The geometry of some Beaufort group sandstones and its relationship to uranium mineralisation

Bibliography: pages 175-191.From field evidence gained whilst working on the Ptistetognathus/ Diictodon Assemblage Zone west of Beaufort West it is found that three discrete sandstone types called: A) straight channel sandstones, B) low sinuosity channel sandstones and C) transitional sandstones can be identified, in what has previously been considered as high sinuosity channel facies association sediments. Palaeocurrent analysis has demonstrated that the transitional sandstones were high sinuosity and were the larger fluvial systems; the straight channel and low sinuosity channel sandstones were generally much smaller and had as their names imply a much lower sinuosity. The palaeo-variability of current vectors in these systems, in the study area, is such that a standard deviation of greater than +40° is considered diagnostic of the transitional sandstones. Moreover in the study area only this type of sandstone is of importance in uranium exploration. From the palaeocurrent data and the reduction-oxidation states of the sandstone types it is proposed that the transitional sandstones represent semi-perennial fluvial systems flowing across an arid intracratonic basin, whilst the straight and low sinuosity channel sands are intrabasinal tributaries of the transitional sandstones. From this study of the sandstone geometry has evolved a new model of uranium mineralisation. It is proposed that the mineralisation is syngenetic and generated by reduction of uranyl carbonates on carbonaceous material. The carbonaceous material must however be lying closely below or within a weak REDOX front, since it is vital to transport complexes in an oxidising environment, and yet such an environment will not allow reduced uranium to be preserved for any length of time, as the carbonaceous material on which it reduced will eventually oxidize. Such a REDOX front, it is proposed, is created by the coalescence of two discrete sands. The upper sand is an oxidising active channel. The lower sand has been buried for some time and is weakly reducing due to anaerobic breakdown of its carbonaceous material by bacteria. Fluids in the two sands mix at the point of coalescence and uranium in transport in the upper sand is transported to and fixed at the REDOX front by carbonaceous material in the lower sand. Borehole data suggests that the correlation between coalescence of two discrete sandstones (the lower being a transitional sandstone) and mineralisation is very good. It is believed that the three sandstone types, whilst representing major fluvial systems and their tributaries, were undergoing water loss along their length such that distal portions of even the major (transitional) sandstones were likely ephemeral. Evidence for this is found in the arid climate, lack of in-channel vegetation, dominance of flat-bedded sandstones and clear evidence for bedload transport