Controls on the formation of the Changgou gold deposit and implications for exploration within the North Hanyin area, South Qinling Mountains, China

This paper focuses on the area around the Changgou gold mine within the North Hanyin area of the South Qinling Mountains, China. We combine new large-scale field geological, tectonic, and lithofacies data with the results of previous research and present new information on the geological characteristics of mineralization within this region. These data indicate that the Changgou gold mine is a brittle-ductile shear zone associated gold deposit and that the key controls on the formation of the deposit can be used to identify areas in this region that are favourable for future exploration. The key control on the spatial distribution of mineralization within the Changgou gold mine area is the location of the DSZ3 brittle-ductile shear zone, with secondary faults and fractures controlling the specific locations of gold orebodies. The structural deformation is dominated by the development of an S-2 foliation that strikes 290-350 degrees (predominantly inclination 230 degrees and dip 30 degrees). Gold-bearing pyrite is preferentially aligned along the S-2 foliation, and individual orebodies are preferentially located along the S-2 foliation. Therefore, this foliation controlled the location of mineralization at both micro- and macroscopic scales. Mineralization also occurs in S-2-phase quartz veins that contain visible granular gold, whereas the S-3-phase quartz veins are generally barren and are not related to the location of gold mineralization. Fluid inclusion microthermometry indicates that gold-bearing quartz veins homogenized at 190-260 degrees C and have salinities (in NaCl equivalent terms) of 6-12%. All of these fluid inclusions contain gas and liquid phases dominated by CO2 and H2O, respectively, indicating that they are CO2-H2O type. The hydrothermal fluids that formed the Changgou deposit are similar to those that generate epithermal deposits (i.e., moderate- to low-temperature, moderate- to low-salinity, and low-density CO2-H2O fluids). Our research into the geological characteristics of the quartz veins that formed at different times in the study area and their associations with different styles of mineralization and alteration indicates that the hydrothermal fluids associated with the S-2 period of deformation were the main ore-forming fluids associated with the Changgou gold deposit.