Significance of diagenesis for the origin of Witwatersrand-type uraniferous conglomerates

Abstract

A petrographic study of pyrite may be the key to the understanding of the Witwatersrand (South Africa) gold-uranium deposits: the sediments of the Witwatersrand Supergroup contain at least nine types of pyrite, namely (1) Laminated pyrite seams; (2) pyrite nodules in shales; (3) pyrite nodules in quartzite and conglomerate; (4) pyrite as overgrowths on carbonaceous filaments; (5) pyrite filling pore spaces and replacing clasts; (6) pyrite replacing detrital magnetite; (7) allogenic fragments of laminated pyrite; (8) allogenic fragments of pyrite nodules; and (9) allogenic fragments of coarse-grained pyrite. Types 1-6 probably formed during diagenesis of the sediment due to the activity of sulphate-reducing bacteria; types 7 and 8 are transported fragments of diagentic pyrite; type 9 may be of diverse origin, but may also in part be transported fragments of diagenetic pyrite. Pyrite petrography suggests a multi-stage history of ore enrichment: diagenetic precipitation of gold, uraninite and pyrite in sediments containing organic matter, followed by erosion, transport of allogenic fragments of ore minerals for short distances, and concentration in lag gravels at channel bottoms and unconformities. Repeated cycles of weathering, diagenetic precipitation from weathering solutions, erosion, minor transport and redeposition may have caused the extraordinary enrichment of the ores on major unconformities in the Upper Witwatersrand Supergroup.