopic compositions and calculated TRD and TMA ages.

Location Sample nameSample size

(g)Re

(ppb)Os

(ppb) 187Re/188Os 187Os/188Os TRD(Ga) TMA(Ga)

Western Areas WA-Au-1 0.316 — — — 0.10604(2)** 3.4 NA1 repeat — — — 0.10613(2) 3.4 NA1 repeat — — — 0.10609(2) 3.4 NA1 mean — — — 0.10621 3.4 NAstd dev 0.0045%WA-Au-2 0.271 — — — 0.10604(2) 3.4 NAWA-Au-3 0.070 — — — 0.10562(1) 3.5 NAWA-Au-4 0.069 — — — 0.10649(1) 3.4 NAWA-Au-5 0.102 — — — 0.10641(3) 3.4 NAWA-Au-6 0.012 — — — 0.10673(4) 3.3 NAWA-Au-7 0.012 27 10350 0.013 0.10695(8) 3.3 3.4WA-Au-8 0.011 18 6989 0.012 0.1099(1) 2.9 3.0WA-Au-9* 0.002 — 910 — 0.1065(6) 3.4 NAWA-Au-10* 0.002 — 224 — 0.106(4) 3.4 NAWA-Au-11* 0.004 3 1095 0.013 0.1071(4) 3.3 3.4WA-Au-12* 0.006 31 818 0.185 0.1071(1) 3.3 5.9WA-Au-13* 0.006 3 1387 0.010 0.1083(3) 3.1 3.2median 1095 0.1067 3.3

Vaal reef VR-Au-1 0.030 10 4162 0.011 0.10848(4) 3.1 3.2VR-Au-2* 0.002 2.5 73 0.151 — NA NAVR-Au-3 0.032 — — — 0.10533(2) 3.5 NAVR-Au-4* 0.009 32 — — 0.1062(4) 3.4 NA

VR-pyrite-1* 0.119 1.7 0.68 14.00(28) 0.840(14) NA NAVR-pyrite-2* 0.205 2.2 0.30 48.54(160) 2.55(8) NA NAVR-pyrite-3* 0.187 2.6 0.30 60.08(229) 3.20(11) NA NAVR-pyrite-4* 0.252 2.8 0.26 87.31(325) 4.68(17) NA NA

* Free gold (10–100 gold grains) or pyrite that has been hand-picked.** Numbers in parentheses are 2-sigma uncertainty in the last decimal places of187Os/188Os ratios using machine error or by varying blank from

1–4 pg, whichever was greater.– Not measured.NA not applicable.

2153Origin of gold in the Witwatersrand basin

ratios to correct for the growth of the187Os/188Os ratio from thedecay of 187Re. The TMA age is the age at which the Re-corrected187Os/188Os ratio is equal to the187Os/188Os of themantle evolution curve (line A-C, in Fig. 5a). As187Re/188Osapproaches zero, the TMA age approaches the TRD age. Modelages (TMA) are older than TRD ages, and this either reflectsincomplete removal of Re at the time of sample formation orover-corrections for187Os growth due to additions of Re, afterthe original Re depletion event (e.g., Walker et al., 1989a).

Both TRD and TMA ages make a number of assumptions thatmust be fulfilled if the calculated ages are to be meaningful.One of the assumptions is that the Os evolution curve of themantle is known. The mantle Os evolution curve is constrainedby the initial187Os/188Os ratio of the mantle at 4.56 Ga and the187Os/188Os of the present day mantle. The initial187Os/188Osof the mantle at 4.56 Ga has been determined by measurementsof iron meteorites, with187Os/188Os ranging from 0.0948 to0.09604 (reviewed in Shen et al., 1996). The187Os/188Os of the

present day mantle has been established from studies of mantle-derived rocks such as abyssal peridotites, MORB, OIB, andmantle xenoliths (e.g., Meisel et al., 1996; Shirey and Walker,1998). Accepted187Os/188Os values for the present day mantlehave varied (see review in Shirey and Walker, 1998). The TRD

ages presented here use a present day mantle187Os/188Os valueof 0.12966 0.001 from a recent study by Meisel et al. (2001)on suites of mantle xenoliths from around the world. The errorsassociated with the TRD ages are calculated from the errorassociated with present day mantle187Os/188Os value of Meiselet al. (2001). If the previously accepted present day mantle187Os/188Os value of 0.1276 (Shirey and Walker, 1998) is used,the TRD ages of the gold are approximately 0.28 Ga younger.However, this would have no effect on the pyrite isochron age.

