Alternative Model for the Derivation of Gold in the Witwatersrand Supergroup

J. geol. Soc. London, Vol. 141, 1984, pp. 263-272, 3 figs., 3 tables. Printed in Northern Ireland.

Alternative model for the derivation of gold in the Witwatersrand Supergroup

Thomas 0. Reimer

SUMMARY: Present models of the derivation of the gold in the Witwatersrand conglomerates of South Africa as detrital grains directly from primary deposits in a source area consisting mainly of Archaean schist belts meets with a volume problem. Witwatersrand deposits have yielded about 923 kg/km2 Au compared to about 65 kg/km* Au in the richest known schist belts. Differences in the extent of mining activities cannot account for these differences in yield. Gold fineness, grain size, and morphology of the gold particles are also difficult to reconcile with a purely detrital derivation. Data on the mobility of gold in the hydrosphere and in the weatheringcycle are used to erect a model which predicts that mostof the gold in the Witwatersrand conglomerates was derived from a source area in trace amounts in a ‘dissolved’ form and precipitated at or close to the edge of the depositional basin, possibly under organic influences. Subsequent sedimentary reworking and metamorphiddiagenetic remobilization led to the formation of the complex association of minerals observed now in the conglomerates.

Since the dawn of civilization an estimatedtotal of data irreconcilable with the ‘mechanical’ model of about 65 000 tons of gold has been produced. Of this derivation of the Witwatersrand gold. Atthe same about 55% has beenderived fromthe auriferous time an alternative model for the processes which led conglomerates of the early Proterozoic Witwatersrand tothe spectacularconcentration of gold in these Supergroup (2.3-2.7 Ga.). Including the proven re- sediments is proposed and its feasibility in the light of serves, this sedimentary basin with a totalarea of geochemical data is shown. about 39 000 km2 (Fig. 1) accounts for about 60% of all gold produced and available (Pretorius 1974). Gold yield The spectacular gold content of these conglomerates was first explained by Hatch (1906) as having been Data on gold productionfrom various types of derived from the auriferous quartz lodes in the ancient deposits are compiled in Table1.In the Wit- schist belts. This model was refined by Viljoen et al. watersrand gold fields atotal of about36000 t has (1970) who related secular changes in the mineralogy beenproduced fromthe conglomerates of the Wit- of the sediments to certain stratigraphic successions in watersrand Supergroup proper. If this is related to the the source area.The model was rejected as an total size of the basin, including the non-producing oversimplification by Reimer (197%) mainly onthe areas, a specific gold yield of 923 kg/km2 is found. For ground that it would require an unfolded condition for individual mines such as the new Elandsrand Mine on the rocksin theArchaean schist belts for up to the West WitsLine (Fig. 1) which exploits a single 600 Ma. after their deposition. conglomerate-in this case theVentersdorp Contact Koeppel & Saager (1974) found a similarity between Reef-the specific yield of mineable gold is 38 200 kg/ theU/Pb-isotope ratios in allogenic pyrites of the km2 over an area of about 19 km2. For the Western Witwatersrand and pyrites from gold deposits in the Deep Levels Mine on the West Wits Line, which Barberton schist belt. Trace element dataon these exploits up to three conglomerate beds over an area of pyrites do not support such similarities (Utter 1978). about 40 km2, the specific yield of mineable goldis However, it is generally agreed that the gold was about 32 000 kg/km2. deposited largely as detrital grains,although some The greatconcentration of gold in the Wit- precipitation from solutions under organic influences watersrand is also underlined by data on other has been proposed, first by MacGregor (1953), later precious elements in the conglomerates.For the by Pretorius (1966) and Hallbauer (1975). In columnar various elements of the platinumgroup enrichment thucholite,akerogen contained in certain of the ratios (concentration in conglomerates: concentration conglomerates, the last author observed cellular in averagecrust) of up to 133 have beennoted structures consisting of gold.Based on gold produc- (Reimer 1979). For other detritalcomponents the tion data and certain geological considerations Reimer ratios are comparable. The respective value for gold, (197%) rejected the purely mechanical derivation of however, is 6100. The high gold yieldin the the gold from the source in favour of a transport in Witwatersrandsediments, representing the largest solution to the depositional area. concentration of gold known on Earth, together with It is the purpose of this paper to present additional the extraordinarily high enrichmentfactor suggests

