Elucidating pentavalent Uranium (U5+) in mineralised carbonate veins from a porphyry-epithermal system

U-Pb dating of carbonate veins reveals the timing and longevity of mineralized fluid systems in ore deposits. Mechanisms of U incorporation in vein carbonate remains poorly understood, but crucial for robust application of the chronometer, and a new approach to tracing fluid origins. Pilot U M5-edge U XANES data from a porphyry-epithermal deposit indicate that distinct pulses of fluid flow preserving complex U speciation, concentration, and spatial distribution patterns. However, in select samples, pilot XANES data is inconclusive and may suggest a mixture of U4+ and U6+ species or the presence of U5+. Here we aim to distinguish between U species using the novel U M4-edge HERFD-XANES, the only method able to elucidate the presence of U5+. This information will provide a new framework for understanding U-Pb carbonate dating and the speciation and mobility of U in hydrothermal fluids. Preservation of U5+ over geological timescales has implications for the storage of radioactive waste.