Geology, fluid inclusion and stable isotope study of the Huangshan orogenic gold deposit: Implications for future exploration along the Jiangshan-Shaoxing fault zone, South China

Abstract

The Huangshan gold deposit is hosted in the ductile shear zone of the metamorphosed Precambrian Chencai Group in the Jiangshan-Shaoxing fault zone between the Yangtze and the Cathaysia blocks. The mineralization can be divided into three stages: pre-ore stage barren quartz-sericite veins, ore stage quartz-pyrite veins, and post-ore stage calcite-quartz veins. A combination of fluid inclusion microthermometry and Raman spectroscopy analysis reveals three types of fluid inclusions: H2O-CO2-NaCl (type I), CO2-rich aqueous (type II), and aqueous fluid inclusions (type III). Primary fluid inclusions in the pre-ore stage quartz veins are mainly type I with low salinities (≤6 wt.% NaCl equivalents) that homogenized to liquid at moderate temperatures (298 to 371°C). Primary fluid inclusions in the ore stage quartz veins contain the following: type I fluid inclusions with variable CO2 volumetric ratios, which homogenized to liquid at moderate temperatures (287 to 376°C) and with moderate salinity (2.0 to 7.7 wt.% NaCl equivalents), type II (CO2 rich) inclusions of low salinity (1.6 to 4.8 wt.% NaCl equivalents), and type III fluid inclusions of low-moderate salinity (4.7 to 9.5 wt.% NaCl equivalents). In particular, some type II and III inclusions occur in the same isolated cluster or fracture, where type II inclusions homogenized into the CO2 phase (263 to 304°C) and type III inclusions into the aqueous phase (226 to 305°C) at almost the same temperature. This occurrence indicates that fluid immiscibility took place during ore-forming processes. Aqueous secondary fluid inclusions in the trans-granular fracture of quartz grains and primary inclusions in the post-ore stage calcite display low homogenization temperature (135 to 227°C and low to moderate salinity (0.2 to 5.5 wt.% NaCl equivalents). Trapping pressures of the immiscible fluid inclusion assemblages from ore stage veins are between 87 and 162MPa, indicating the gold-bearing quartz veins were precipitated deeper than 3.2km. These pressures are too high and exclude the previously proposed epithermal model for this deposit. Raman spectroscopy shows that volatile compositions in the CO2-bearing inclusions are primarily CO2 with minor N2. The oxygen and hydrogen isotope compositions (δD=−56‰ to −67‰; δ18O=3.3‰ to 4.4‰) imply a likely metamorphic origin of the ore stage fluids. Sulfur (+4.1‰ to +5.4‰) and lead (206Pb/204Pb: 17.606 to 17.704; 207Pb/204Pb: 15.490 to 15.536; 208Pb/204Pb: 37.701 to 38.083) isotopes of the ore minerals show similar signatures to the host rocks of the Chencai Group. These data strongly indicate that the Huangshan gold deposit is an orogenic-type deposit. Combined with previously published geochronology data and the geological setting, we stress the existence of a Caledonian orogenic gold belt along the Jiangshan-Shaoxing fault zone and thus suggest an extensive target area for further gold exploration along the Jiangshan-Shaoxing fault zone.