Alternative model for the derivation of gold in the Witwatersrand Supergroup

Abstract

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 weathering cycle are used to erect a model which predicts that most of 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.

Understanding Mineral Deposits

“…Gold transported from the source area involved some particulate gold (under oxygen-deficient conditions) but most of it was in solution or in colloidal form (Reimer 1984, Hutchinson & Viljoen 1988. Gold was deposited in braided alluvial fans from which it was, at least in part, reworked and concentrated mechanically in the conglomerate horizons (Mossman & Dyer 1985).…”

Origin of placer gold nuggets and history of formation of glacial gold placers, Gold Creek, Granite County, Montana

“…For example, Reimer (1984) suggests that gold in Witwatersrand deposits, South Africa, was dissolved in trace amounts from rocks in the source area and transported in solution to the basin where it was precipitated and reworked, although detailed studies of Witwatersrand gold (Hallbauer and Utter, 1977) have tended to support a detrital origin for the gold. In another case, Eyles and Kocsis (1989, p. 49) have favored gradual accretion of gold particles in the Cariboo district, Canada, as a result of deep weathering and supergene enrichment of sulfide-bearing quartz veins during the Tertiary, although little petrographic data are offered to support this assertion.…”

Witwatersrand Metallogenesis: The Case for (Modified) Syngenesis

“…Furthermore, the changes in temperature and pH in this situation are far less important mechanisms than oxidation in governing the precipitation of gold. These conditions are precisely those favored by facultative aerobic prokaryotic microbacteria, bringing to mind the scenario described by Reimer (1984), Mossman and Dyer (1985), Southam and Beveridge (1994), and Mossman et al (1999), whereby transport of gold occurred as a solution or colloid, stabilized by humic acids or by sulfur cycle intermediates, with biologically induced precipitation of gold ensuing as a result of oxygen produced in localized environments by microbial communities. In this setting, the strongly electronegative nature of gold would also result in its attraction to associated clay particles.…”

“…Indeed, any case against the wholesale detrital derivation of gold in particulate form from the hinterland that is formulated on the basis of the immense tonnage involved grows with the realization that even supposing that ;39,000 t of gold remain to be recovered (Frimmel and Minter 2002, p. 17), there likely remains at least an equivalent amount of unpayable gold ''reserves'' in excess of the estimated 2 3 50,000 t ultimately recoverable. Other workers (e.g., Reimer 1975Reimer , 1984Pretorius 1991) have also argued that there is too much particulate gold in the WWR conglomerates to have been entirely derived by detrital processes from a ''fertile'' hinterland rich in hydrothermal gold deposits. In contrast, Loen (1992) calculated that the total gold embedded in the WWR could easily have been derived by weathering of a source exhibiting no unusual concentrations of gold.…”

“…Friable, banded, and porous, this pyrite has all the characteristics of a chemical precipitate, including trace amounts of a range of metals present as major constituents in accompanying sulfide minerals Koen 1975, Hallbauer 1986). The sedimentary control and remarkable stratigraphic continuity of this complex ore assemblage, in common association with carbon and the sheer mass of gold found, led to the alternative source concept that the key components gold and partly also uranium were derived from the hinterland, carried in solution to the edge of the basin, and precipitated there under reducing conditions (e.g., Sharpe 1949;Koen 1961;Reimer 1975Reimer , 1984Simpson and Bowles 1977;Mossman and Dyer 1985;Horscroft 1989Horscroft , 2004. Note that in this scenario, any gold precipitated outside of the preserved basin could have been reworked FIG.…”