Genesis of the Baimadong carbonate-hosted uranium deposit, Guizhou, SW China: Constrains from geology, fluid inclusions, and C-O isotopes

The Baimadong uranium deposit is situated in the Yangtze Craton, SW China, and is one of the most representative carbonate-hosted uranium deposits in China. Uranium mineralization has spatial and genetical relationships with black and red alterations that has been documented by previous studies. However, the signature, source, and evaluation of ore-forming fluids producing the alterations and mineralization remain poorly understood. Fluid inclusions hosted in different generations of hydrothermal calcite have been analyzed in this study, including Cal 1 (pre-ore stage), Cal 2 (early ore stage), and Cal 3 (main ore stage). Three types of fluid inclusions were identified: liquid-dominated biphase, vapor-dominated biphase, and vapor-only monophase. Based on fluid inclusion microthermometry using the FIA method, the fluids associated with Cal 2 are characterized by relatively higher temperature (similar to 243 degrees C) and salinity (5.3 wt% NaCl equiv.) compared with fluids associated with Cal 1 (T-h = similar to 221 degrees C; Salinity = 1.9 wt% NaCl equiv.) and Cal 3 (T-h = similar to 144 degrees C; Salinity = 2.5 wt % NaCl equiv.). The results of stable isotopes (C, O, and S) indicate that the carbon in pre-ore stage fluids was dominantly originated from host marine carbonate rocks (delta C-13(fluid-PDB) = -3.0 to 0.5 parts per thousand; delta O-18(fluid-SMOW) = 4.8 to 11.3 parts per thousand), the carbon and sulfur in early ore-forming fluids were mainly derived from the Niutitang Formation with minor from deep crust (delta C-13(fluid-PDB) = -11.2 to-8.4 parts per thousand; delta O-18(fluid-SMOW) = 7.9 to 11.4 parts per thousand; delta S-34(v-CDT) = -11.3 to + 2.1 parts per thousand), whereas the main ore-forming fluids were sourced from meteoric fluids (delta C-13(fluid-PDB) = -8.6 to-4.9 parts per thousand; delta O-18(fluid-SMOW) = 0.4 to 8.6 parts per thousand). These results reveal that redox reaction as well as fluid-rock reaction are key factors controlling the large-scale uranium mineralization at Baimadong. Combined with these observations, geological setting, and previous geochemical features, we suggest that tectonic system together with overprinting zone of black and red alterations are indicators for exploration of carbonate-hosted uranium deposit in South China and elsewhere worldwide.