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

• The Chaihulanzi gold deposit is a lode gold deposit. • The primary fluids were an oxidizing mesothermal low salinity NaCl–H 2 O ± CO 2 system. • The evolution of the fluids into a reductive NaCl–H 2 O–CH 4 –CO 2 system by interaction with wall rocks. • The phase separation, methane reductants, and sulfidation resulted in the gold precipitation. 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: H 2 O–NaCl (type I), H 2 O–NaCl–CH 4 –CO 2 (type II), and CH 4 –CO 2 (type III). The primary type I FIs in stage I indicate that the initial hydrothermal fluids were a mesothermal low salinity NaCl–H 2 O–CO 2 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 (CO 2 –rich), and IIIb (CH 4 –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–H 2 O–CH 4 –CO 2 system. Abundant CH 4 –rich FIs in stage III indicate that the fluid was transformed into a medium-to-low temperature and low-salinity NaCl–H 2 O–CH 4 ± CO 2 system. The properties of stage IV FIs indicated a low-temperature and low-salinity NaCl–H 2 O 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–H 2 O–CO 2 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–H 2 O–CH 4 –CO 2 system. The precipitation of Au was attributed to the combined effects of phase separation, reducing methane agent, and sulfidation of iron-containing minerals.