LA-ICP-MS zircon U-Pb ages, geochemistry and Hf isotopic composition of Jinbaogou granite porphyry in eastern Hebei and their geological implications

The Jinbaogou gold deposit in eastern Hebei has been recently identified as a large porphyry gold deposit. The orebodies mainly occur in Jinbaogou granite porphyry and the contact zone between the granite porphyry and the biotite-hornblende plagiogneiss of Archean Qian'xi Group. In order to confirm the age, geochemistry,magma source characteristics of Jinbaogou granite porphyry and its relationship with Yu'erya and Niuxinshan granites, the authors conducted LA-ICP-MS zircon U-Pb dating of the two granite porphyries, which yielded ages of 169.9±1.0Ma and 170.4±2.0Ma, suggesting the products of Middle Jurassic. Jinbaogou granite porphyry falls into the peraluminous and shoshonite series, its total values of REE are 38.17×10^-6~136.51×10^-6; and the rock is characterized by enrichment of large lithophile elements (Rb, K) and Ba, Th, U but depletion of high-field strength elements (Ta, Nb, Ti) and P, Sr, displaying the characteristics of typical island arc or magmatic rocks of active continental margin. Zircon Hf isotope studies show that the ε_Hf(t) values and T_DM2 ages of two granite porphyry samples are -12.8~-7.4 and -14.4~-8.8, 1685~2028Ma and 1773~2130Ma, respectively, implying representatives of the Paleoproterozoic crustal reservoir. Integrated with the regional geological materials, it is considered that the Middle Jurassic granites in the eastern Hebei, including the Jinbaogou granite porphyry and volcanics of Tiaojishan, were under the tectonic settingsof crustal thickening and the subduction of ancient Pacific plate towards Eurasian plate, and were emplaced during the relaxation after extrusion stress.