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/2/263/4888393/gsjgs.141.2.0263.pdf by guest on 30 September 2021 264 T. 0. Reimer

0post-Wltwatersrand rocks Wltwatersrand basln 0 (outcrop.suboutcrop) pre-Wltwalersrand basement lncl Domlnlon Reef, Zoetl~ef, Pongola, Messma Groups E] schtst belts 8 transport dlrectlons

FIG. 1. Geological sketch map of the Kaapvaal Craton showing position of Witwatersrand basin

unusual conditions for the formation of these deposits. the gold yields can amount to up to 17 000 kg/km2, as The most important pre-Witwatersrand gold source for the Consort Mine in theBarberton belt. In known in southern Africa is represented by the WesternAustralia about 1100 t gold have been hydrothermaland sedimentarydeposits in the produced from the 5 km2 in the Golden Mile of the Archaean schist belts (Table 1). Forthe cratonic Kalgoorlie area, equivalent to a gold yield of about areas, consisting of granites, gneisses, and schist belts, 220 t/km2. gold yields of 3.1-11.2 kg/km2 are found. Forthe The data on gold yields in Table 1 illustrate a major schist belts themselves, after deduction of the general- discrepancy. The Witwatersrand sediments contain 14 ly ‘barren’ granitelgneiss terranes,the average gold times more gold than the average for the Zimbabwe yields vary between 21 and 65 kg/km2. Valuesfor schist belts which per unit area represent the richest individual schist belts are in the same range. Based on known Archaean gold province. Differences in the this distribution of gold yields it can be assumed that extent of mining activity alone are insufficient to the richest schist beltshave orhad yields of upto explain the differences in the gold yield. Even if it is 150 kg/km2. assumed thatthe source area of the Witwatersrand Within the schist belts themselves the majority of sediments extended over twice the area of the present gold occurrences mined are of very small size, the bulk basin, resulting ina maximum transport distance of of the production usually coming from less than about 100 km (Fig. l), the enrichmentfactor would five mines (Anhaeusser 1976). For these richer mines still be about 7:1.

G old production Yield production Gold area (km2) (5, Period (kgikm2)

(A) Sediments Witwatersrand 36.000 39.000 date to 923 Witwatersrand, incl. proven reserves 39,000 reserves proven 51,000 - 1308 (B) Cratons (Archaean parts only) Kaapvaal 80,662 Kaapvaal date240 to 3.0 Zimbabwe 195,762Zimbabwe date2190 to 11.2 Western Australia 700,000 Australia 1975 2200 3.1 Canada 2,200,000 Canada date5370 to 2.4 (C) Schistbelts averages: K aapvaal' (K.) 6 (K.)Kaapvaal' 240 684 date to 36 Z imbabwe (Z.)Zimbabwe 33,735 2190date to 65 Western A ustralia 100,000 Australia21 1975 2100 C anada 150,000 Canada date5370 to 36 Individual belts: B arberton (K.)Barberton 210 3300 date to 64 Murchison (K.) 1600 8 to 1955 5.0 E ersteling (K.)Eersteling 250 to 1.5 1937 6.0 G atooma (Z.) Gatooma 135 4300 31 to 1951 Shamva (Z.) 43 4100 to 1930 10.5

Sources: Reimer (1975a); Anhaeusser (1976); Various mining statistics.

