A sedimentological and geochemical study of the uranium mineralization at Rietkuil Mine, Beaufort West, Karoo Basin
Despite a vast amount of research conducted on the Karoo Uranium Province there is no clear definitive model for the uranium mineralization. This thesis aims to create a mineralization model for the uranium deposit at Rietkuil mine making use of detailed field mapping and geochemical analyses. This information can possibly contribute to a revised uranium mineralization model for the Karoo Uranium Province. The sediments from Rietkuil mine forms part of the Abrahamskraal Subgroup in the lower Beaufort Group (Late Permian age). It hosts uranium mineralization in horizontally bedded, very fine to fine-grained sandstone with a moderate sorting. Clay pellet conglomerate, cross-bedded sandstone, horizontally bedded sandstone, ripple cross-bedded sandstone, massive sandstone, parallel laminated siltstone and mudstone, massive mudstone, and mudstone with ball-and-pillow structures were the eight lithofacies identified at the study location. With the aid of petrographic and XRF data, the sandstone was classified as lithic-wacke and lithic-arenite. The distribution of the carbonaceous matter was strongly controlled by lithology. Fieldwork revealed that the carbon was strongly associated with horizontally bedded units. LECO analysis (total carbon), Mebius titrations (organic carbon) and field observations show that there was a decrease in the amount of carbon upwards in the lithology. Field and petrographic observations produced evidence of a Ca-rich hydrothermal fluid that intruded into the wacke. The porosity of the wacke was increased by tectonic features (joints, folds, and faults), lithological features (thin horizontally bedded layers, and interfingering sandstone lenses), mineralogical features (solution seams, absorption by a muddy matrix, and adsorption onto carbonaceous matter). These factors controlled the distribution of the uranium mineralization. The geochemical data obtained from Rietkuil mine was evaluated by using Pearson's correlation coefficients indicating elemental relationships of identified minerals. It revealed strong positive relationships between elements hosted in detrital minerals as well as elements hosted in hydrothermal minerals. The elements of the two mineral parameters though show strong negative correlations with each other. Strong positive relationships between pyrite and organic carbon were observed. Uranium showed strong positive correlations with CaO (inorganic carbon). The uranium mineralization was confined to zones with high amounts of carbon. This indicated that the carbon and pyrite produced a suitable environment for the precipitation of uranium minerals. Evidence suggests that the uranium mineralization at Rietkuil mine was a secondary uranium deposit. The uranium sourced from eroded granitic material and tuff deposits contained in underlying sedimentary rocks. XRD analysis revealed that the uranium occurred in the U+6 oxidation state thus being transported as uranyl complexes with ions like SO4-2, PO4-2, VO4-2, and AsO4-2 by a Ca-rich low-temperature hydrothermal fluid. The joints were the feeder mechanism for the transportation of fluids. When these fluids encounter the reduced environment produced by the carbon, uranium and sulfide minerals precipitated from the solution.