A Review of Uranium Deposits in the Karoo Supergroup of South Africa

Doug Cole
Council for Geoscience, South Africa, Cape Town, South Africa

Uranium is present in the form of fluvially-deposited, sandstone-hosted peneconcordant tabular deposits in the Late Permian lower Beaufort Group (Adelaide Subgroup) and Late Triassic Molteno and Elliot Formations within the main Karoo Basin and Late Permian coal-hosted deposits in the Springbok Flats Basin. The sandstone-hosted deposits generally contain less than 1000 t U in situ, the largest deposit having 6791 t U. Average recoverable grades are 0.76 kg U/t. Metallogenesis is thought to have been dependent upon uranium source, palaeoclimate and availability of a reductant. Basement granite and volcanic ash have been proposed as possible uranium sources for the Adelaide Subgroup and granite for the Molteno and Elliot Formations. The warm, semi-arid palaeoclimate of all these stratigraphic units implies an oxidising environment, which was a prerequisite for the leaching and mobilisation of uranium from the above sources. Uranium-bearing solutions moved through the sand bodies with precipitation occurring in relatively sparse reduced zones that contained carbonaceous debris. Coffinite and less abundant uraninite are the principal ore minerals. Molybdenite, which is confined to ore in the Adelaide Subgroup in the southwestern part of the main Karoo Basin, forms a secondary economic commodity. The coal-hosted deposits have grades of between 0.16 and 1 kg U/t over 1 m width. Uranium is disseminated in the form of organo-metallic compounds, coffinite, oyamalite and auerlite. It was mobilised from basement granite by oxidised groundwater and was adsorbed by the coal several million years after peat accumulation associated with a lacustrine environment. The sandstone-hosted deposits have an estimated resource of 31000 t U and the coal-hosted deposits 55000 t U.

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