Zircon geochronology and Hf isotope geochemistry of the granitoids in the Yangshan gold field, western Qinling, China:implications for petrogenesis, ore genesis and tectonic setting

摘要

The Yangshan gold deposit, Gansu Province, is the largest gold deposit in China and the best representative of the western Qinling gold province. Granite dykes can be observed in the orefield, of which some are spatially associated with the gold orebodies. Gold mineralization was debated between magmatic hydrothermal and metamorphic hydrothermal models in previous studies due to a shortage of precise isotopic ages of the granite dykes in the orefield. In this contribution, we report in-situ zircon LA-ICP-MS U-Pb ages combined with Hf isotope and REE data. The age of granite dykes is constrained to be 220.2 ± 3.1 Ma, coeval with the regional development of Triassic granitoids, which resulted from the northward subduction of the Mian-Lue oceanic plate. The εHf(t) values of inherited and new zircon grains from the granite dykes show that the granite dykes have a complex source dominated by sediments which were sourced from the North China Craton, Yangtze Craton and Qinling Orogen, and accumulated in a fore-arc setting. The granite dykes were derived from partial melting of the fore-arc sediment prism induced by the fluids sourced from metamorphic devolatilizaiton of the underthrust Mian-Lue oceanic plate. The Yangshan gold deposit was formed at about 190 Ma, some 30 Myr later than the intrusion of the granite dykes, ruling out the possibility that gold mineralization was caused by Triassic granitic magmatism. The Yangshan gold deposit is a transitional system between Carlin- and orogenic-types that resulted from continental collision orogeny, metallogeny and fluid flow.