Abstract

Problems with the placer model for genesis of the Witwatersrand gold include the lack of Fe oxides but abundance of pyrite, unusual sulfides in the reefs, detrital transport of uraninite grains, the inference of hydraulic equivalence of reef minerals, and the source of the gold itself. Other features not predicted by the current placer model are mineralization in the immediate footwall of the reefs, apparent structural control of payshoot orientations, and the basinwide chemical associations that include Au-S-Fe-As-C and U-C-Ti.

The supposed total predictive capacity of the placer model is questioned here, and an alternative viewpoint is presented that emphasizes the possible importance of postburial fluids during diagenesis, deformation, and metamorphism. Chloritoid and pyrophyllite independently suggest that greenschist facies conditions (T = 350 ±50 °C) were reached in all the Witwatersrand gold mines. A fluid in equilibrium with pyrite and having potential to move gold is inferred from the mineral assemblages, but the effect of this fluid on the reefs has not yet been quantified.

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