Geochemistry as indicator of Petrographically the Witwatersrand Supergroup is a source area composition highly siliceous, arenaceous sequence containing about 60% quartz. This compares with about 18% quartz in Geochemical and petrographical dataon the Wit- watersrand sediments allow further conclusions as to TABLE2; Ni and Cr contents of early Proterozoic and the feasibility of the detrital derivation of the gold. As Archaean sediments the schist belts are characterized by large amounts of mafic to ultramafic lavas rich in Ni andCr, the Cr(ppm) Ni (ppm) CriNin concentrations of thesetraces in the Witwatersrand Witwatersrand Supergroup sediments are of great interest (Table 2). Calculations 1 GovernmentReef have indicated that the source area of the Archaean shales 249 830 5 3.30 Fig TreeGroup (Ni, Cr values in Table 2) in Jeuuestowntheshales 2 218 706 3.24 5 Barberton schist belt contained about 40% of mafic to 3 Kimberley ultramafic rocks of Onverwacht Group age with an ouartzites 25 90 3.60 27 average of 1520ppmCr and 890ppm Ni (Reimer 4 kmberley conglom- 19756). If the Ni content of the average Witwatersrand erates 45 90 2.00 27 sediment (33% shale, 67% sandstone, quartziteand 5total Witwaters- conglomerate) is comparedto the Fig Treedata a rand (weighted) 319 95 3.36 proportion of about 10% mafic to ultramafic rocks in Dominion Group (-2.8 Ga) the Witwatersrand source area seems to be indicated.Conglomerates 6 282 549 1.94 20 Adding to this the sedimentary and intermediate to Fig Tree Group (-3.3 Ga) felsic volcanic portions of the schist belt sequences, the 7 Greywackes 291 563 1.93 30 total contribution of schist belts to the WitwatersrandShales 8 94 1 543 1.73 11 sediments can beestimated as about 15-20%. This figure includes second-cycle schist belt materialde- Sources: 1 + 2, Hofmeyr (1971); 3 + 4,Rasmussen & Fesq rived from pre-Witwatersrand sedimentary sequences. (1973); 5, this paper; 6, Hiemstra (1968); 7 + 8, Reimer (1975b).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/2/263/4888393/gsjgs.141.2.0263.pdf by guest on 30 September 2021 266 T. 0. Reimer average crustal material. This high quartz content and together with the development of tidal sediments in the widespreadoccurrence of orthoquartzites in the the lowerpart of the Witwatersrandsupergroup lower part of the Witwatersrand Supergroup suggest (Eriksson et al. 1981) suggests thatthe depositional the presence of an extensive arenaceous component in basin must have been open to the contemporaneous the source area. ocean at least during part of the depositional history The average chemical composition of the Wit- and cannot have been a closed intra-montane basin as watersrand sediments is shown in Table 3 (Column 1). frequentlyproposed. In a similar budget calculation In order to obtaina quartz content in the sediments of carried out for the Fig Tree Group of the Barberton 20%,about 60% arenites have tobe deducted as schist belt comparable losses of MgO and especially sourcecomponents. The resulting composition is CaO have been noted (Reimer 197%). presented in column 2, while in column 3 the From the above discussion it can be concluded that composition after further deduction of the 10% mafic the source area of the Witwatersrandsediments to ultramafic rocksreferred to above is given. This consisted of15-20% schist belts (possibly also as residualcomposition is compared tothe average second-cycle material derived from pre-Witwatersrand composition of theCanadian Shield andthe upper sediments); 60% arenitic sediments (possibly correla- crust. For Si02,TiO2, and A1203 there is a reasonable tives of thepre-3 Ga. Pongola Supergroup of the agreement.The high Fe203content could be ex- south-eastern part of South Africa and Swaziland); plained either by a strong banded ironstone compo- 20-2570 other material such as granites and gneisses of nent in the source areaor by the development of the Kaapvaal Craton, as well as felsic to intermediate conditions favourable for the deposition of such iron volcanic rocks of the Dominion Group and Zoetlief formations within the basin itself. The occurrence of ‘System’ (Reimer 1975~). banded ironformations such as the so-called ‘Con- The conclusions from the budget calculations are tortedBed’ in the lower part of the Witwatersrand thus compatible with regional geological arguments Supergroup supports the latter possibility. against the existence of extensive schist belt material The strong deficit in MgO and the extremely strong in the source area of the auriferous sediments (Reimer deficit in CaO is noteworthy. It can be ascribed to a 1975~).If the Witwatersrand gold had been derived large extent to the preferential removal of the alkali from schist belts as detrital grains and if