Uranium mobility in non-oxidizing brines: field and experimental evidence

Abstract

Solution, transport and precipitation mechanisms for U which are dependent on the formation of soluble uranyl U 6+ ions and complexes under oxidizing conditions and of insoluble uranous U 4+ species under reducing conditions do not adequately explain U mobility under consistently reducing conditions. Uranium deposits formed under deep-seated metamorphic conditions and some hydrothermal deposits fall into this category. This paper presents field and experimental evidence supporting the mobility of U in strongly reduced polycationic brines. The Wollaston Group of Early Proterozoic sediments in northern Saskatchewan, Canada, locally contains series of associated semi-pelitic gneiss, calc-silicate gneiss and marble which have major and trace element characteristics and identify them as having formed in an evaporative environment. Associated with the evaporative metasediments are Na-rich and Cl-rich metasomes, albitite and scapolite calc-silicate rocks, respectively, which are the products of brine-rock reactions centred on the saliferous zones. Uranium and Mo appear to have been deposited syngenetically with the primary sediments but have been redistributed during metamorphism resulting in the formation of numerous mineralized zones which have attracted considerable prospecting attention. The present distribution of U can be related to the movement of the brines as revealed in the Na Ca Mg Cl-metasomes. Experiments were conducted at 60 and 200°C under stringently non-oxidizing conditions using solvents ranging from distilled water to a Ca Na K brine formulated to simulate the major element composition of the Salton Sea geothermal brines. The experiments were conducted on natural pitchblende (UO 2.67) and synthetic uraninite (UO 2.01). Natural pitchblende was more strongly dissolved than the synthetic uraninite, and the synthetic Salton Sea brine was a more potent solvent than distilled water, 1:4 diluted Salton Sea brine, or pure NaCl brine. Within analytical limits of detection the dissolved U is present in the uranous (U 4+) state. The evidence demonstrates empirically the mechanism of dissolution of naturally occurring U minerals in reduced brines and describes a geological case where this appears to have happened. Redeposition of the U presumably occurs in zones where the uraniferous brines are diluted or experience any changes in their cationic or anionic compositions which lessen U solubility factors.