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

Abstract

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 CH₄–N₂-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 CH₄–H₂O fluids (296–359 °C), which evolves into medium-temperature N₂–CO₂–H₂O fluids (251–323 °C) in stage II, further shifting into low-temperature NaCl–H₂O 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. CH₄ in the stage I fluid is generated by the thermal decomposition of organic matter, possibly from Jurassic coal-bearing strata. CH₄-rich fluids facilitate the activation and migration of gold from the surrounding strata. The simultaneous trapping of single-phase CH₄ and H₂O inclusions indicate that fluid effervescence is the primary mechanism of metal precipitation in stage I. The conversion of CH₄ to CO₂ in stage II fluid is indicative of increased oxygen fugacity. N₂, likely owing to the breakdown of NH₄⁺-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 CH₄–N₂-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.