GENESIS OF STRATIFORM U-MO DEPOSITS IN THE KAROO BASIN OF SOUTH-AFRICA

Evaluation of existing theories on the formation of uranium orebodies in the Karoo shows that no single model can adequately explain their distribution, size and geochemistry, so that a combination of various ore-forming processes should be considered instead. Extra-basinal leaching of granitic and volcanic source rocks and the expulsion of fresh, uraniferous pore waters from tuffs interbedded with the argillaceous overbank and lacustrine sediments during early diagenesis probably account for most of the uranium, although the breakdown of feldspars, glass shards and heavy minerals within the sandstones probably contributed to the widespread occurrence of uranium deposits in the Karoo. Mineralization in some of the main uranium-bearing sandstones in the Beaufort Group seems to correlate in time with major paroxysms of the Cape orogeny. Tectonism and uplift in these provenance areas probably coincided with an increase in volcanic activity as well as the orographic rainfall, which would have enhanced chemical weathering and erosion of the source rocks and amplified the volume of ash and detritus supplied to the basin. Low-temperature ore fluids migrated down the paleoslope along permeable conduits in channel sands, possibly driven in part by seismic pumping associated with the early stages of the Cape orogeny. Locally, the uranium was precipitated by H2S accumulating along the permeable pathways and in the deeper portions of channels where plant material was protected from oxidation by stagnant groundwaters. Some mineralization could have taken place at the interface between slightly oxidized ore fluids and reduced groundwaters. Calcite gangue introduced by the mineralizing fluids drastically reduced the permeability of the host sandstones and effectively sealed the mineralization in place in the Beaufort Group, but in the Molteno and Elliot Formations, coarser grain sizes and a lack of carbonate allowed the redistribution of uranium by circulating groundwaters. This process apparently continued until at least after the intrusion of dolerite dykes and sills during the Jurassic.