Origin of the unique volcanic-related uranium-phosphate mineralization in Shengyuan uranium ore field, South China: Constraints from zircon Hf-O isotopes, mineral chemistry and sulfur isotope

• Early Cretaceous (135–131 Ma) crust-derived felsic volcanics provided a U-fertile source. • Uranium is chiefly incorporated into collophane rather than discrete U minerals. • Biogenic sulfide and organic matter acted as key reducing agents. • A shallow, steam-heated genetic model explains the U-phosphate co-precipitation. The Xiongjia U deposit in the Shengyuan volcanic basin, East Jiangxi Province, South China, is a unique volcanic-hosted uranium-phosphate (U-P) ore system where U is primarily hosted in collophane. However, key aspects such as the timing of volcanic emplacement, U source and occurrence, hydrothermal fluid sources, and the mechanism of U-P co-precipitation remain poorly understood. This study addresses these gaps through detailed petrography, zircon U-Pb dating with Hf-O isotopes, ore and gangue mineral chemistry analysis, and in situ sulfur isotope measurements. Zircon U-Pb dating on tuff and ignimbrite constrains the volcanic activity in the Shengyuan basin to ca. 135–132 Ma, which is nearly contemporaneous with the U-P mineralization. Zircon ε Hf (t) values (−17.9 to −1.8; mean = −11.2) and elevated δ 18 O (7.17 to 9.26; mean = 8.18) indicate its derivation from ancient crustal sources. The ore-hosting tuffs display an advanced argillic, dickite-rich halo. Ore-stage collophane is F-rich (2.4–3.9 wt%) and sulfur-bearing (SO 3 = 0.36–0.77 wt%), and contains high concentrations of U (3788–4596 ppm), elevated Th, high Y/Ho (≈32–35) and pronounced negative Eu anomalies (δEu ≈ 0.07–0.09), consistent with precipitation from oxidized, halogen-rich acidic fluids. Pyrite is As-rich (to ∼0.7 wt% As) with Co/Ni < 1, indicating low-temperature, epithermal-grade conditions. Furthermore, in-situ sulfur isotope data from ore-stage pyrite (−23 to −16 ‰) suggest that hydrogen sulfide was generated through the microbial reduction of sulfate in an organic-rich lake environment. Based on these findings, we propose a volcanic-lake model for the Xiongjia U deposit. In this model, the acidic and halogen-rich magmatic vapors degassed into a crater lake. Subsequent processes, including evaporation, fluid-rock interaction which released calcium and increased pH, and organic-mediated reduction, collectively drove the rapid co-precipitation of U and P as the gel-like collophane.