the source earths from the sedimentarymaterial during erosion area was twice the size of the basin, a mean gold yield and transport and their possible deposition as carbon- in the range of 230@3100 kg/km2 would have been ate rocks in a part of the depositional basin no longer needed. Such a value appears unrealistic in view of the preserved. Na20 also shows strong depletion, while data presented in Table 1. for K20 the loss is not so pronounced, considering the Aspointed outabove, 60-70% of the gold in a problemsassociated with such budget calculations. particular schist belt is usually produced by less than This strong removal of themore solubleelements, five mines. A notable increase of the gold yield would thus require an increase in the number of large mines. TABLE 3: Chemical composition of Wit- Based onthe data from the Barberton area it was watersrand sediments estimated that in order to obtain thehigh gold yield of the Witwatersrand, the potentialnumber of large l 2 3 4 5 producing deposits in the schist belts would have to be increased by a factor of 20-30. Even if one assumes Si02 78.01 64.08 69.27 64.93 65.5 extraordinary gold yields such as theone for the TiOz 0.34 0.55 0.51 0.52 0.70 Golden Mile of WesternAustralia, it would still A120, 9.20 13.04 14.56 14.63 14.8 require one such mining district per 35-50 km2. FezOs 5.13 10.56 10.53 4.11 5.1 2.07 4.0 0.60 2.24 2.30 Some of the discrepancy might be explained by a MgO relativeenrichment of gold owing tothe loss of CaO 0.54 0.96 (-1.81) 4.12 4.00 Na20 0.88 0.94 1.89 3.46 3.30 dissolved earth alkalis and clay from the sedimentary K20 1.831.68 3.403.10 2.38 material accumulating in the Witwatersrand basin. L.O.I. 1.38 2.853.00 However, preliminary balance calculations show that it is unlikely thatthe amount of material lost 1, averagecomposition of Witwatersrand sedi- accountedfor more than about 10% of the source ments(33% shale,67% sandstone, quartzite, material. conglomerate); 2, As 1, calculated after deduction Even if one allows for the fact that the gold yield is of 60% arenites; 3, As 2, calculatedafter deduction of 10% mafic to ultramafic volcanic rocks governed by non-geological factors such as the gold (negative value for CaO indicates strong deficit); 4, price, the enrichmentfactor is still of a magnitude Averagecomposition of CanadianShield (from which casts severe doubtson the purely detrital Shaw et al. 1967); 5, Average composition of Upper derivation of the gold proposed in the earlier models Crust (Wedephohl 1967). of the source of the Witwatersrand mineralization.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/2/263/4888393/gsjgs.141.2.0263.pdf by guest on 30 September 2021 Derivation of gold in the WitwatersrandSupergroup 267 Gold fineness Fineness values of gold from schist belts and from the Witwatersranddeposits have been compiled from a variety of sources (Fig. 2a,b). The data for schist belt gold are from total mineproductions, ore samples, and individual gold grains. The mediansrange from fineness values of 825 to 950, and for each of the occurrences the individual values arerather poorly ‘sorted’. In contrast to this the medians for gold from the Witwatersrand and recent placers range mainly from 850 to 925 and the distributions are considerably better sorted. Detailed investigations by Utter (1978) have shown thatthere is no correlationbetween grain size and fineness in gold from the Vaal Reef in the Klerksdorp area of the Witwatersrand. The same author furthermore observed that there is no significant difference in fineness betweendetrital gold, post-depositionally recrystallized gold, and gold in carbonaceous matter. It was also noted that the fineness of gold locally is FINENESS (A%xlOOO) controlled by sedimentary depositional facies. The differencesin fineness and itsdistribution between gold from schist belts and the Witwatersrand (b) Placer gold: 1, Witwatersrand,Viljoen (1971). 2, Yukon 99 area (Hester 1970). 3, Alaska (Schneiderhohn 1962). All others from Utter (1978). 97 95 sediments are in keeping with observations on modern 90 placers which usually have higher fineness values than the possible primary gold sources. This is ascribed to a - 80 preferential solution of silver under present oxidizing vf 70 atmosphericconditions. Hallbauer & Utter (1977) 4 60 rejected the possibility of such processes in the Witwatersrand due to a supposed lack of free oxygen 0%so E 40 in theatmosphere at the time. They, as well as 30 Schidlowski (1968) noted an absence of margins 5 20 depleted in silver in the detrital gold grains of the z Witwatersrand. These, and the other data presented 10 I above indicate that the gold must have gone through 3 v5 some process of homogenization before its incorpora-

