Characteristics of Quartz In Situ Trace Elements and Fluid Inclusions in the Jinshan Gold Deposit, Jiangxi Province, and Their Implications for the Ore-Forming Process

The Jinshan gold deposit is a super-large orogenic gold deposit located in the eastern segment of the Jiangnan Orogenic Belt. The Au-bearing quartz veins developed in the ore body record the physical and chemical conditions of the polysulfide-quartz-ferromagnesian carbonate stage (main mineralization stage). However, the lack of microscale process analyses has limited in-depth understanding of the mineralization process. To reveal the fine mineralization process of mineral deposits, this article uses in situ microanalytical techniques, including scanning electron microscope-cathodoluminescence (SEM-CL), fluid inclusion microthermometry, laser Raman spectroscopy analysis, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis based on detailed petrographic observations to explore the evolution of ore-forming fluids and mineral precipitation mechanisms. Research has shown that quartz in Au-bearing quartz veins can be classified into early generation quartz (Qz1) and late generation quartz (Qz2) through SEM-CL image features, with gold mineralization closely related to Qz2. The LA-ICP-MS trace element analysis results of quartz show that Al mainly exists in quartz in the form of electricity compensation by binding with alkali metal Li, and the cathodoluminescence intensity is jointly controlled by the content of Al and Li. The Al content in Qz1 is lower than that in Qz2, and Ti content is higher than that in Qz2, recording the process of pH increase caused by fluid–rock interaction and temperature decrease. In Qz1, H2O inclusions (type III) are mainly developed, with homogenization temperatures of 342–391 °C. In Qz2, H2O-CO2 inclusions (type I), CO2-rich inclusions (type II), and H2O inclusions (type III) are mainly developed. Different types of inclusions in Qz2 coexist in groups, with similar homogenization temperatures (268–324 °C) and a significant difference in salinity (2.39–8.51% NaCleqv and 6.30–12.28% NaCleqv), indicating the occurrence of fluid immiscibility. The homogenization temperature of Qz1 to Qz2 inclusions gradually decreases, and the corresponding estimated pressures are 92.0–107.2 MPa and 73.8–85.9 MPa, recording the process of decreasing temperature and pressure of the ore-forming fluid. Therefore, fluid immiscibility caused by decreases in temperature and pressure, together with increased fluid pH caused by fluid-rock interaction and CO2 phase separation, were the main causes of gold precipitation in the Jinshan gold deposit.