Geochemical modelling of uranium redistribution in the Osamu Utsumi mine, Poços de Caldas

Abstract

Abstract Substantial mobilisation of uranium has resulted from the oxidation of alkaline intrusive rocks now exposed in the Osamu Utsumi mine, Pocos de Caldas, Brazil. Groundwaters percolating through the sequence have promoted dissolution of pyrite and primary pitchblende ore with subsequent redeposition of uranium in the form of discrete concretionary nodules below a sharply defined redox front. Above the front, uranium is found at lower concentrations in association with finely dispersed iron oxyhydroxides. A surface complexation model for uranyl species on goethite has been used in an attempt to quantify the extent to which surface adsorption can account for the uranium levels found in oxidised rock. The results of these calculations approach, but generally underestimate, observed concentrations, suggesting that a more complex mechanism than equilibrium adsorption is responsible. A combined diffusion-precipitation model has been used to quantify uranium movement across the redox front itself. The results indicate a sharp uranium reprecipitation zone in reduced rock leading to concentrations in the region of 4,000 ppm. These findings are in broad agreement with the observed occurrence of uranium in the mine.