Pure CO2 fluids in the Sarekoubu gold deposit at southern margin of Altai Mountains in Xinjiang, West China

Sarekoubu gold deposit occurs in metamorphosed acid volcanic rocks and volcaniclastic rocks of Devonian Kangbutiebao Formation along the southern margin of Altai Mountains in Xinjiang, West China. Pyritization, silicification and carbonatization are developed in wallrocks, and main gold mineralization stages are pyrite-quartz stage (II) and polymetallic sulfide stage (III). Primary fluid inclusions are often seen in main gold mineralization stages, which are almost pure CO2 liquid inclusions ({ie333-01}) under room temperature with high densities of 0.85–1.07 g/cm3. CO2-rich inclusions ({ie333-02}) and H2O-rich inclusions ({ie333-03}) are subordinate. Fluid inclusions in stage I (polymetallic sulfide stage) are more complex than in stage II, containing CO2-CH4 system with 78.1–61.9°C of solid CO2 melting temperatures and 33.7–17.7°C of partial homogenization temperatures. The trapping pressures of high densities are estimated to be 150–350 MPa. Laser Raman microprobe was used to confirm the composition of pure CO2 liquid inclusions. δ13C values of CO2 inclusions range from 10.73‰ to 21.15‰, which are similar to some of fluids in the mantle-derived minerals that contain surface organic carbon by subduction recycling. These characteristics differ from many hydrothermal gold deposits. CO2 fluids have a regional source related to post-collisional orogeny.