Geology and geochemistry of the Bulong quartz-barite vein-type gold deposit in the Xinjiang Uygur Autonomous Region, China

Two types of fluid inclusions, namely H2O-NaCl and H2O-CO2 (± CH4/N2)-NaCl types, have been recognized in quartz, barite and calcite. Homogenization temperatures of fluid inclusions vary from 159 to 390 °C, with Th during the main stage of gold mineralization ranging from 200 to 340 °C. Salinity ranges from 2.4 to 46.2 wt.% NaCl equiv. Ore fluid densities range from 0.73 to 1.13 g/cm3. The δ34S values of pyrite associated with gold mineralization range from 14.6‰ to 19.2‰ and those of barite from 35.0‰ to 39.6‰, indicating that the sulfur was probably derived from the sedimentary host rocks. The 3He / 4He ratios of fluid inclusions in pyrite are 0.24-0.82 (R / Ra), approximating helium ratios of the crust. The 40Ar / 36Ar ratios vary from 338 to 471, slightly higher than those indicative of atmospheric argon. The 40Ar / 4He ratios of the ore fluids range from 0.02 to 0.41 with a mean of 0.15. Helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the Bulong gold deposit were mainly derived from the crust. The δ13CPDB values of fluid inclusions in vein quartz from the Bulong gold deposit define a narrow range of − 4.6‰ to − 1.4‰. The δ18OSMOW values of vein quartz range from 17.2‰ to 21.1‰, with corresponding δ18Ofluid values of 6.7‰ to 14.7‰, and δD values of fluid inclusions of between − 70‰ and − 55‰. The combined isotopic data imply that the ore-forming fluids of the Bulong gold deposit were mainly derived from basinal fluids, with some minor contributions from magmatic fluids and meteoric water, and the carbon in the ore fluids was mainly derived from marine carbonate rocks. The temperature and pressure decrease, local ore-forming fluid boiling, water-rock exchange, changes in composition of ore fluids played important roles in ore-forming processes of the Bulong gold deposit.