The Xicheng (Xihe-Chengxian) ore concentration area in Gansu Province is one of the most important polymetallic ore concentration areas in the West Qinling orogen. There are mainly lead-zinc deposits and subordinately gold, copper and molybdenite deposits (ore spots). The Changba biotite monzo-granite is located in the mid-east of the Xicheng ore concentration area, and the molybdenum-bearing coarse-grained quartz veins occur on its southern margin. The subhedral molybdenite occurs within the quartz veins in lumpy form. The Re-Os isotopic dating of 7 molybdenum samples yielded model ages ranging from 207.7±3.0 to 209.8±2.8 Ma, with the average model age of 208.9±1.1 Ma and the isochron age of 209±15 Ma, indicating that the Mo mineralization on the southern margin of the Changba pluton took place in the Late Triassic, slightly later than the formation of the Changba pluton. The low content of Re (18.82×10^-6~21.97×10^-6) shows that the material was mainly derived from the crust, with the mixture of a little mantle material. In combination with the ages (200~225 Ma) of the regional magmatism, it is held that gold, lead-zinc deposits and molybdenite mineralization on the south margin of the Changba pluton resulted from the tectonism, magmatism and fluid metallogenic event in the late Triassic period. The Re-Os dating of the molybdenite provides a new thinking for exploration of the Indosinian quartz vein type Mo deposits in the contact zone of the late Triassic granitoid plutons in the West Qinling orogen.