3 i4r’i i 1A-i i i i ~ 1 j tion into the sediments. Sedimentary facies control of 31 ’6l!!Il!!l the gold fineness can best be explained by formation of 1 [!v!! the gold particles, possible from solutions, almost 650 700 750 800 850 900 960 contemporaneously with or slightly prior to becoming FINENESS (AxlOOO) subjected tothe depositional processes because the AAA9 grain size distribution (Fig. 3) shows similarity to FIG. 2. Distribution of gold fineness. unequivocal placersand may indicate reworking of (a) Schist belt deposits: precipitated gold. Although this would be in contrast 1, Mazoe schist belt (Zimbabwe), mines with the usual assessment of gold as a noble element, (Phaup & Dobell 1938). 2, Umfuli schist belt, there is supporting data available on the mobility of as(1). 3,Zimbabwe, ore samples (Eales gold under surface conditions. 1961). 4, Barberton schist belt. mines (Anhaeusser 1969). 5, Jessie Mine(Zim- babwe), gold grains (Eales & Viljoen 1973). 6, Grain size Gwanda schist belt (Zimbabwe), mines (Eales 1961). 7,Barberton schist belt, gold grains Proponents of the purely detrital derivation of the gold (Viljoen 1971). 8, Barberton schist belt,ore concede that considerablea portion, if not the samples (Gay 1968). majority presentof itsit in form originated from

