Uranium mineralization in the Karoo Basin, South Africa

Abstract

Uranium mineralization in the fluviatile Upper Permian Beaufort Group of the southwestern Karoo basin, South Africa, is mainly confined to one sandstone-rich unit. The reason for this stratigraphic confinement of the mineralization can be related to the geomorphic evolution of the basin and the deposition of the host sandstones at a time when gradients and sediment input were declining in response to weathering and denudation of an initially uplifted, volcanically active source area. As a result, orographic rainfall declined, promoting deeper, more intense weathering of the source rocks and slower rates of sedimentation at the depositional site. This provided optimum conditions for leaching and solution of metals while increasing the residence time of the mineralizing solutions at the depositional site, and the possibility of precipitating minerals from the solution on encountering a suitable reducing agent. Mineralization is mainly confined to the base of the thicker, more permeable channel sandstones trending down the regional paleoslope. Erosion of the deposits was effectively prevented by deposition of a thick overlying sequence of flood basin mudstones and by early compaction and diagenesis of the sediments rendering them impervious to later fluid movement. Thus, mineralization has retained its original character and was relatively unaffected by later remobilization and reconcentration processes.The uranium was probably leached from intrabasinal volcanic detritus in the host sediments by weathering and diagenesis in an arid climate, and was mobilized as uranyl carbonate complexes by oxygenated alkaline ground waters. These moved through the more permeable channel sand bodies, and on encountering bacterially generated H 2 S at sites of differential permeability, uranium was precipitated as coffinite and uraninite. Postburial changes in ground-water chemistry caused the primary oxidized sandstones to be reduced, a process that was concentrated in the more permeable sandstones leaving the mudstones relatively unaffected.Mineralization occurred shortly after deposition but before deep burial and final reduction of the host sandstones. The timing of mineralization is indicated by the replacement of undeformed plant structures by ore minerals, the presence of laumontite, and the estimated depths at which bacterial activity and decarboxylation can occur. These factors suggest an early diagenetic origin for the mineralization which must have occurred within a comparatively short period of time after deposition at temperatures of less than 50 degrees C and depths of less than about 1,400 m.