Genesis of the South Zhuguang uranium ore field, South China: Fluid inclusion and H–C–O–S–Sr isotopic constraints

Abstract

The South Zhuguang uranium (U) ore field in South China, which was discovered in the 1950s, contains an estimated >20,000 t of recoverable U. The U ore is hosted by the Zhuguang granite massif, which is located in the South China Block. The main uranium-bearing mineral, pitchblende, is found mainly in veins along high-angle normal faults, associated with hematite, quartz, fluorite, calcite, and locally pyrite. The hydrothermal alteration is generally silicification, illitization, hematization, and chloritization that developed adjacent to the ore-bearing faults and outwards over hundreds of meters into the unaltered granites.The ore-forming fluids in the South Zhuguang U ore field can be classified into two stages. Stage I fluids were acidic and oxidizing, with fluoride and sulfate complexes of UO22+ being the dominant species in the fluid. Stage II fluids were alkaline and oxidizing, with carbonate and hydroxide complexes of UO22+ being the dominant species. δ18O values of syn-ore quartz range from +6.29‰ to +14.30‰, with calculated δ18OW-SMOW values from −1.85‰ to +5.35‰. δDW-SMOW values of the ore-forming fluids were −104.4‰ to −23.1‰. δ18O values of the altered granites increase gradually from ∼+4‰ close to ore, to ∼+12‰ in the nearly unaltered granites. The δ13C and δ18O values for syn-ore calcite range from −10.3‰ to −4.3‰ and −22.41‰ to −16.8‰ relative to Pee Dee Belemnite, respectively, with calculated δ18O and δ13C values for water and CO2 of −2.97‰ to +2.83‰ and −11.2‰ to −5.2‰, respectively. The δ34SV-CDT values of syn-ore pyrite range from −17.1‰ to −3.4‰. Initial 87Sr/86Sr ratios of syn- and post-ore fluorite vary from 0.719250 to 0.721327 and 0.714598 to 0.716299, respectively.The ore-forming fluids in the South Zhuguang U ore field were sourced primarily from Cretaceous–Paleogene redbed basins in the south, and initially derived from meteoric waters. Hydrothermal alteration associated with stage I fluids released considerable amounts of Ca2+ and F− into the ore-forming fluids, increased the fluid pH, decreased the stability of the dominant fluoride complexes, and lowered the saturation solubility of the uranyl ions. Reduction of the ore-forming fluid converted U6+ into U4+, which resulted in the deposition of pitchblende–fluorite–quartz veins of the stage I ore in fractures along with pyrite. During stage II, the U mineralization was controlled mainly by CO2 degassing, which resulted in decomposition of the carbonate complex UO2(CO3)2–, and deposition of U in pitchblende–calcite–quartz veins as the ore-forming fluid was reduced.