An integrated U-Pb-Zn mineralization system in the Xiangshan ore field, South China: Insights from petrography, geochemistry, and geochemical modeling

Hydrothermal U mineralization is commonly associated with other elements, such as Co, Ni, As, and Cu, which form U-polymetallic deposits. Enrichment of Pb and Zn has also been reported in some deposits, but these metals are generally subordinate compared to other metals. Recently, significant Pb-Zn mineralization has been reported in association with U mineralization in the Xiangshan volcanic basin in South China. This basin represents the third largest volcanic-type U mineralization center in the world. Several deposits in this basin are characterized by shallow U-dominated orebodies (SUO) and deep Pb-Zn-dominated orebodies (DPZO). The nature of the U-Pb-Zn association, especially the timing and spatial relationships between U and Pb-Zn, is examined in this study through petrographic and geochemical analyses as well as geochemical modeling. Petrographic observation and TESCAN Integrated Mineral Analyzer (TIMA) mapping demonstrate that galena and sphalerite are locally present in the SUO, while minor amounts of pitchblende and brannerite also occur in the DPZO, and all developed during the same paragenetic stage. Moreover, both SUO and DPZO share similar alteration, including illitization, chloritization, and carbonatization. Geochemical analyses indicate that galena, sphalerite, and gangue minerals in the SUO and DPZO share similar S-Pb-C-H-O isotopic compositions, which suggests that the U and Pb-Zn ores were formed by the same fluid. Geochemical modeling results indicate that the ore-forming fluids are acidic, oxidized, and S-poor, capable of carrying high concentrations of U, Pb, and Zn. U is precipitated under more oxidized conditions than Pb-Zn sulfides, with a decrease in oxygen fugacity (fO2), and Pb-Zn deposition requires additional mixing with deep H2S-bearing reduced fluids. This distinct precipitation mechanism leads to spatial separation of the U and Pb-Zn orebodies. The temporal and spatial coupling of SUO and DPZO suggests that the U-Pb-Zn mineralization is related to a basin-scale thermal convection system and associated fluid mixing. Based on these findings, we propose a hydrodynamic-geochemical mineralization model for the Xiangshan U-polymetallic ore field. In this model, convecting fluids mix with oxidized meteoric water, leaching U and other metals from host rocks, with subsequent ore precipitation driven by fluid-rock interactions and/or fluid mixing. This mineralization model has implications for further exploration of U-polymetallic deposits in the Xiangshan basin and in other volcanic basins with similar geological backgrounds.