The Xiaodonggou porphyry molybdenum deposit located in the Xarmoron molybdenum metallogenic belt is chose as the research area. The Xiaodonggou pluton is rich in silicon, potass, zirconium, and low in REE. It has no or small negative Eu anomaly and shows an isotopic composition high in epsilon(Nd)(t) and low in Sri, indicating its magma origining from the melting of juvenile thicken lower crust. In the meanwhile, it contained the features of high temperature, quick melting, quick segregation. The SHRIMP zircon U-Pb dating gives a result of 142 +/- 2Ma, corresponding to the big tectonic transition period of 140Ma, when the major stress field changing from NS to WE. At that time, the Da-Hinggan Mountains area was under an extensive tectonic background, underplating proceeded and mantle materials could add into the magmas forming in the lower crust. The common lead analysis of the pluton orthoclases and molybdenites shows that the former transfer from orogen to mantle and the latter come from mantle, demonstrating that the rock and ore-forming materials of the deposit have different sources, magma from the lower crust mixing with mantle fluid. From the above analysis, with the physical experiment results of the water-magma reaction, we propose the following model for the Xiaodonggou porphyry molybdenum deposit. In the Early Cretaceous period, the Da-Hinggan Mountains area was under an extensive tectonic background, the adding of mantle fluid containing ore materials into heated lower crust made it melt to produce magmas. Following more mantle fluid got into the magma room and urged the magma to escape from the source quickly. The fluid and magma uplifted together, when they arrived at shallow depth, the fluid-magma became unstable and the latter was broken into many small agglomerates with fluid connecting them in the interspaces. Because of the H+, K+ and various elements existing in the fluid, it would reacted with the magma and the rock through alteration and ore minerals crystallized out, forming the Xiaodonggou porphyry deposit with disseminated mineralization phenomenon.
