Different hydrothermal fluids inducing alteration and uranium mineralisation in the Baquan deposit of the Xiangshan uranium ore field: Constraints from geochemistry of altered rocks and ores

The Baquan uranium deposit is one of the hydrothermal uranium deposits in the Xiangshan volcanic uranium ore field and is hosted in subvolcanic granitic porphyry rocks (GPRs). To constrain sources of the hydrothermal fluids and metals for alteration and mineralisation, respectively, we analysed the chemical compositions of chlorites and pyrites formed during alteration and mineralisation processes, in-situ sulfur isotope compositions of pyrites, whole-rock compositions and sulfur and molybdenum isotopes of the least-altered and altered GPRs, and mineralised GPRs (i.e. uranium ores). The results show distinct differences in mineralogical and geochemical features of the hydrothermally altered GPRs and uranium ores, which are as follows: (1) In the altered GPRs, hydrothermal chlorites (chlorite-I) are mainly daphnite with higher Fe/(Fe + Mg) (0.70–0.75) but lower Fe3+/Fe2+ (∼0.014), and hydrothermal pyrites (pyrite-I) have low Co and Ni contents (