Genesis of the Bianbianshan Cu-polymetallic deposit of the Xilamulun Cu-Mo metallogenic belt, Inner Mongolia, China: A fluid inclusion and O-H-S Pb isotope study

The Bianbianshan Cu-polymetallic deposit is located in the Xilamulun Cu-Mo belt of Inner Mongolia, NE China, and occurs within Late Jurassic andesite, rhyolite and tuff. Cu mineralization is observed in various types of hydrothermal disseminated veins exhibiting siliciification, sericitization and chloritization. The ore-forming process occurred in four stages: (1) chalcopyrite-pyrite-quartz, (2) pyrite-chalcopyrite-quartz-vein, (3) sphalerite-galena-quartz-vein, (4) and quartz-calcite-vein. Three types of fluid inclusions were identified in the deposit, including C-type (CO2-NaCl-H2O) as well as V- and L-type (NaCl-H2O) inclusions, where only L-type inclusions are observed in the fourth stage. Homogenization temperature of fluid inclusions in the four stages are 325-365 degrees C, 282-335 degrees C, 268-312 degrees C, and 195-259 degrees C, respectively, all of which exhibit salinities lower than 13 wt% NaCl equiv. Trapping pressure estimated from C, V- and L-fluid inclusions are between 550 and 900 bars, corresponding to minimum depths of 2.5-3.7 km. Hydrogen and oxygen isotopic compositions of quartz from different stages (delta D = 109.7 similar to 107.6%o, delta O-18 = 7.6-9.6%o, V-SMOW) suggest that the mineralizing fluid gradually evolved from a magmatic to a meteoric origin. Sulfur (delta(34)(Sv-CDT) = 0.7 to 0.4%o) and lead isotopes ((208)pbi(204)pb = 37.746-38.137%o; Pb-208/Pb-204 = 15.52-15.56%; Pb-208/Pb-204 = 17.75-18.36%) indicate that ore-forming materials were derived from mantle with minor additional participation of crustal reservoirs. Our data suggests the ore-forming fluids of the Bianbianshan Cu-polymetallic deposit are attributed to a moderate temperature, moderate-low salinity, and moderate density system, where decreases in pressure, phase separation and local mixing with meteoric water drove precipitation of metals.