Genesis of the Xiangshan Uranium Ore Field: Implications from Tescan Integrated Mineral Analyzer and Micro-X-Ray Fluorescence Mapping and Thermodynamic Modeling

Hydrothermal alteration provides critical information for both the exploration and scientific research of hydrothermal uranium deposits. The Xiangshan uranium ore field, the largest volcanic-hosted uranium deposit in China, is characterized by different alterations, including hematitization, illitization, sericitization, chloritization, carbonation and silicification. However, the mineralogical and geochemical characteristics of hydrothermal alterations and their relationships with uranium mineralization remain unclear. In this study, we conducted detailed petrography, TIMA mapping, μ-XRF analyses, mass balance calculations and thermodynamic modeling on the hematitized and illitized porphyritic lava from the Zoujiashan deposit in the Xiangshan ore field. During hematitization, hematite and albite are produced, while quartz, K-feldspar, chlorite, sericite and biotite are consumed, consistent with the increase in Na2O, Al2O3, Fe2O3-T, U, As, Pb, Cu, Sc, V, Zr, Y, Hf and Th and the loss of K2O, MgO, Li, Zn, Ni and Ba. The production of hydrothermal hematite, illite and sericite indicates that the ore fluids are acidic and oxidized. Such physiochemical conditions are favorable for uranium transport as UO2Cl2(aq), UO2SO4(aq) and UO2OH+. Geological processes such as fluid–rock interactions, fluid mixing and fluid boiling could cause fO2(g) decrease, pH increase and temperature decrease and therefore result in the decrease in uranium solubility and mineralization.