3.1.1. TRD/TMA results

Calculated TRD and TMA ages are shown in Table 2, Figure3 and Figure 5b. The TRD ages of Western Areas gold rangefrom 3.5 Ga to 2.9 Ga, with a median age of 3.3 Ga. The TRD

ages of the Vaal Reef gold samples are 3.1 Ga, 3.4 Ga and 3.5Ga, agreeing well with Western Areas gold samples.

TMA ages are approximately 0.1 Ga older than the TRD agesfor all but sample WA-Au-10, whose TMA age is unreasonablyold at 5.9 Ga. Because rhenium is relatively mobile undersurficial conditions, during weathering processes and in hydro-thermal environments (e.g., McCandless et al., 1993; Chesleyet al., 2000; Peucker-Ehrenbrink and Hannign, 2000), thehigher Re concentration and187Re/188Os ratio, but unradio-genic187Os/188Os ratio of sample WA-Au-10, is therefore mostlikely a result of Re addition subsequent to original crystalli-zation. For this reason TRD ages are used in the figures anddiscussion.

Re-Os isotope data of pyrite from the Vaal Reef define alinear array on an isochron diagram (Fig. 6) that corresponds toan age of 2.996 0.11Ga (MSWD5 0.77). The initial187Os/188Os ratio determined for these samples is 0.1246 0.037.

4. DISCUSSION

To evaluate our data and the relative proportion of detritalversus hydrothermal gold, we review the three major modelsproposed for the origin of the gold in the Witwatersrand basinand the possible consequences to the Os systematics of thegold. Model 1, describes an unmodified placer deposit, model2 describes a modified placer deposit and model 3 represents ametamorphic/hydrothermal model.

4.1. Model 1-(Unmodified Placer)

This model assumes that all of the gold in the basin isdetritus from a source area(s) older than the end of basinsedimentation with no significant hydrothermal input or mod-ification (e.g., Mellor, 1916; Hallbauer and Utter, 1977). Theage of the Witwatersrand basin (West and Central Rand groups,Fig. 1) is constrained by volcanic rocks that cover and underliethe basin sediments. U-Pb ages on igneous zircons (summa-rized in Robb and Meyer, 1995) in the volcanic rocks of theDominion Group, which lie below the basin, estimate the onsetof Witwatersrand basin formation at 3.07 Ga. The end of basinsediment deposition and Central Rand group deposition is

Fig. 5. Model ages. (A) Graphically demonstrates the differencebetween TRD and TMA model ages. Path A to B represents a TRD ageand assumes no ingrowth of radiogenic187Os and hence is a minimummodel age. Path A to C represents a TMA age calculated on the samehypothetical sample. The correction for Re decay makes TMA agesolder but the ages approach TRD ages as187Re/188Os ratios approachzero. (B) Illustrates the range and median187Os/188Os ratios of Wit-watersrand gold analyzed, as well as the calculated TRD ages based onthese ratios. The187Os/188Os ratios of samples range from 0.1056 to0.1099 and correspond to TRD ages between approximately 3.5 Ga and2.9 Ga, with a median age of 3.3 Ga. Ages were calculated with valuesof 187Re/188Os 5 0.401 (Walker et al., 1994),187Os/188Os 5 0.1296(Meisel et al., In Press) for the present day mantle; and a187Re decayconstant ofl 5 1.666E211 y21 (Smoliar et al., 1997).

2154 J. Kirk et al.

obtained by the age of the overlying Ventersdorp lavas at 2.71Ga. The majority of gold production is centered in the CentralRand group (e.g., Phillips et al., 1989), whose maximum age isconstrained by an igneous zircon age of 2.92 Ga on the Crownlava, which occurs near the top of the West Rand group. Theage of the sediments supplied to the Witwatersrand basin areconstrained by detrital zircon and monazite U-Pb ages, andrange from approximately 2.8 Ga to 3.3 Ga, with most agesoccurring at 3.20 Ga and 3.08 Ga (Robb and Meyer, 1995 andFig. 3). Analysis of the chemistry of shales and the presence ofboth heavy minerals, zircon and chromite, suggest that thesource area for the sediments (as well as gold in a placer model)was a mixture of variable proportions of felsic and mafic/ultramafic rocks (e.g., Pretorius, 1974).

The best evidence for a detrital origin for gold comes from:

1. The effective confinement of gold to conglomerate layers(reefs) and the relationship between gold production and theconglomerate thickness and amount of sorting.

2. The morphology of pyrite, quartz clasts and gold (e.g.,Minter, 1976; 1999; Phillips et al., 1989).

3. Association of gold with unconformity surfaces (footwall of

conglomerate beds) (e.g., Mellor, 1916), inferred fluvialchannels, deflation surfaces (e.g., Robb and Meyer, 1995)and with cross-bed forsets (Minter et al., 1993).

