The nature and timing of ore formation in the Budunhua copper deposit, southern Great Xingʹan Range: Evidence from geology, fluid inclusions, and U–Pb and Re–Os geochronology

The Budunhua copper deposit is located in the southern Great Xingʹan Range where the stockwork ore bodies are mainly hosted in the Lower Permian Dashizhai Formation. The mineralization occurs in two blocks – the Jinjiling and Kongqueshan – and both are characterized by four main stages: arsenopyrite–pyrite–(molybdenite)–quartz, chalcopyrite–pyrrhotite–quartz, galena–sphalerite–chalcopyrite–quartz and calcite–fluorite–quartz. Wall-rock alteration, which is related to mineralization, is dominated by silicification, sericitization, and chloritization. Fluid inclusion studies show that the ore-forming fluids from Cu stages in both blocks are broadly identical with trapping temperatures of 280 to 400 °C for the Kongqueshan, and 300 to 420 °C for the Jinjiling. Estimated trapping pressures for the Kongqueshan are 15.0 to 26.6 MPa and for the Jinjiling are 14.9 to 29.4 MPa, corresponding to entrapment depth of 1.5 to 2.7 km and 1.5 to 2.9 km below the paleowater table, respectively. Zircon grains from the tonalite porphyry in the deposit yielded a weighted 206Pb/238U mean age of 152 ± 0.7 Ma, which is consistent with a molybdenite Re–Os model age of 150 ± 2.2 Ma, indicating that both the porphyry intrusion and the copper deposit are of Late Jurassic age. Our data suggest that the Budunhua deposit is an atypical porphyry deposit related to a Late Jurassic magmatic-hydrothermal system.