Hydrothermal apatite as fingerprints of ore-fluid evolution and U-Th-REE enrichment in the Xiangshan volcanic-related uranium ore field, South China

The mechanisms controlling the mobilization and co-enrichment of uranium (U), thorium (Th), and rare earth elements (REEs) in uranium-associated hydrothermal systems remain unclear, despite their economic and scientific significance. We present extensive textural, geochemical, and in situ Sr isotopic analyses on magmatic apatite and three generations of hydrothermal apatite (Ap I, Ap II, and Ap III) from the Shannan U deposit, located in China’s largest volcanic-related uranium ore field at Xiangshan, which is also enriched in Th and heavy REEs (HREEs), aiming to clarify fluid compositions and metal sources. Ap I formed via dissolution-reprecipitation of pristine magmatic apatite. Pronounced Th and REE depletion, elevated Sr, and high 87Sr/86Sr ratios (0.71917−0.72016) were likely caused by interaction with oxidizing fluids. Alteration of allanite and garnet in the altered wall rocks further released substantial U, Th, and REEs into the hydrothermal system. Ap II crystallized during the early ore stage. These apatites are characterized by moderate U enrichment, Th and REE depletion, and intermediate 87Sr/86Sr ratios (0.71467−0.71634), suggesting oxidizing basinal brines transporting U as uranyl hydroxy and carbonate complexes. Ap III precipitated during the main ore stage with abundant fluorite-illite, marked by extremely high Sr (6−9 wt%) and high 87Sr/86Sr ratios (∼0.723), along with high F but negligible Cl contents. These geochemical features likely indicate low-pH, F- and SO42−-rich fluids that underwent intense fluid-rock interaction. Trace-element mapping of Ap III demonstrates that the influx and mixing of basin-derived brine and oxidized surface fluids charged with magmatic solutes, combined with water-rock interaction, collectively facilitated the coprecipitation of U, Th, and HREEs. Our findings highlight apatite and other accessory mineral phases as robust proxies for tracing fluid evolution and multi-element enrichment in volcanic-related U systems.