Abstract:

The Xiaodonggou porphyry Mo deposit is located in the southern part of Da Hinggan Mountains, with a distance of 25 km from the Xarmoron River huge fault northward.The Sr-Nd isotope and Pb isotope analyses of the Xiaodonggou intrusion show that the value of ⁸⁷Sr/⁸⁶Sr(t) ranges from 0.705,0 to 0.705,5, and εNd（t） ranges from-2.4 to -2.8.Re-Os isotopic age dating for six molybdenite sampled from the major orebody gives an isochron age of (138.1±2.8) Ma, indicating that this deposit formed in the early Cretaceous.Pb isotope analyses results show that the rock and ore-forming materials of the Xiaodonggou porphyry molybdenum deposit are from different sources.The rock-forming materials are from juvenile crust materials originating from subducted oceanic crust, while the ore-forming materials are from mantle fluid.Considering the tectonic history of this area, we explain the origin of the Xiaodonggou porphyry Mo deposit as following: as the Da Hinggan Mountains began to raise in the early Cretaceous, delamination of the lithosphere, upwelling of the materials from the asthernosphere, underplating of basic magmas and mantle fluid led the lower crust rocks melting, following with more mantle fluid entering the magma room.Carrying the mantle fluid,magma uplifted along the EW, NE huge fault and evolved into granite rich in silicate and potash,then invaded into the Permian strata and finally precipitated into Mo deposit.   

Key words: Sr-Nd isotope, Pb isotope, Re-Os isotope, mantle fluid, Xiaodonggou porphyry molybdenum deposit, Inner Mongolia