Role of methane-rich fluids in mesothermal gold mineralization: Insights from the Chaihulanzi gold deposit, North China Craton

Lode gold deposits in low-grade greenschist belt account for an estimated 40–45 % of the global gold endowment. Au-migrating fluids are commonly metamorphic, low-salinity, and aqueous-carbonic in nature, and flow along high-permeability fault zones where methane-rich fluids may appear under special physicochemical conditions. The Chaihulanzi deposit located in the northern margin of the North China Craton is a large lode Au deposit characterized by abundant methane-rich inclusions. In this study, we examined the nature and isotopic composition of ore-forming fluids to identify their origins, evolution, and roles in Au mineralization. Based on their nature and phase transition patterns, three types of fluid inclusion (FI) were identified: H₂O–NaCl (type I), H₂O–NaCl–CH₄–CO₂ (type II), and CH₄–CO₂ (type III). The primary type I FIs in stage I indicate that the initial hydrothermal fluids were a mesothermal low salinity NaCl–H₂O–CO₂ system. Stage II fluids are characterized by coexisting assemblages of type I, IIa (carbon phase occupying 20–50 vol%), IIb (carbon phase occupying 50–80 vol%), IIIa (CO₂–rich), and IIIb (CH₄–rich) FIs, which display different homogenization modes at similar homogenization temperatures. The wide range of X_CH4 suggests the addition of a foreign methane-rich fluid, indicating that the ore-forming fluids evolved into a medium-to-low-temperature and low-salinity NaCl–H₂O–CH₄–CO₂ system. Abundant CH₄–rich FIs in stage III indicate that the fluid was transformed into a medium-to-low temperature and low-salinity NaCl–H₂O–CH₄ ± CO₂ system. The properties of stage IV FIs indicated a low-temperature and low-salinity NaCl–H₂O system. The H–O–C isotope data of stage I suggest that the primary fluids were derived from a dominant magmatic origin. The increasingly depleted H–O–C isotope data indicate the progressive involvement of a foreign methane-rich fluid in stage II. The fluids in stage III show an increased degree of fluid mixing. In conclusion, our data confirmed that the primary ore-forming fluids were oxidizing mesothermal low-salinity NaCl–H₂O–CO₂ systems. With the mixing process of wall-rock buffered fluids, the main metallogenic stage fluids evolved into a reductive medium-to-low temperature and low-salinity NaCl–H₂O–CH₄–CO₂ system. The precipitation of Au was attributed to the combined effects of phase separation, reducing methane agent, and sulfidation of iron-containing minerals.