Fluid Inclusions, Stable Isotopes, and Geochronology of the Haobugao Lead-Zinc Deposit, Inner Mongolia, China

The Haobugao deposit, located in the southern segment of the Great Xing'an Range, is a famous skarn-related Pb-Zn-(Cu)-(Fe) deposit in northern China. The results of our fluid inclusion research indicate that garnets of the early stage (I skarn stage) contain three types of fluid inclusions (consistent with the Mesozoic granites): vapor-rich inclusions (type LV, with V-H2O/(V-H2O+L-H2O) < 50 vol %, and the majority are 5-25 vol %), liquid-rich two-phase aqueous inclusions (type VL, with V-H2O/(V-H2O+L-H2O) > 50 vol %, the majority are 60-80 vol %), and halite-bearing multiphase inclusions (type SL). These different types of fluid inclusions are totally homogenized at similar temperatures (around 320-420 degrees C), indicating that the ore-forming fluids of the early mineralization stage may belong to a boiling fluid system. The hydrothermal fluids of the middle mineralization stage (II, magnetite-quartz) are characterized by liquid-rich two-phase aqueous inclusions (type VL, homogenization temperatures of 309-439 degrees C and salinities of 9.5-14.9 wt % NaCl eqv.) that coexist with vapor-rich inclusions (type LV, homogenization temperatures of 284-365 degrees C and salinities of 5.2-10.4 wt % NaCl eqv.). Minerals of the late mineralization stage (III sulfide-quartz stage and IV sulfide-calcite stage) only contain liquid-rich aqueous inclusions (type VL). These inclusions are totally homogenized at temperatures of 145-240 degrees C, and the calculated salinities range from 2.0 to 12.6 wt % NaCl eqv. Therefore, the ore-forming fluids of the late stage are NaCl-H2O-type hydrothermal solutions of low to medium temperature and low salinity. The delta D values and calculated delta O-18(SMOW) values of ore-forming fluids of the deposit are in the range of -4.8 to 2.65 parts per thousand and -127.3 parts per thousand to -144.1 parts per thousand, respectively, indicating that ore-forming fluids of the Haobugao deposit originated from the mixing of magmatic fluid and meteoric water. The S-Pb isotopic compositions of sulfides indicate that the ore-forming materials are mainly derived from underlying magma. Zircon grains from the mineralization-related granite in the mining area yield a weighted Pb-206/U-238 mean age of 144.8 +/- 0.8 Ma, which is consistent with a molybdenite Re-Os model age (140.3 +/- 3.4 Ma). Therefore, the Haobugao deposit formed in the Early Cretaceous, and it is the product of a magmatic hydrothermal system.