The origin and role of CH4–N2 in the formation of Yantongqiaozi gold deposit, Jilin Province, NE China

• CH 4 in fluid inclusions are from thermal decomposition of organic matter. • N 2 is from the breakdown of NH 4 + -containing minerals in the metamorphic strata. • Fluids undergo metamorphic rock compositional exchange and meteoric water dilution. • Fluid effervescence, water–rock interaction and dilution give mineral precipitation. The Yantongqiaozi gold deposit, located in the southern Jilin Province of Northeast China, is hosted within a breccia pipe and surrounded by Archean metamorphic rocks. This deposit has four paragenetic stages: stage I pyrrhotite–pyrite–quartz, stage II gold–pyrite–quartz, stage III sphalerite–pyrite–quartz, and stage IV pyrite–quartz–calcite. Additionally, it contains CH 4 –N 2 -rich fluid, which is associated with gold mineralization; however, the origin and evolution of this fluid remains unclear. Fluid inclusion petrography and microthermometry revealed that stage I is characterized by medium–high temperature CH 4 –H 2 O fluids (296–359 °C), which evolves into medium-temperature N 2 –CO 2 –H 2 O fluids (251–323 °C) in stage II, further shifting into low-temperature NaCl–H 2 O fluids (165–254 °C) in stages III and IV. The H–O–C–N isotopes record that the initial magma fluids were contaminated by organic matter from the surrounding strata, followed by water–rock interaction and gradual dilution by meteoric water. CH 4 in the stage I fluid is generated by the thermal decomposition of organic matter, possibly from Jurassic coal-bearing strata. CH 4 -rich fluids facilitate the activation and migration of gold from the surrounding strata. The simultaneous trapping of single-phase CH 4 and H 2 O inclusions indicate that fluid effervescence is the primary mechanism of metal precipitation in stage I. The conversion of CH 4 to CO 2 in stage II fluid is indicative of increased oxygen fugacity. N 2 , likely owing to the breakdown of NH 4 + -containing minerals formed during metamorphism. Isotopic fractionation patterns suggest that extensive water–rock interactions lead to gold precipitation in stage II. Substantial temperature decreases and volatile losses in the stage III and IV fluids indicate continuous dilution by meteoric water, resulting in mineral precipitation. Unlike that in reduced porphyry and orogenic gold deposits, the gold mineralization at Yantongqiaozi derives from the combined effects of magmatic and metamorphic processes. Together, our findings highlight the pivotal role of CH 4 –N 2 -rich fluids in the migration and precipitation of gold and support a porphyry–orogenic transitional mineralization model for the Yantongqiaozi deposit. This model is expected to provide insights regarding the genesis and exploration of similar deposits.