Uranium mineralization of the Witwatersrand and Dominion Reef systems

Abstract

Uranium-bearing minerals in the Witwatersrand and Dominion Reef sediments have been studied by ore microscopic, electron microprobe, fission track and neutron activation analytical methods to determine the controls of uranium mineralization. In the Dominion Reef, which represents a high-energy banket type of depositional environment, allogenic thorian uraninite occurs in hydraulic equivalence with allogenic pyrite, quartz and possibly also gold in the sediments which have uraniumthorium ratios between 3.1 and 5.6 indicating substantial amounts of thorium-rich resistate minerals. The Witwatersrand sediments have uranium-thorium ratios ranging between 7.1 and 19.6 indicating lesser amounts of resistates which is consistent with the lower energy depositional environment. The proximal or nearshore deposits are of banket type but are distinguished from the Dominion Reef by the abundance of concretionary pyrite formed within the Basin and the presence of carbonaceous matter. The distal deposits formed at greater distance from the shoreline contain decaying organic material which has precipitated both uranium and gold from solution. Subsequent metamorphism has resulted in the formation of carbonaceous material bearing finely disseminated low-thorium pitchblende and a fine dissemination of gold associated with sulphides and arsenides. Further evidence of the existence of uranium in solution is to be found in the banket deposits. In this case fine disseminations of uranium (> 500 parts/10 6 ) occur in clay minerals within concretionary pyrite nodules and in lenticles formed of clay minerals in Witwatersrand banket deposits. They represent reduction-deposition of the soluble uranyl ion below the sediment-water interface where conditions are reducing. Allogenic thorian uraninite from the present-day Indus river has a texture, composition and association with gold and pyrite similar to allogenic uraninite in the Witwatersrand and Dominion Reef System. Thorian uraninite is a stable phase over large distances in this river. Hence it would appear quite unnecessary to postulate a reducing atmosphere for the transportation of detrital uraninite. Moreover the retention of sulphate and uranyl ions in solution in the model proposed here suggests that the atmosphere was oxidizing at the time of deposition. This conclusion indicates the likely occurrence in younger sediments of mineralization of this type provided the necessary geological criteria are met.