Abstract

Uraniferous Upper Permian sediments of the Beaufort Group, Southern Karoo (Gondwana) Basin, South Africa, cover an area of about 25,000 km², attain a maximum thickness of 2,700 m and consist predominantly of sandstones and mudstones. A systematic upward decrease in the ratio of these two lithologies, together with changes in their sedimentary properties enable the succession to be divided into low sinuosity channel, high sinuosity channel and floodbasin facies associations. This overall fining in the succession may be fundamentally linked to decreasing sediment supply and channel gradients in response to denudation of the source area and the migration (or the changing locations) of the depositional system. The facies associations probably represent the proximal, intermediate and distal components of an extensive fluvial depositional system which built out from a distant source to the south and west.

Significant uranium mineralisation only occurs in high sinuosity channel sandstones, probably because of more numerous permeability barriers. The distribution of mineralised zones suggest that local permeability differences and carbonaceous debris were the major controls of mineralisation. Host sandstones are arkosic wackes containing carbonate cement, sulphides and volcanic fragments, interbedded with occasional cherts of tuffaceous origin. The uranium was probably derived by the leaching of volcanic material and transported by mildly reducing alkaline ground water as uranyl carbonate complexes. Channel sands provide permeable pathways for these solutions and on encountering a strong reductant uranium was precipitated. Remobilisation and further concentration has played no part in the genesis of these deposits, which probably formed in situ, shortly after deposition and before compaction and diagenesis had proceeded to completion.