4.2. Model 2-(Modified Placer)

This model assumes most of the gold in the basin wasultimately detrital in origin, but with subsequent modificationby metamorphic/hydrothermal fluids (e.g., Robb and Meyer,1995). Varying degrees of local mobilization and re-precipita-tion of gold have been used to explain the two distinct appear-ances of gold within the basin and can be related to the amountof inferred fluid to rock interaction (Frimmel and Gartz, 1997).Two scenarios have been proposed to account for the authi-genic appearance of much of the gold and the observed litho-logic and sedimentary associations. It has been suggested thatthe mobilization of gold takes place by complete dissolutionand re-precipitation on a very small scale, or that detrital goldacted as a site of nucleation around which secondary gold wasprecipitated (e.g., Frimmel et al., 1993; Robb and Meyer,1995).

Fig. 6. Re-Os isochron diagram for Vaal Reef pyrite samples. Data yields age of 2.996 0.11 Ga (n5 4, MSWD5 0.77).Error crosses for samples determined by varying the Os blank between measured values of 1 to 4 pg.

2155Origin of gold in the Witwatersrand basin

4.3. Model 3 (Metamorphic/Hydrothermal)

Proponents of a metamorphic/hydrothermal model assumethat gold was transported in solution from a source outside ofthe basin, with deposition occurring sometime between 2.7 and2.0 Ga, from metamorphic or hydrothermal fluids. Pressure-temperature conditions are thought to have been 350°C650°C, pressures up to 3 kbars and gold transported as a bisulfidecomplex (e.g., Phillips and Law, 1994). Subsequent to fluidinflux into the basin, reactions with lithologic and sedimento-logically associated Fe-S and C phases are called upon for goldprecipitation. Hydrothermal fluid flow is thought to be in small-scale structures along lithologic boundaries such as beddingplanes and sedimentary foresets (Barnicoat et al., 1997). Fluidstransporting gold are thought to be influenced by larger faultsas well (e.g., Phillips and Myers, 1989). The nature of thismodel requires a relatively large fluid flux from outside of thebasin, with significant interaction with mid to lower crustalmaterials (e.g., Phillips and Myers, 1989; Kerrich and Cassidy,1994)

4.3.1. Os consequences to a placer model

Gold introduced into the basin through sedimentary pro-cesses would, like the sediments, be older than the end of basinsedimentation and their age would presumably reflect that ofthe detrital source (Fig. 2). If detrital gold remained a closedsystem since its crystallization, then the187Os/188Os measuredin the gold (187Os/188Os)m, would be a function of its initial187Os/188Os ratio,(187Os/188Os)i, and would change over timebecause of addition of187Os from the decay of187Re. For goldsamples with low187Re/188Os, such as those analyzed from theWitwatersrand basin, the (187Os/188Os)m approximates the(187Os/188Os)i ratio. Therefore, the (187Os/188Os)m would be amaximum estimate of the (187Os/188Os)i. If the gold is detritalwith no subsequent modification, then the (187Os/188Os)m ratiosreflect the Os isotopic composition originally incorporated intothe gold before placer transport into the basin.

The Witwatersrand gold samples analyzed have (187Os/188Os)m ratios analogous to the mantle at 3.5 to 2.9 Gyr ago(Table 2 and Fig. 5b). The;3.0 Ga age pyrite isochron age isconsistent with the range of TRD ages of the gold. The medianTRD age of 3.3 Ga for the Witwatersrand gold and the;3.0 Gaage of the pyrite are within error of the peaks of detrital zirconand monazite ages of Witwatersrand sediments (Robb andMeyer, 1995) and are in good agreement with the age ofosmiridium grains. In this model, the range of Os concentra-tion, 187Os/188Os ratios and calculated TRD ages probablyreflects a mixture of gold with different Os concentrations fromsource rocks of different ages and therefore different187Os/188Os ratios for the gold.

4.3.2. Os consequences to a modified placer model

As in model 1, the187Os/188Os ratio of the detrital goldreflects the ultimate age and source of the preplacer gold.However, subsequent hydrothermal modification by crustal flu-ids is likely to increase the187Os/188Os ratio because of therapid growth of187Os in crustal materials. Unlike other isotopicsystems, where both parent and daughter elements are incom-patible, Re behaves relatively incompatibly, while Os is com-

patible during mantle melting events (e.g., Hertogen et al.,1980; Luck and Allegre, 1980; Walker et al., 1988). Thisbehavior results in mantle melts and crustal materials havingsmaller Os concentrations and larger Re/Os ratios than themantle. Therefore, the higher Re/Os ratios of crustal materiallead to a preferentially rapid growth of187Os/188Os ratiosrelative to the mantle. Fluids dissolving and transporting goldfrom crustal material would also dissolve and transport Re andOs (see below) and would take on the elevated187Os/188Osratios of the crustal material.

