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Alternative Model for the Derivation of Gold in the Witwatersrand Supergroup

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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 the Ventersdorp 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 up to 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. 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 Witwatersrand Supergroup 265 TABLE 1. Gold production from Archaean schist belts and Proterozoic sediments 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%.
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