URANIUM AND THORIUM ISOTOPES IN GROUNDWATERS FROM THE OSAMU UTSUMI MINE AND MORRO-DO-FERRO NATURAL ANALOG SITES, POCOS-DE-CALDAS, BRAZIL

The dissolved uranium isotopes in aquifer waters at the Osamu Utsumi uranium mine suggest that isotopically normal very deep water flows upwards and becomes mixed with downward percolating oxidized water. The latter is acidic and enriched in leached uranium. This flow regime results in some uranium migrating downwards where it enriches the ore body at the redox front; some also escapes with surface run-off. At the present time there are no waters which have the uranium isotopic signatures which would be associated with flow-through redox fronts. The uranium isotopes in shallow aquifer waters at the Morro do Ferro thorium/rare-earth deposit confirm the inferred rapid flow rates, although the groundwater in the well at the base of the hill has elevated activity ratios suggesting an inflow from deeper aquifer waters. The thorium content of water in the Morro do Ferro aquifer is somewhat above normal for groundwaters because of percolation through the ore body. Uranium isotopic modelling of groundwater flow at both sites confirms the flow regimes as deduced from hydrogeological and geochemical studies, and supports interpretations of uranium dissolution and redeposition along redox fronts.