Fluid inclusion studies of the Chinkuashih high-sulfidation gold–copper deposits in Taiwan

Epithermal high-sulfidation Au–Cu deposits at Chinkuashih in northeastern Taiwan are found both within Pleistocene calc-alkaline volcanic rocks and the Miocene sedimentary rocks. The deposits consist of gold–copper veins of the Penshan group and breccia pipes of the Tushihshan–Changjen group. Primary two-phase inclusions in hydrothermal quartz and secondary two-phase inclusions in hydrothermal quartz as well as igneous quartz phenocrysts probably represent fluids related to mineralization. These inclusions have homogenization temperatures generally between 190°C and 280°C and salinities mostly from 0.5–5.0 wt.% NaCl eq. In the Penshan group, microthermometric data indicate that variable amounts of boiling, mixing and cooling occurred in the hydrothermal system. Based on the Th of inclusions trapped from boiling fluids and the CO2 contents of the inclusions, the hydrostatic pressure is estimated at 6.9–7.9 MPa. In the Tushihshan–Changjen group, the microthermometry of sulfur gas-bearing secondary two-phase fluid inclusions from hydrothermal quartz shows bimodal contents for a certain Th value, perhaps due to the intermittent incursion of brine at depth by phreatic or phreato-magmatic explosions. The presence of poly-phase (liquid+vapor+halite±sylvite±opaques) fluid inclusions with high Th (307°C–484°C) and high salinities (30–51 wt.% NaCl eq.) in one sample suggests a dense brine of probable magmatic derivation circulated in the Chinkuashih system. Halite-bearing inclusions which were homogenized by vapor disappearance coexisting with low-salinity vapor-rich inclusions in this sample may have been trapped in an immiscibility field at vapor-pressures from 7.5 to 21.5 MPa. Halite-bearing inclusions which were homogenized by halite dissolution after the vapor disappearance may have been trapped under even greater pressures. Fluid inclusion gas in samples were analyzed with a QMS. The results of bulk analyses indicate that these inclusions contain 99.45–99.67 mol% H2O, 0.27–0.52 mol% CO2, 0.015–0.066 mol% N2, 0.0015–0.015 mol% CH4 and small amounts of SO2 in three samples. These data suggest that the inclusion fluids are dominantly meteoric waters that interacted with host rocks and accumulated upward-fluxing magmatic volatiles. The fluid inclusion data indicate that the hydrothermal fluids related to the ore deposition of the high sulfidation Au–Cu deposits at Chinkuashih are characterized by the interaction of magmatic fluids with meteoric water under epithermal conditions.