Hambleton-Jones reported in his Ph.D. dissertation for the University of Pretoria that This study comprises a geological and geochemical investigation of the uranium deposits in the region near the Swakop River which extends from the Langer Heinrich Mountain in the east to the end of the Turner River in the west, and part of the region between the Khan and Swakop Rivers. The Langer Heinrich area, which includes the Bloedkoppie Flats, Gawib River Valley and surrounding regions totaling 180 km2 in size, was mapped in detail.
Most cf the area is covered by superficial deposits which unconformably overlie basement rocks of the Nosib and Swakop Groups. It is proposed that the informal term Namib group be used to include all these rocks, which were further subdivided into the chronostratigraphic Langer Heinrich calcrete and Tumas gypcrete formations. The Gemsbok calcrete formation unconformably overlies the Langer Heinrich calcrete formation.
The general geology of the basement rocks in the Langer Heinrich region only is discussed. The main lithological units present are the Etusis, Chuos and Tinkas Formations; remaining rock types are the Salem, Rloerikoppie, Gawib and Hoiebis Granites and pegmatites. The Bloedkoppie Granite, which shows cross-cutting features to the Tinkas and Etusis Formations, is the main granite found in the area. Unusual weathering features are characteristic of the Bloedkoppie Granite, in which alveoli and tafoni are developed.
The occurrence of uranium in the region of study follows a unique geochemical cycle, and the geochemistry at each stags in the cycle was examined. The first stage in the uranium-geochemical cycle was the basement rocks. The rare-earth distribution between these rocks provided evidence concerning their paragenesis, and indicated that the Gawib and Bloedkoppie Granites were the result of partial melting
of the Etucis formation; also, that they are late-syntectonic in age.
The second stage in the geochemical cycle of uranium was the groundwater. Predictions regarding mineral equilibria could be made from the chemical composition of the water. It was found that the uranium minerals soddyite and carnotite were the stable phases in this environment. From the prospecting aspect, 30 ppb uranium were considered anomalous, while values below this were regarded as back= ground.
The third stage in the geochemical cycle of uranium concerns its occurrence in the duricrust deposits. The segregation of carnotite at the Langer Heinrich was brought about by upward diffusion of dissolved uranium caused by sol suction, and was precipitated by the nnr. leation of the uranyl cation on montmorillonite, and subsequent complexing with vanadate ions. Mechanisms for the formation of calcrete and gypcrete are discussed. A new mechanism for the precipitation of gypsum is proposed which resulted mainly from the precipitation of calcium sulphate brought in by onshore fogs. Isotopic disequilibrium measurements showed that uranium is still migrating, and that the age of the carnotite precipitation is about 30,000 years, based on the open-system model of uranium migration.
In the final stage of the geochemical cycle, the geochemistry of uranium in seawater and the diatomaceous muds is discussed. Organic matter was the main precipitating agent of uranium from seawater, and its heterogeneous distribution is due to selective uptake by particular
organic compounds.
A classification system for the uranium deposits near the Swakop River, based on genetic relationships, is proposed and described in terms of the geochemical cycle of uranium, the mode of transport and mode of deposition. The relationships between the duricrust uranium deposits
and the other uranium deposits of South Africa are compared.
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