U-Pb, Re-Os, AND 40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY Sn ± Cu ± Mo AND POLYMETALLIC (Ag-Pb-Zn-Cu) VEIN MINERALIZATION AT BIANJIADAYUAN, INNER MONGOLIA, NORTHEAST CHINA: IMPLICATIONS FOR DISCRETE MINERALIZATION EVENTS

Many new discoveries of base and precious metal (Ag-Pb-Zn-Cu) veins in the Great Hinggan Range ore district have made this region the most important Ag-Pb-Zn metallogenic belt in northern China. The newly discovered Bianjiadayuan deposit, which is located in this district, is characterized by diverse mineralization types, including porphyry Sn +/- Cu +/- Mo, breccia Sn-Pb-Zn, and base metal veins. Geochronological studies have revealed two separate mineralization events. The earliest magmatic-hydrothermal event is recorded by two robust zircon U-Pb ages of 140.8 +/- 0.9 and 140.2 +/- 0.6 Ma for the quartz porphyry intrusion. These ages are indistinguishable, within error, from the mean Re-Os age of 140.0 +/- 1.7 Ma for molybdenite veins/veinlets hosted by the porphyry. A slightly younger 40Ar/39Ar age of 138.7 +/- 1.0 Ma was obtained for sericite in base and precious metal (Ag-Pb-Zn) veins hosted by slate adjacent to the porphyry. On the basis of the U-Pb, Re-Os, and 40Ar/39Ar data, we infer that the above mineralization occurred at similar to 140 Ma and was caused by fluids released from the magma that crystallized the quartz porphyry. A late hydrothermal event is evident from numerous Cu-Pb-Zn-mineralized veins hosted in a gabbro intrusion with zircon U-Pb ages of 133.2 +/- 0.9 and 133.0 +/- 0.8 Ma. The late base metal veins are interpreted to be coeval with or postdate diorite dikes that intruded both gabbro and slate; the diorite yielded a zircon U-Pb age of 130.0 +/- 2.8 Ma. The discovery of separate but superimposed hydrothermal events has important implications for local and regional exploration.