Mineral modifications observed in uranium-bearing reefs of the Witwatersrand, South Africa

The Witwatersrand Basin was filled with sediments during the Early Proterozoic when vegetation was absent and the atmosphere was probably deficient in oxygen. During the erosion-sedimentation cycle, and after burial, the sediments were subjected to a number of consecutive mechanical and geochemical prosesses, each changing or even erasing the records of the preceding one. Although the information provided by the current mineral assemblages indicate that most of the mineralogical transformations required reducing conditions, some of them could also have occured in a weakly oxidizing environment. Notwithstanding the difference in terrestrial, atmospheric, and hydrospheric conditions during the Early Proterozoic, it is possible from the mineralogical records to establish the processes and chemical mechanisms that operated before and after the Witwatersrand sediments were deposited