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/2/263/4888393/gsjgs.141.2.0263.pdf by guest on 30 September 2021 268 T. 0. Reimer The abovedistance of30 km would also have to include the considerable ‘distance’ covered by a gold grain during strong,but more or less stationary, sedimentary reworking. The possible distance of the gold sourcefrom the site of deposition is thus still further reduced. Gold mobility Data on the behaviour and abundance of gold in the weathering cycle and the biosphere is plentiful and indicates that gold can be called a noble element only with limitations (Boyle 1979). Gold readily forms complex ions with chlorides, thiosulphides, and especially cyanides. The last appearto be rather wide- spread in soils where they originate by hydrolysis of cyanogenic plants, of which several hundred, including FIG.3. Grain size distribution of gold. fungi, are known(Lakin et al. 1974). The resulting Primary gold: gold-cyanide complexes may be incorporated to some 1, LonelyMine, Zimbabwe (Eales 1961). 2, extent in the organism, although the process of OlympusMine, (as for 1). 3, Barberton area, incorporation is not fully understood yet. A compila- SouthAfrica (from Hearn (1943) & De Villiers tion of literature data in Mossman & Harron (1983) (1957). shows values of up to 125 pprn Au in higher plants, Placer gold: some of which can be used asindicators of gold 4, Tarkwa River, Ghana (Sestini 1973). 5, Protero- concentrations in the soil. For lichens values of up to zoic Tarkwa conglomerates, Ghana (as 4). 6, (as 5). 7, Witwatersrand(compiled from Liebenberg 1ppm Au havebeen found and 11.6 pprn Au for (1955) Schidlowski(1968), Hallbauer & Joughin fungi. Marine organisms contain up to 126 ppb of gold 1973)). Others from Utter (1979). (see Dexter-Dyer et al., 1984). Other indicators for the mobility of gold in the weathering cycle and hydrosphere are theformation of gold crystals and nuggets in post-depositional recrystallization (Liebenberg 1955). placers and the enrichment in gossans. Leube (1968) The detailedinvestigations of Utter (1979) present reports that in certain tropical areas the mobility of grain size data on detritalgrains from the Vaal Reef of gold is so high that it can be re-enriched in previously the Klerksdorp area (Fig. 1). These are shown in Fig. worked placers to such an extentthat they can be 3 together with size dataon detrital gold grains profitably exploited again after 15-20 years. compiled from various literature sources. The popula- Gold is furthermore enriched in carbonaceous tions arerather well sorted, with mediansbetween sediments.Values of up to112ppb have been 0.05 and 0.15 mm Size data on gold from schist belt reported by Viljoen et al. (1969) fromcarbonaceous deposits are rare, but the populations are usually of tuffs of theArchaean Onvenvacht Group of the smaller size and less well sorted (Fig. 3, No. 2) than in Barbertonarea. Mossman & Harron (1983) report the case of the placer gold. However, the number of values of up to 7.2ppm from pelitic sediments ofof size analyses from schist belt gold are too few to allow theHuronian Supergroup in Canada. Their median an assessment. for samples from 24 localities is about 350 ppb. The most important process for transporting gold in Grain shape waterappears to be in the form of metal colloids stabilized by hydrophilic organic molecules forming a From a comparison of the grain shape of gold from protective coating (Ling & Swanson 1969). Deposition the Witwatersrand and from recent auriferous streams of gold from these colloids will occur when different in the Barberton schist belt, Hallbauer & Utter (1977) chemical environments areencountered. This illus- concluded thatthe Witwatersrand gold was trans- trates clearly, as pointed out by several authors, that ported overnot morethan about 30 km. This biogeochemistry cannot be disregarded as an impor- observation implies thatthe source of the gold was tant factor for the concentration of gold in deposits within a belt of not more than about 50 km around the such as the Witwatersrand. The observation of presentouter edge of the gold occurrences in the cell-shaped gold in columnar thucholite by Hallbauer upper Witwatersrand. As part of this belt is and was (1975) is a point in support of this statement, though underlain by rocks of the lower part of the group the much disputed. actualsource belt would have been still narrower, Indications onthe weathering conditions in the possibly less than 10 km. Witwatersrand source area could be obtained from the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/2/263/4888393/gsjgs.141.2.0263.pdf by guest on 30 September 2021 Derivation of gold inWitwatersrand the Supergroup 269 distribution of the platinoidelements in the con- is visualized: some detrital gold was derivedfrom a glomerates. Although the behaviour of these ‘noble’ number of sources such as primary deposits in the elements in the weathering cycle is not completely schist belts,secondary enrichments in arenaceous understood, it has beennoted that palladium is sediments possibly of Pongola (3.2-3.0Ga.) or of relatively mobile under oxidizing conditions. Platinoid Dominion age (2.85 Ga.) and gold concentrated in placers are usually depleted in Pd against the corres- carbonaceoussediments pre-dating the Witwaters- ponding protores (Mertie 1969). Aspreferential rand.The majority of the gold most probably was mechanical removal of the Pd minerals is unlikely, this