Transport and deposition of uranium in hydrothermal systems at temperatures up to 300°C: geological implications

Abstract

Many geologists accept that certain uranium deposits may have formed at elevated temperatures in the presence of aqueous fluids. Such deposits may range in nature from typical veins and fracture fillings to deposits in metamorphic terrains that exhibit a variety of structural controls. Fluid inclusion studies and mineral stability relationships from a variety of occurrences indicate that these deposits form at temperatures up to 300°C. The purpose of this paper is to outline the physicochemical conditions under which uranium is mobile and how it is transported, and also the conditions under which uranium is deposited, and to determine possible mechanisms of precipitation. Existing thermodynamic data were used to evaluate the relative stability of various uranium complexes in hydrothermal solutions up to 300°C. Subsequently, the conditions under which various aqueous uranium species and solids are stable were calculated in terms of oxidation potential (fugacity of oxygen,f_o2) and pH. The stabilities of uranium species and minerals are compared against those of alteration and gangue minerals commonly associated with uranium mineralization. These natural assemblages are then used to determine the conditions of ore deposition, and possible mechanisms of deposition are proposed.