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 0.91 g/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 8DV-SMOW value of the H isotope show values of-102.8‰ to-96.7‰,and the 813CV-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.
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