LA-ICP-MS zircon U-Pb geochronology and geochemical characteristics of the quartz syenite porphyry in the Xiaoxigong gold deposit and their geological implications

The Xiaoxigong quartz syenite porphyry is located in the south of the Beishan orogenic belt, Northwest China. LA-ICP-MS zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 247.5±2.2Ma for the quartz syenite porphyry, implying middle Triassic intrusion. Geochemical analyses show that the porphyry is enriched in Si, K, Na, and REE, and depleted in Mg and Ca, with SiO2 being 65.8%-66.2%, Na2O+K2O 8.99%-9.41%, CaO 1.72%-2.19%, and MgO 0.63%-0.70%. REE are characterized by negative Eu anomalies. The porphyry is enriched in Rb, Th, U, K, Pb, Nb, Zr and Hf and depleted in Ba, Sr, P, Ti and Eu. The quartz syenite porphyry has high Zr+Nb+Ce+Y abundances and 104?Ga/Al ratios. Petrologic and major and trace elements data all indicate that the Xiaoxigong quartz syenite porphyry is a typical A-type granite. The magma probably originated from partial melting of the crust due to underplating mantle-derived magmas and was emplaced in a post-orogenic extensional tectonic setting.