Ore-forming conditions and genesis of the Huogeqi Cu–Pb–Zn–Fe deposit in the northern margin of the North China Craton: Evidence from ore petrologic characteristics

The Huogeqi Cu–Pb–Zn–Fe deposit is located in the Langshan district in the western segment of the northern margin of the North China Craton. The deposit is hosted by upper greenschist-lower amphibolite facies (550–650 °C) rocks of the Langshan Group. Most orebodies are hosted in a shear zone that developed in parallel with the sedimentary bedding of the Langshan Group. Fe was precipitated coeval with the deposition of the host rocks (i.e., a syngenetic origin), whereas Cu, Pb and Zn were precipitated in hydrothermal systems postdating formation of the host rocks (i.e., an epigenetic origin). The hydrothermal mineralization process can be subdivided into two stages: 1, a main stage with predominant Cu–Pb–Zn mineralization stage where ore-forming structures are characterized by brittle–ductile and brittle-shear deformation. The main stage sulfide veins cut the mylonite fabrics of the host rocks. Mineral paragenesis and compositions indicate that main-stage Cu mineralization took place at 330–440 °C and 3.4–3.9 kbar. This pressure–temperature (P–T) condition is consistent with that of the brittle–ductile transition zone but lower than that of the peak metamorphism of the host rocks. Compared with Cu mineralization, the main stage Pb–Zn mineralization took place at relatively lower temperature and pressure. The syngenetically formed Fe orebodies and iron formations in the Huogeqi deposit were favorable sites for epigenetic Cu precipitation. The H2S-rich Cu fluids would have reacted with Fe in the host rocks when flowing through Fe orebodies and iron-formations. Such a reaction would have led to a reduction in ɑH2S of the ore-forming fluids and consequently Cu precipitation; 2, the late stage Cu-mineralization is of less economic importance, characterized by open-space filling textures and a low-temperature mineral assemblage that constrains the ore-forming temperature as lower than 330 °C. This late stage mineralization took place in an epithermal-like hydrothermal system at a shallower crustal depth.