Rare Earth Element Partitioning Between Fluids and Uraninite At 50−700 °C

ABSTRACT Uranium deposits are globally diverse, occurring in a wide variety of geological settings and ranging in age from Archean to Holocene. As a result, understanding the mechanisms involved in the genesis and subsequent alteration of these complex deposits is challenging. Building on recent work on the geochemical signatures of uraninite, a series of experiments were designed to document the partitioning of rare earth elements between uraninite and fluids over a range of temperatures and to explore the impact of O and H diffusion, under reducing conditions, on U-Pb isotope systematics and rare earth element concentrations in uraninite. Our results show that O and H diffusion in the presence of a rare earth element-rich fluid, under reducing conditions, has no effect on rare earth element concentrations and patterns or U-Pb isotopic compositions of uraninite. Our results also show that temperature (300 to 700 °C) has no effect on the rare earth element patterns, indicating that the dominant control on rare earth element concentration in uraninite is the metal source(s), the ability of the fluids to transport rare earth elements without inducing fractionation, and the degree of recrystallization. These results have implications for nuclear forensics, as well as for our understanding of the genesis of uranium-bearing ore deposits.