Abstract: The Xiaobeigou gold deposit is located in the middle of Jiapigou metallogenic belt and is a quartz vein-altered rock type gold deposit. The ore-bearing surrounding rock of the mining area is mainly high amphibolite and granulite facies plagioclase amphibolit. The gold-bearing quartz veins are small, discontinuous parallel veins or lenticular bodies found within secondary brittle shear fractures superimposed on an early ductile shear zone trending NW. There are also some low-grade altered rock type orebodies (altered mylonite and gylite) developed on the sides of the upper and lower plates, and at the apex. The study of fluid inclusions from the main ore-forming stage shows four types of primary fluid inclusions: CO2, CO2-rich, CO2-containing and gas-liquid two phases. The homogenization temperatures range from 226.8 to 362.5 ℃, with salinity w(NaCl) between 3.57% and 11.95%, and density of 0.65 to 0.91g/cm3. The ore-forming fluid generally belongs to the CO2-H2O-NaCl system, characterized by low temperature, low salinity and low density. The ore-forming pressure ranges from 70 to 206 MPa, corresponding to a depth of about 9.8 km. The δ18OV-SMOW composition of the O isotope ranges from 10.8‰ to 11.3‰, while δDV-SMOW value of the H isotope show values of -102.8‰ to -96.7‰, and the δ13CV-PDB values of the C isotope are between -20.6‰ and -8.3‰, indicating that the ore-forming fluid source of the main ore-forming stage is of mantle origin. The homogenization temperature of CO2 and CO2-rich inclusions are notably lower at -24.7 to 26.6 ℃ compared to other nearby deposits (such as Sandaocha gold deposit of 21.9 to 29.7 ℃, Erdaogou gold deposit of 23.8 to 30.9 ℃, etc.), suggesting a deeper mineralization. The geological, fluid inclusions and H-O-C isotopic characteristics of Xiaobeigou gold deposit indicate it is a medium-temperature hydrothermal vein type gold deposit, transitioning from the brittle zone to the deep ductile-brittle zone. There remains potential for further deep exploration, though the ore type may change from quartz vein type to tectonic-altered rock type or tectonic-altered mylonite type.

Key words: ore-forming fluid')">

ore-forming fluid, fluid inclusions, H-O-C isotopes, mesothermal vein gold deposit, Xiaobeigou gold deposit, Jiapigou metallogenic belt