The physicochemical controls on gold precipitation in orogenic gold deposits remain poorly constrained, with traditional fluid inclusion and isotopic studies often yielding ambiguous results due to overprinting or incomplete records. This study addresses this challenge using chlorite—a sensitive mineral proxy for fluid conditions—as a quantitative sensor in the Jinshan orogenic gold deposit (>200 t Au) of the Jiangnan orogenic belt, South China. Hosted in Neoproterozoic phyllite within NE–NNE-trending ductile–brittle shear zones, Jinshan features auriferous quartz–polymetallic sulfide veins with prominent chlorite alteration. Integrating high-resolution SEM-EPMA analyses of multi-generational chlorite with thermodynamic modeling, we reconstruct the temporal evolution of temperature, oxygen fugacity (fO₂), pH and sulfur fugacity (fS₂) during ore formation. Four paragenetic stages are identified: Stage 1 (ankerite–quartz), Stage 2 (pyrite–arsenopyrite–quartz), Stage 3 (quartz–gold–polymetallic sulfide), and Stage 4 (chlorite–carbonate–quartz). Electron microprobe analysis reveals that the chlorite composition changes from Fe-rich chamosite (Stage 2) to Mg-rich clinochlore (Stage 3) and then to Fe-rich chamosite (Stage 4). Chlorite from Stage 2 (Chl-1) formed metasomatically at low fluid/rock ratios, while Stage 3 and 4 chlorites (Chl-2 and Chl-3) precipitated directly from higher fluid/rock ratio fluids. Chlorite compositions record a critical Stage 2–3 transition involving cooling from ~320 °C to ~260 °C, reduction (log fO₂ from −33.6 to −39.7), and alkalinization, and sulfur fugacity remained stable within a narrow range (log fS₂ = −13.6 to −8.0), followed in Stage 4 by minor reheating to ~280 °C, re-acidification, and a slight rebound in oxygen fugacity. Thermodynamic simulations reveal that the destabilization of Au(HS)₂⁻ complexes, primarily driven by the synergistic effects of cooling, pH increase, and decreasing oxygen fugacity, triggered gold precipitation during the main ore stage. Results demonstrate that abrupt cooling coupled with fluid alkalinization and reduction exerted the dominant control on gold precipitation in Jinshan, resolving long-standing debates on ore-forming mechanisms and highlighting chlorite as a robust quantitative sensor for fluid evolution.

Hydrothermal cobalt (Co) deposits are a significant source of Co; however, the sources of Co and hydrothermal fluids for such deposits remain poorly understood. This study addresses this issue through an investigation of the geology, mineralogy, and in situ sulfur isotopes of the Qibaoshan Co-Pb-Zn-Cu deposit, a typical hydrothermal Co deposit in South China, to constrain the occurrence of Co and the sources of Co and hydrothermal fluids. Detailed scanning electron microscopy (SEM), TESCAN Integrated Mineral Analyzer (TIMA), and electron microprobe (EPMA) mapping analyses reveal that Co in the Qibaoshan deposit occurs predominantly as Co-bearing minerals in veinlet mineralization, mainly including cobaltite, skutterudite, and smaltite. EPMA elemental mappings reveal that cobaltite grains commonly show a compositional evolution from Ni-S-rich and As-Fe-poor cores to As-Fe-rich and Ni-S-poor rims. This evolution indicates a decrease in fluid temperature and Ni content, coupled with an increase in the As/S ratio during ore-forming processes. In situ S isotope analyses of various sulfides (pyrite, chalcopyrite, sphalerite, galena, and arsenopyrite) yield a wide range of δ³⁴S_V-CDT values from 0.24‰ to 19.08‰, with two dominant clusters at 2–5‰ and 15–17‰. This suggests two end-member sources for sulfur and hydrothermal fluids in the Qibaoshan deposit: magmatic and sedimentary sources. Arsenopyrite, which is closely associated with Co minerals, yields δ³⁴S_V-CDT values ranging from 2.17‰ to 5.99‰, pointing to a magmatic origin for Co in the Qibaoshan deposit. The Pb-Zn and Cu mineralization of the deposit was also likely mainly derived from magmatic sources, with the incorporation of sedimentary sulfur and fluids during the ore-forming processes. This study demonstrates that magmatic–hydrothermal fluids derived from depth can serve as sources of Co, even in hydrothermal deposits where no magmatic rock is exposed, which provides crucial implications for the metallogenic models and mineral exploration of hydrothermal Co deposits.