The relationship of silver and gold in the basal reef of the Witwatersrand System, South Africa

Abstract

Ag/Au ratios of individual gold particles and of ore-samples obtained from the Basal Reef conglomerate horizon of the Upper Witwatersrand System in the Orange Free State Goldfield (South Africa) are discussed. The Ag/Au values of individual gold grains were found to be exceptionally constant at about 0.0812, whereas the Ag/Au ratios of the ore-samples have markedly higher values and show a large variation. These values indicate that the detrital gold has been homogenized during transportation and sedimentation and that part of the original silver content has been redistributed. It is likely that the first refining of detrital gold particles occurred in streams which eventually drained into the Witwatersrand basin. Further redistribution of the silver relative to the gold took place in the littoral zone of the basin, where the conglomerates were reworked by wave action and erosion. It is contended that supergenic enrichment of gold might also have taken place close to the original shore-line. These processes are believed to account for an increase in the Ag/Au values of ore-samples towards the northwest in the area investigated. Finally, the gold has been homogenized during the metamorphic period of the Witwatersrand System, when pseudohydrothermal transport and redeposition of certain constituents took place. It is suggested that the main portion of the silver content of the original alloy, which was redistributed during the various processes outlined above, is now present as finely dispersed silver minerals. The hypothesis of an elevation control of the Ag/Au values has been found to be improbable.