The mobility of gold under a variety of hydrothermal andmetamorphic conditions has been well documented (e.g.,White, 1981; Maiden, 1984; Seward, 1984). The presence ofeconomic gold deposits of hydrothermal and metamorphic or-igin show that natural fluids are capable of mobilizing andconcentrating large amounts of gold. A number of ore depositsdisplay characteristics that indicate that both Au and the PGEshave been transported in the same fluid (Mogessie et al., 1991;Watkinson and Ohnenstetter, 1992; Mernagh et al., 1994; Birdet al., 1997). However, experimental data (Xiong and Wood,2000), emperical evidence from hydrothermal sulfides (Walkeret al., 1989b; Frei et al., 1998; Mathur et al., 1999; McInnes etal., 1999;) and gold (Mathur et al., 2000; Kirk, UnpublishedData), as well as direct evidence from volcanic ridge fluids(Sharma et al., 2000), all suggest that hydrothermal fluids canonly carry ppt to low ppb concentration levels of Os. Highwater:rock ratios in combination with efficient precipitationmechanisms of Os from a hydrothermal fluid with low Osconcentration could explain the high Os concentration of theWitwatersrand gold, however, as discussed below, can notaccount for the unradiogenic187Os/188Os ratios.

4.3.3. Local remobilization and precipitation

The Os isotopic composition and Os concentration in locallymobilized and re-precipitated gold would be a function of theisotopic composition and concentration of the Os from thedissolved detrital gold and that of the mobilizing fluids. How-ever, the elevated187Os/188Os ratios of the mobilizing fluids inisotopic equilibrium with the dissolved detrital gold wouldsignificantly alter the original187Os/188Os ratio of the re-precipitated detrital gold.

Hydrothermal/metamorphic fluids in the Witwatersrand ba-sin are thought to be derived from either burial metamorphismof crustal materials within the basin (e.g., Frimmel et al., 1993)or from the devolitization of crustal basement rocks (Phillipsand Myers, 1989). Provenance studies suggest that the majorityof basin sediments were formed at 3.2 and 3.0 Ga, in additionthe majority of the basement rocks formed before 3.1 Ga (Robband Meyer, 1995). Therefore, models of basin dewatering ordevolitization of basement rock between 2.7 to 2.0 Ga (e.g.,Phillips and Myers, 1989) require evolution of crustal sourcerocks for a minimum of 0.5 to 1.0 Gyr. The high Re/Os ratiosin crustal rocks suggest that the Os of a fluid in equilibriumwith these rocks will be very radiogenic (187Os/188Os 5 0.52after 0.5 Gyr and0.95 after 1.0 Gyr; using average upper conti-nental crust187Re/188Os values of 50, Esser and Turekian, 1993).Mixing calculations between the Os dissolved from postulateddetrital gold (Os51ppm,187Os/188Os50.105) and the Os carriedin a mobilizing fluid (Os5100 ppt,187Os/188Os5 0.52 or 0.95)

2156 J. Kirk et al.

illustrate that to retain the low187Os/188Os values measured in thegold, water:Au ratios must be kept below approximately 100:1.This implies that the fluids responsible for gold mobilizationhad an anomolously high carrying capacity for Os relative tothe ppt to low ppb levels found in natural fluids (Sharma et al.,2000) and experimental data (Xiong and Wood, 2000) or thatwater:Au ratios exceeded 100:1. However, if the water:Auratios exceed 100:1, then the unradiogenic Os from the dis-solved detrital gold would be sufficiently diluted by the radio-genic Os from the mobilizing fluid to increase the187Os/188Osratios beyond what is found in the Witwatersrand gold (Fig. 7).

A modified placer model is compatible with our data and thetextural and spatial associations of gold. However, Au mustremain largely in-place, as the high concentration of Os andunradiogenic187Os/188Os ratios measured in gold rule outsignificant hydrothermal transport. If there is an outside hydro-thermal component added to the gold and Os, then it is likelyas an overprint and by virtue of the low concentration of Os inhydrothermal fluids, would not significantly alter the low

187Os/188Os ratios and old TRD ages of the high Os concentra-tion core of the unmobilized detrital gold.

