The molybdenum potential of the sandstone-hosted uranium accurrences in the Beaufo

Abstract

The sandstone-hosted uranium deposits of the southern Karoo basin in the Republic of South Africa were investigated as a possible source of molybdenum. A summary of global resources, production, co-products (Re and Se) and uses of molybdenum, as well as the geochemical cycle of the metal is given. The investigation comprised mapping and sampling of selected known uranium occurrences in the Karoo. Initial field analysis of samples was done by means of a portable XRF analyser, followed by laboratory XRF analysis. The calibration of the portable and the laboratory XRF analysers by means of spikes is described. It is concluded that the portable XRF is a useful field tool in exploration. Ore specimens were examined by means of auto radiography, and reflected and transmitted light microscopy. The statistical analysis of the geochemical results on the basis of field parameters shows that the south-eastern palaeostream direction has the highest molybdenum potential expressed in terms of mean Mo concentration, followed by the southwestern direction. The north western direction has the highest uranium potential. Of the ten sandstone units investigated the Poortjie Sandstone Member has the highest combined Mo U potential, followed by the Barberskrans Sandstone Member. The rock type with the highest Mo potential is a fissile sandstone. The Mo-potential of a rock increases by about an order of magnitude in terms of the mean Mo concentration if visible organic matter is present. The carbonate-cemented sandstone has a low molybdenum potential. Mineralization associated with laminated sandstone is normally found near the base of sandstone bodies. Accompanying elements with a sympathetic concentration distribution to that of molybdenum are U, V, Co, As and Pb; Zn is indifferent and Cu antipathetic. The elements sympathetic to uranium are Mo, V, Co, As, Zn and Pb, and Cu behaves antipathetic. The evaluation of the results of the Geological Survey's stream-sediment sampling of the Karoo conducted in the late 1970's revealed that only the neutron activation analytical technique for uranium returned anomalies that could be matched with known uranium occurrences. Plant toxicity to livestock as an exploration tool was investigated. Although this gives a good mapping of doleritederived copper, the method is unsuitable to U\Mo exploration. An early-diagenetic model based on the movement of oxygenated water through the still unconsolidated sediment is proposed. Where this water passes through pockets where anaerobic conditions prevail due to the presence of decaying plant material, heavy metals borne in solution or as adsorptions are precipitated and ore-bodies may develop. The conclusion is reached that economic exploitation of the uraniferous sandstones for their molybdenum content is unlikely because of the small volume of individual orebodies, the irregular distribution of mineralization and grade within them as well as their small thickness and their horizontal attitude which makes extraction of the ore difficult.