Uranium Metallogenesis in South China and Its Relationship to Crustal Extension during the Cretaceous to Tertiary

South China is rich in vein-type hydrothermal uranium deposits hosted in granitic, volcanic, and carbonaceous and siliceous pelitic sedimentary rocks. The uranium deposits are spatially associated with extensional structures and/or mantle-derived mafic dikes. Both the uranium deposits and mafic dikes are Cretaceous to Tertiary in age, temporally coincident with the crustal extension. Carbon isotope analyses of calcite deposited in the main-stage mineralization in the veins from 12 representative uranium deposits yield δ13C values of ore-forming fluids mainly from −4 to −8 per mil, which are permissive of a mantle origin for the CO2 in the ore-forming fluids. A mantle origin is consistent with the association of the deposits with mafic dikes and the 3He/4He ratios of ore-forming fluids (e.g., 0.10–2.02 Ra for the volcanic-hosted Xiangshan uranium deposit). Isotopic compositions of H and O demonstrate that water in the ore-forming fluids is predominantly meteoric in origin. Ore-forming temperatures ranged approximately from 150° to 250°C. Uranium-rich crustal rocks in South China may have been the sources for the uranium. Crustal extension and associated mafic magmatism are considered to have heated the rocks and allowed CO2 (possibly from mantle sources) to migrate upward and to mix with CO2-poor meteoric water. The CO2-rich hydrothermal fluids mobilized uranium from the source rocks and then the uranium was deposited in various host rocks to form the uranium deposits.