Origin and evolution of ore-forming fluid for the Gaosongshan gold deposit, Lesser Xing'an Range: Evidence from fluid inclusions, H-O-S-Pb isotopes

Epithermal gold deposits are typical precious metal deposits related to volcanic and subvolcanic magmatism. Due to the lack of direct geological and petrographic evidences, the origin of the ore-forming fluid is deduced from the spatial diagenesis-mineralization relationship, chronological data, physicochemical characteristics of mineral fluid inclusions, mineral or rock elements and isotopic geochemical characteristics. By objectively examining this scientific problem via a geological field survey and petrographic analysis of the Gaosongshan epithermal gold deposit, we recently discovered and verified the following points:(1) Pyrite-bearing spherical quartz aggregates (PSQA) occur in the rhyolitic porphyry; (2) the mineralization is structurally dominated by WNW- and ENE-trending systems and occurs mostly in hydrothermal breccias and pyrite-quartz veins, and the ore types are mainly hematite-crusted quartz, hydrothermal breccia, massive pyrite-quartz, etc.; (3) the alteration types consist of prevalent silicification, sericitization, propylitization and carbonation, with local adularization and illitization. The ore minerals are mainly pyrite, primary hematite, native gold, and electrum, with lesser amounts of chalcopyrite, magnetite, sphalerite, and galena, indicating a characteristic epithermal low-sulfidation deposit. The ore-forming fluid may have been primarily derived from magmatic fluid exsolved from a crystallizing rhyolitic porphyry magma. Further zircon U-Pb geochronology, fluid inclusion, physicochemical and isotopic geochemical analyses revealed that (1) rhyolitic porphyry magmatism occurred at 104.6±1.0 Ma, whereas the crystallization of the PSQA occurred at 100.8±2.1 Ma; (2) the hydrothermal fluid of the pre-ore stage was an exsolved CO₂-bearing H₂O-NaCl magmatic fluid that produced inclusions mainly composed of pure vapor (PV), vapor-rich (WV) and liquid-rich (WL) inclusions with a small number of melt-(M) and solid-bearing (S) inclusions; mineralization-stage quartz contains WL and rare PV, WV and pure liquid (PL) inclusions characterized by the H₂O-NaCl system with low formation temperatures and low salinities; (3) the characteristics of hydrogen, oxygen, sulfur, and lead isotopes and those of rare earth elements (REEs) provide insight into the affinity between PSQA and orebodies resulting from juvenile crust or enriched mantle. Combined with previous research on the mineralogenetic epoch (99.32±0.01 Ma), we further confirm that the mineralization of the deposit occurred in the late Early Cretaceous, which coincides with the extension of the continental margin induced by subduction of the Pacific Plate beneath the Eurasian Plate. The formation of the ore deposit was proceeded by a series of magmatic and hydrothermal events, including melting of enriched juvenile crust, upwelling, the eruption and emplacement of the rhyolitic magma, the exsolution and accumulation of magmatic hydrothermal fluid, decompression, the cooling and immiscibility/boiling of the fluid, and mixing of the magmatic fluid with meteoric water, in association with water-rock interaction.