Genesis and mineralization of gold-bearing quartz veins in the Xiao Qinling area, central China

Gold-bearing quartz veins of the Taihua Group consisting of Archean metavolcanic rocks are a main gold deposit type in the Xiao Qinling area, one of the three biggest gold production areas in China. The quartz veins experienced strong alteration characterized by a typical mesothermal hydrothermal altered mineral assemblage. The grade of gold is affected by the contents of sulphides, e.g. galena, pyrite and chalcopyrite. Results of minor elements analysis for the of gold-bearing quartz veins indicate higher contents of Au and high contents of Ag, Pb, Cu, Cd, W, and Mo. Abundant fluid inclusions were found in the gold-bearing quartz veins. Three types of fluid inclusions were identified: (1) aqueous inclusions; (2) CO₂-bearing inclusions; and (3) daughter crystal-bearing fluid inclusions. Homogenization temperatures ranged from 110 to 670°C with low and high peaks appearing at 160?C180°C and 280?C300°C, respectively. The salinity of aqueous inclusions varies between 1.8 wt% and 38.2 wt% NaCl. The homogenization temperature and salinity show a positive correlation. The H and O isotopes of fluid inclusions in the gold-bearing quartz veins indicate that magmatic solution and metamorphic hydrothermal solution, together with meteoric water, were involved in the formation of gold-bearing fluid. Mesozoic magma activities related to granite intrusions should be the main source of CO₂ fluid with higher temperature and salinity.