In a modified placer model the large range of Os concentra-tion and 187Os/188Os ratios in the gold suggests that this iseither a result of original variation in the detrital gold or a resultof samples averaging gold grains with differential amounts ofhydrothermal addition and/or remobilization.

4.3.4. Os consequences to a metamorphic/hydrothermalmodel

As discussed in model 2, a metamorphic or hydrothermalfluid carrying gold can also complex osmium, although at muchlower concentrations (see also Marcantonio et al., 1994; Panand Wood, 1994). The Os isotopic composition of hydrother-mal gold precipitated from an aqueous fluid should solely be afunction of the Os isotopic composition of the fluid from whichit precipitated. If gold is transported into the basin by a hydro-thermal fluid between 2.7 to 2.0 Ga (e.g., Phillips and Myers,1989), the minimum values of187Os/188Os would be from

Fig. 7. Mixing diagram between Os derived from dissolved detrital gold (Os5 1ppm,187Os/188Os5 0.105) and the Osin a mobilizing fluid (Os5 100 ppt,187Os/188Os 5 0.52 or 0.95). The Os isotopic composition in the mobilizing fluid(assumed to be in isotopic equilibrium with crustal material which formed at 3.2 Ga with a187Os/188Os initial of 0.1066and allowed to age for 0.5 or 1.0 Gyr) was calculated using average upper continental crust187Re/188Os ratios (187Re/188Os5 50, Esser and Turekian, 1993). Percentages refer to the proportion of the total Os derived from the mobilizing fluid,and were used to calculate approximate water: Au ratios and the Os concentration of a resulting fluid, which contains Osderived from the mobilizing fluid plus the Os derived from the dissolved detrital gold. Water: Au ratios below 100:1 areunlikely because the concentration of Os in the resulting fluid would exceed the carrying capacity of natural fluids(references in text). This figure illustrates that if water: Au ratios exceed 100:1, then the unradiogenic Os in the detrital goldis sufficiently diluted by the radiogenic Os from the mobilizing fluid to increase the187Os/188Os ratios beyond what is foundin the Witwatersrand gold.

2157Origin of gold in the Witwatersrand basin

fluids derived directly from the convecting mantle. However, at2.5 Ga for example, the mantle187Os/188Os ratio was about0.112 (TRD age of 2.5 Ga), elevated above the highest187Os/188Os value found in the Witwatersrand gold analyzed. A fluidwith Au and Os derived from crustal interaction would havesignificantly higher187Os/188Os ratios (0.52–0.95; see hydro-thermal end-members, Fig. 7).

Any fluid bringing gold into the basin at 2.7 to 2.0 Ga, fromany well understood reservoir, would take on the elevated,suprachondritic,187Os/188Os ratio of the source area and wouldhave 187Os/188Os ratios much higher than the subchondriticratios measured in the Witwatersrand gold samples.

The pyrite isochron indicates that the Re-Os system wasclosed with respect to pyrite for approximately 3 billion years,which would not be predicted from the high fluid to rock ratiosassumed in this model. The older age of the pyrite isochron andthe model ages from gold, predate the end of basin formationand are thus inconsistent with gold introduction by hydrother-mal fluids derived from outside of the basin.

5. CONCLUSIONS

Gold from the Witwatersrand basin have very high concen-trations of Os and low Re/Os ratios. The unradiogenic187Os/188Os ratios, old TRD/TMA ages of the gold and the isochronage of pyrite are older than the end of basin sedimentation andare in agreement with ages of detrital zircon, monazite andosmiridium grains. The combination of these data all support anoriginally detrital origin for the bulk of gold deposition in theWitswatersrand basin. The above data in conjunction with thelimited solubility of Os in hydrothermal solutions and the highconcentration of Os measured in the gold indicate that modelsinvolving significant hydrothermal transport of gold after theformation of the Witswatersrand basin are unlikely.

Acknowledgments—Analytical work has been funded through the Na-tional Science Foundation Grants EAR 9708361, and EAR 9628150.Re-Os analyses were performed at the W. C. Keck laboratory at theUniversity of Arizona. We would like to thank Mark Baker for man-aging the isotopic laboratory. Nic Fox, of AngloGold for help inproviding Vaal Reef samples. We appreciate thoughtful reviews by T.McCandless, J. Patchett, P. Klipfel and R. Mathur, as well as reviewsby W. E. L. Minter and an anonymous reviewer for Geochimica etCosmochimica Acta that have greatly improved this manuscript.

Associate editor:S. A. Wood

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2159Origin of gold in the Witwatersrand basin