Some aspects of the uranium minerals in Witwatersrand sediments of the Early Proterozoic

Abstract

Uraninite was introduced into the Witwatersrand sediments as detrital grains, and hydraulic sorting processes were responsible for the selective concentration of the minerals. When the sedimentary trap was favourable for the growth of primitive plants, they entrapped uraninite grains, at the same time breaking them up into fragments. Owing to the continuous multiplication of the micro-organisms, the uraninite fragments gradually disintegrated to increasingly smaller sizes. Several textural features of the intergrowth of kerogen with uraninite and with phyllosilicates strongly suggest that the multiplication of organisms was responsible for the fragmentation of the uraninite grains but that chemical interaction was negligible. Before consolidation of the sediment, uraninite and leucoxene reacted with post-depositional intrastratal solutions to form uranous titanates of variable composition. These compounds also precipitated at grain boundaries and in available interstitial voids, and could have formed from very fine-grained uraninite and leucoxene that, during the transport cycle, were abraded from the detrital grains and became entrapped in the organic matter or matrix.