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

Abstract

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 (<0.05% and <0.04%, respectively) and a narrow range of δ34S (9.11–9.87‰). In contrast, hydrothermal chlorites (chlorite-II) in the ores are ripidolite and brunsvigite with lower Fe/(Fe + Mg) (0.43–0.62) but higher Fe3+/Fe2+ (∼0.056), and hydrothermal pyrites (pyrite-II) have high Co and Ni contents (0.02–0.3% and 0.01–0.11%, respectively) and a wide range of δ34S values (2.32–13.8‰). (2) The altered GPRs have Zr and Hf contents (135–187 ppm and 3.39–4.34 ppm, respectively) similar to those of the least-altered GPRs (162–169 ppm, and 3.79–4.01 ppm, respectively), whereas the ores have elevated Zr and Hf contents (184–548 ppm and 4.17–11.6 ppm, respectively). (3) The δ98Mo values of the altered GPRs (0.39–0.99‰) and uranium ores (0.52–1.74‰) show contrasting correlations with K2O or Na2O contents. These differences suggest that hydrothermal alteration and mineralisation in the Baquan deposit may have been caused by two fluids of different origins. Based on these characteristics and comparisons with possible sources, we suggest that the fluid responsible for the alteration of GPRs most likely originated from the subducting slab, whereas the mineralising fluid forming the Baquan uranium deposit likely originated from the brine of a nearby Cretaceous basin. The uranium in the deposit mainly originated through leaching of the volcanic rocks by the brine-derived hydrothermal fluid.