P-T-X conditions, origin, and evolution of Cu-bearing fluids of the shear zone-hosted Huogeqi Cu–(Pb–Zn–Fe) deposit, northern China

Huogeqi is a shear zone-hosted epigenetic deposit within the greenschist-amphibolite facies of the Mesoproterozoic Langshan Group in the Langshan district on the northern margin of the North China Craton (NCC). Copper mineralization in the Huogeqi deposit was formed in two stages: a main-stage controlled by the shear zone and characterized by brittle-ductile ore-forming structures and a lower greenschist facies mineral assemblage, and a late stage characterized by open space-filling textures and low-temperature minerals. Based on microthermometric and Raman microprobe analysis, the main-stage Cu-bearing fluid was mesothermal, low-salinity and H2O–CH4-dominant, and was generated by an interaction between a deep-crustal metamorphic fluid and graphite-bearing host rocks. This interaction resulted in a more CH4-rich fluid, which was more amenable to be immiscible. We showed that immiscibility of the H2O–CH4 fluid occurred due to temperature decrease, prior to the main-stage Cu mineralization; Cu was finally precipitated from the resultant H2O-rich aqueous fluid. Main-stage Cu mineralization temperature was obtained using various methods: 310–370 °C by intersection of isochors of coexistent CH4 and aqueous inclusions; 364 ± 41 °C on average by pressure correction of the homogenization temperatures of aqueous inclusions; and 362 ± 26 °C using the chlorite geothermometer. Pressure during Cu-deposition fluctuated between lithostatic and hydrostatic at depths of ca. 10–12 km, but it seemingly had no effect on the mineralization process. The late-stage Cu-bearing fluid was a low temperature, low salinity, H2O of meteoric origin.