The Laojiezi PbAg deposit, located in the middle section of the Cenozoic Red RiverAilaoshan alkalirich porphyry belt, is the typical alkalirich volcanicmagmatic hydrothermal deposit in central Yunnan province. A 70mwide syenite porphyry and molybdenitepyrite vein with high CuMo abnormal pyritization was firstly discovered in the deep part of the deposits (3# vein of 2073 level). It is believed that obvious alkalirich porphyry and magmatichydrothermal CuMo superimposed mineralization occurs at depth of the deposit. This study conducted ReOs isotopic analysis for molybdenitepyrite samples from molybdenitepyrite veins, pyritegalena veins and high CuMo syenite porphyry from deep adits in the Laojiezi PbAg deposits. An isochron age of 3371± 029Ma and a weighted average age of 3399±022Ma were obtained for molybdenite, indicating that the mineralization of the PbAg polymetallic deposit took place in Oligocene, which is consistent with the diagenetic age of the syenite porphyry. The initial Os value of 03049~15549 and the γOs value of 1405~1126 suggest that the metallogenic material was derived from partial melting of the crustmantle transition layer. The metallogenic age of the Laojiezi porphyry PbAg polymetallic deposit is roughly the same as that (30~40 Ma) of regional alkalirich porphyry and magmatic hydrothermal polymetallic deposits. It shows that, after the collision between the India plate and Eurasia plate, the change of tectonic stress field resulted in largescale strikeslip faulting system, which induced partial melting of crustmantle transition zone within the stable cratons, forming alkalir-ich intrusive rocks and metallogenic fluids, and the fluid formed alkalirich porphyry and magmatichydrothermal polymetallic deposits along the weak structures.