The gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa)

Abstract

An investigation of the gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa) has confirmed that the bulk of this metal is concentrated within the bottom parts of the “reefs”, mostly near the footwall contacts. This, together with other criteria, suggests that the distribution of the metal was governed by sedimentary processes. On the other hand, the shape of the gold grains, as well as the general fabric of the gold-enriched layers, often lack conspicuous sedimentary characteristics. Subsequent metamorphism has obviously produced marked structural modifications and a limited textural rearrangement of the gold and its host-rock, thus obliterating to a certain degree the primary alluvial character of the metallization. The large-scale reconstitution of the gold fraction was effected either by a mechanical reshaping of the malleable and ductile gold particles, or by an intermediate solution process followed by reprecipitation. These processes took place more or less in situ. Accordingly, the original sedimentary distribution pattern was not profoundly affected, whereas the individual allogenic gold grains were transformed into authigenic ones or sometimes even into major gold aggregates. Electron-probe microanalysis established that the gold contains on the average between 9.9 and 12.4% silver, which is also suggestive of an alluvial origin. In spite of some uncertainties arising from the camouflaging effect of “pseudohydrothermal” reconstitution processes and from the absence of hydraulic equilibrium between gold and several demonstrably detrital heavy minerals, an integration of all available data strongly supports a modified placer theory for the origin of the Witwatersrand gold.