Geological, fluid inclusion, and H–O–C–S–Pb isotopic constraints on the genesis of the Shuangqishan gold deposit, Fujian, China

The Shuangqishan Au deposit lies in the Dehua–Youxi–Yongtai Au metallogenic belt in central eastern Fujian Province, SE China. The orebodies are hosted mainly in the Neoproterozoic Dalingyan Formation. Based on the characteristics of mineralization, alteration mineral assemblages, fluid inclusions, and H–O–C–S–Pb isotopic data, the deposit is classified as a granitoid-related deposit. The primary metallic minerals are native gold, electrum, pyrite, chalcopyrite, galena, tellurbimuth, and tetradymite; the main gangue minerals are quartz, calcite, sericite, and illite. The main types of wall-rock alteration are silicification, pyritization, carbonatization, and sericitization. Fluid inclusions in quartz from auriferous polymetallic sulfide-bearing quartz veins from the main ore stage were studied. Two-phase liquid-rich (type I), CO2-rich (type II), and pure CO2 (type III) inclusions were identified. Their homogenization temperatures (196 °C–295 °C) and salinities (0.8–5.0 wt% NaCl equivalent) indicate that the ore-forming fluids of the Shuangqishan deposit were of a medium-temperature, low-salinity H2O–CO2–NaCl system. Gold deposition at Shuangqishan was likely caused by immiscibility of fluids. H isotopic compositions (δDH2O = −68.0‰ to −64.8‰) of inclusion water in quartz and O isotopic compositions (δ18OH2O = 3.8‰ to 6.3‰) of fiuid, which was calculated from δ18O of quartz, imply that ore-forming fluids were mainly of magmatic origin. C isotopic compositions (δ18CPDB = −15.7‰ to −5.7‰) of the fluid inclusions indicate that the CO2 in the ore-forming fluids was predominantly magma-derived, but was contaminated by organic carbon from the Neoproterozoic Dalingyan Formation. δ34S values of sulfide minerals have a narrow range of–2.7‰ to −0.9‰, suggesting that S was derived from a homogeneous magmatic source. Pb isotopic ratios of gold-bearing ores are similar to those of late Mesozoic granodiorite porphyry of the Xianyang pluton, indicating that the granodiorite porphyry was the source of Pb for the ores.