transported in solutionfrom the source areato the depletion is most probably due to the breakdown of basin. As probablesources, mafic to ultramafic the Pd-bearingsulphides in the presence of oxygen volcanic rocks of the schist beltsand the low-grade and the subsequent removal of the Pd in solution. gold deposits contained in the them can be visualized. In the Witwatersrand conglomerates so far only a An additional important source might have been few isolatedpalladiniferous detrital minerals have present in banded-ironformations of the Archaean been noted and the element is usually not assayed for schist belts which, for example, in Zimbabwe, are in the analyses of the platinoidminerals extracted known to contain up to 124 ppm of gold (Fripp 1976). fromthe ores. From informationobtained from the The high iron content of the reconstructed source area Rand Refinery, the company which processes the gold for the Witwatersrand supports this possibility (Clem- and theother preciouselements from the Wit- mey 1981). watersrand, Reimer (1979) concluded that Pd is The gold was dissolved during weathering in trace present in the ores, albeit in a concentration consider- amounts, possibly under the influence of cyanogenic ably below that of the other platinoid elements. micro-organisms and transported as organic-protected The absence of this element from the Witwatersrand colloids. ores could possibly beexplained by the lack of On encounteringdifferent chemical environments Pd-bearing ores in the source area. However, primary when entering the depositionalbasin, the gold was platinoid deposits with low content of Pd are excep- precipitated from thesesolutions, again possibly tionally rare (Mertie 1969) and the absence of Pd from influenced by organic matter as witnessed by the the conglomerates throughoutthe basin suggests auriferouscolumnar thucolite. Fineness of the gold secondary influences such as depletion of this element particles precipitated was governed mainly by eH and duringweathering which could havebeen of an pH of the hydrosphere and thereby a homogenization oxidative nature, comparable to present day condi- under sedimentary facies control was effected. The tions. If thiswere the case, the absence of a resulting gold grains were then reworked by normal silver-depleted rim aroundthe detrital gold grains sedimentary processes. During metamorphism of the could be cited as evidence against their transport in conglomerates some mobilization and reprecipitation particulate form by fresh water from the source area to of the gold andcertain other detrital minerals has the depositional basin. taken place, resulting in a complex association of primary and secondary minerals. Conclusions The quantitative feasibility of the model of derivation of the gold proposed here can be illustrated by a The above data allow the following conclusions: mass balancecalculation. If it is assumed thatthe Gold yield. The amount of extractable gold in the Witwatersrand gold was dissolved from schist belt schist belts is not large enough to explain the vast rocks covering 20% of the source area, equivalent to amounts of gold in the Witwatersrand, especially in about 12 000 km2 at an average thickness of 2000 m, view of the small proportion (15-20%) of schist belt the 36 000 t gold produced so far from the conglomer- predicted in the source area. ates would represent aconcentration of about 0.56 Fineness. The distribution of gold fineness values in ppb in the source rocks. This is about15% of the the Witwatersrand conglomerates differs notably from crustal average for gold which according to Wedepohl that in the schist belts. These data, together with the (1967) is 4ppb. Considering that the schist belts are sedimentological control of the fineness values suggest enriched in gold against the average crust, the relative formation of the gold particles virtually contempor- proportion of the gold dissolved could have been even aneously with the deposition of the enclosing sedi- lower. ments. Based on preliminarya budget calculation for Grain shape. The shape of the detrital gold grains uranium, similar to the onecarried out for gold above, suggests transport distances of less than about 30 km it canbe concluded that comparable processes of which make it difficult to relate the gold to a source solution in the source area andprecipitation atthe outside of the basin. basin edge could havebeen responsible for the The above observations can best be explained by a uranium mineralization in these conglomerates (Reim- modification of the ‘modified placer model’ for the er1975~). The extent of pre-concentration in pre- Witwatersrand gold. The following sequence of events Witwatersrand arenites in the form of sandstone-type

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/2/263/4888393/gsjgs.141.2.0263.pdf by guest on 30 September 2021 270 T. 0. Reimer depositsmight, however, havebeen much more oxidizing atmosphere, as suggested by investigations pronounced for uranium than for gold. It is possible of the author on paleosols developed above a basalt of thatthe weathering processes tookplace under an the upper part of the Witwatersrand Supergroup.

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Received 20 July 1982; revised typescript received 29 September 1983. THOMAS0. REIMER,c/o Dyckerhoff Engineering GmbH, P.O. Box 2247, D-6200 Wiesbaden-Biebrich, F.R. Germany.

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