The source of hydrothermal fluids for the Sarekoubu gold deposit in the southern Altai, Xinjiang, China: Evidence from fluid inclusions and geochemistry

The Sarekoubu gold deposit, located at the southern margin of the Altai Mountains, Xinjiang, occurs in metamorphic acid volcanic and volcaniclastic rocks of Devonian Kangbutiebao Formation. Echelon-like ore-bearing veins occur as dextrorotation, intersect ore-controlling faults at small angles. Ore-bearing minerals in the vein systems are mainly native gold, electrum, pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, and native bismuth. The main gold mineralization stages are the pyrite–quartz stage (II) and the polymetallic stage (III). Native bismuth was first found in this study by electron microprobe analysis. R-type cluster analysis shows that Bi exhibits a close relationship to Au in the ore samples and wall rock samples, with a correlation coefficient of 0.75. Relatively high amounts of Bi in this area can be used as an important indicator for gold mineralization, and native bismuth may be a sign indicating gold-rich veins. Primary fluid inclusions in main gold mineralization stages are mainly pure liquid CO2 inclusions (LCO2) under room temperatures with high densities of 0.85–1.07 g/cm3. CO2–H2O inclusions (LCO2–LH2O) and aqueous inclusions (LH2O–LCO2) are subordinate. A ICP-MS study shows that the ΣREEs of fluid inclusions in vein quartz is very high which is related to CO2-rich ore-forming fluid, and the fractionation of light REEs and heavy REEs in fluids of vein quartz is not distinct. Carbon isotope analysis on the Sarekoubu gold deposit shows that δ13C of the CO2 fluid inclusions ranges from −10.23‰ to −21.15‰, and the δ13C of CH4 in fluid inclusions varies from −32.02‰ to −34.11‰. It has been concluded that the origin of the CO2 fluids in the Sarekoubu gold deposit may be a deep source during the late Paleozoic collisional orogeny.