Oxidation state and accumulation of uranium in hydrothermal REE minerals

To meet growing demand for renewable technologies that will mitigate climate change, the world requires a sustainable supply of rare earth elements (REEs). Uranium (U) is a common impurity in REE ores that causes socio-environmental challenges during mining and processing. Sustainable REE extraction thus relies on targeting ores with little U, which necessitates an understanding of the conditions under which U accumulates in REE minerals. Here we show that rhabdophane and monazite - two common hydrothermal REE ore minerals - can incorporate U in its oxidised U(V) and U(VI) forms. Contrary to current understanding, this demonstrates that reducing conditions are not required for U to accumulate in hydrothermal REE minerals. Rather, the higher availabilities of U in oxidising and/or Ce-poor fluids can also drive significant accumulation. Four redox scenarios are consequently theorized for predicting the capacity of aqueous-fluid-affected REE ores for accumulating U. This framework may assist in evaluating the environmental sustainability of various REE deposits, thereby guiding decisions on exploration and extraction. Results also confirm the capacity of both minerals to immobilise fluid-soluble U(VI) during formation, with implications for their application as mineral barriers in U-contaminated systems.