College of Earth Sciences, Jilin University, Changchun 130061, China

Mohexiala silver polymetallic deposit is a newly discovered deposit in the East Kunlun metallogenic belt of Qinghai in recent years. The zircon LA-ICP-MS U-Pb dating and geochemical characteristics of granite porphyry in Pingdongkou mine area are studied to reveal the diagenetic age, magma source area and tectonic setting. Zircon dating results show that it was formed in the Late Devonian (371.6±2.9 Ma) of Paleozoic.Elemental geochemistry is characterized by high Si, high alkali, rich in Fe and Al, relatively rich in K and poorin Ca and Mg. The trace elements are characterized by the enrichment of Rb, Th, La, Ce, Nd, Zr, Sm and Gd, but strong loss of Ba, Sr, P,Ti, etc., and relatively poor in Nb and Ta. Rare earth elements are characterized by obvious enrichment of light rare earth elements(LREE)and strong negative Eu anomalies. Comprehensive studies show that the genetic type of Pingdongkou granite porphyry in the mining area is A type of granite, with the forming material mainly from partial melting of felsic crust. The geotectonic setting of its formation is the post-collision extensional environment resulted from the collaging of Wanbaogou oceanic basalt plateau and Kuhai ancient land to the southern margin of Qaidam block in Late Devonian.