Characteristics of Alkali Metasomatism and Implications for Uranium Mineralization in the Xiangshan Volcanic-Related Uranium Ore Field

The Xiangshan ore field is characterized by extensive alkali metasomatism, which represents the early-stage hydrothermal event before the acidic metasomatism during major U mineralization. However, the mineralogical and geochemical characteristics of alkali metasomatism, as well as its association with uranium mineralization, remain poorly understood. This study evaluates these scientific problems by conducting petrographic and geochemical analyses on feldspar, together with thermodynamic modeling. Hydrothermal feldspars are present as veinlets, differing from the magmatic ones with granular and subhedral structures. Hydrothermal albites have lower Na but higher K content than magmatic ones, while hydrothermal K-feldspars have lower K but higher Na content than magmatic ones. In addition, hydrothermal feldspars are significantly depleted in Ca and Sr, likely associated with the consumption of Ca in fluids by fluorite and calcite precipitation. Furthermore, alkali metasomatism is accompanied by intense hematitization, indicating the oxidized properties of ore fluids that are favorable for uranium transport. Thermodynamic modeling further demonstrates that continuous K+ consumption during fluid–rock interaction leads to a pH increase in the fluid, which is buffered by quartz–muscovite–K-feldspar (QMF). Given that quartz solubility is positively correlated with pH, this process induces extensive quartz dissolution in the host rocks. Such dissolution significantly enhances the porosity and permeability of the host rocks, creating ideal physical traps for the subsequent accumulation of uranium-bearing fluids. Consequently, alkali-metasomatized rocks associated with quartz dissolution and hematitization serve as critical indicators for regional uranium exploration.