Geochemistry and U-Pb geochrony of zircon from granite porphyry of Jinchang gold deposit in Heilongjiang,China and its geological significance

The Jinchang gold deposit,located in the east segment of the Central Asian orogenic belt,is a large hydrothermal deposit associated with the Mesozoic magmatic activities.In order to determine the type,age and tectonic setting of the metallogenic magmatic rock,the authors measure the U-Pb dating and in situ trace elements of the zircons from granitic porphyry by cathodoluminescence(CL) images and LA-ICP-MS.All zircon grains have fine oscillatory zonings in CL images,and the ratio of Th/U is 0.4,indicating their magmatic origin.Zircon U-Pb dating is 220 Ma and 103~123 Ma,representing the two-phase magmatic events.The zircon with age of 220 Ma belongs to the captured magmatic zircon.Correspondingly,zircons with ages of 103~123 Ma are derived from the new magma.Geochemistry and U-Pb dating of the zircons suggest that the captured zircon was derived from the Early Triassic granite porphyry.The trace elements of the zircons indicate that the granite porphyry originated from the highly melting and crystallization differentiation and the emplacing in the oxidizing environment in the shallow crust of the Early Triassic granite.The values of negative Eu anomalies in zircons from the new magma decreased in the oxidizing environment.The weighted average U-Pb age(113.5±3.8 Ma) of zircons from new magma is identical with the metallogenic age.Therefore,the Early Cretaceous magmatic event directly resulted in the gold mineralization in the Jinchang deposit.The mineralization occurred in lithospheric extensional setting related to the subduction of the Pacific Plate.