Geological Characteristics and Fluid Inclusions of the Wujiawan Gold Deposits in Northern Hanyin, Shaanxi

The Wujiawan gold deposit is an important gold deposit in northern Hanyin, Shaanxi. It is located at the Shiquan-Shenhe thrust-napped ductile shear zone in South Qinling. The field geological study found that the ore-bearing horizon of the Wujiawan gold deposit is the fourth lithological layer (S₁m₄¹) of the first lithology section of the Meiziya Formation, and this layer is mainly composed of carbon-bearing sericite schist with siliceous rocks; The structure is mainly composed of two layer ductile shear zones (RFa and RFb), which can roughly distinguish the three-phase structural facies of S₁, S₂ and S₃. In which, S₀ is almost completely replaced, and S₂ deformation is closely related to gold mineralization. Gold ore body was mainly formed in the ductile shear zone during S₂ deformation. Based on geological research, the quartz veins from the Wujiawan Gold Mine can be divided into three phases. The study of fluid inclusions shows that these inclusions are dominated by gas-liquid two-phase inclusions, with rare pure gas phase, pure liquid phase and three-phase inclusions. Their homogeneous temperatures range from 176.8℃ to 344.4℃, which is concentrated at 230~280℃. Their salinity vary from 0.35 wt% to 13.51 wt% and are concentrated in 2~10 wt%. The ore-forming fluid belongs to medium-low temperature and low salinity fluid. The fluid capture depth is 1.38~3.47 km and is concentrated at 1.5~2.8 km, and the mineralization is occurred in the middle and shallow parts. The fluid inclusions of quartz vein in S₂ stage are closely related to mineralization, their homogeneous temperatures are mainly concentrated at 240~280℃, which are the fluid in the main ore-forming period. Laser Raman spectroscopy analysis shows that the liquid phase composition of gas-liquid two-phase inclusions is dominated by H₂O, containing a small amount of CO₂ and CH₄. The gas phase components are mainly CO₂, followed by CH₄, N₂ and H₂S. The inclusion types are mainly H₂O-CO₂ system inclusions with rich CO₂. Compared to the properties of fluid inclusions in different areas of northern Hanyin, it is found that the homogeneous temperatures of fluid inclusions range from 190℃ to 260℃, and their salinities vary form 6 wt% to 10 wt%, which are beneficial to mineralization.