Genesis of Late Ordovician granite porphyry at Heicigou gold deposit, East Kunlun: constraints from zircon U-Pb age, geochemistry and Sr-Nd-Pb-Hf isotopic systematics

The Heicigou gold deposit is located in the southern segment of the East Kunlun Orogen.The graniteporphyry in the area which is parallel to the gold metallogenic belt along the NW trending structure, is a newly delineated felsic dike related to the gold mineralization in recent years.Based on LA-ICP-MS zircon U-Pb dating, Sr-Nd-Pb-Hf isotopic systematics, and whole-rock geochemistry, the formation age, petrogenesis, tectonic setting, and magma source of the granite porphyry are discussed in detail.The results show that the zircon U-Pb age of the granite porphyry is 445.8±2.4 Ma(MSWD=0.36), indicating an age of the Late Ordovician.The granite porphyry contains 75.17%~78.94% SiO₂, 10.22%~12.8% Al₂O₃, 5.49%~7.07% K₂O+Na₂O, K₂O/Na₂O ranging from 0.01 to 0.03, 0.05%~0.07% MnO, 0.08%~0.25%MgO, 0.07%~0.08% P₂O₅, and 0.15%~0.18% TiO₂, suggesting that the rock belongs to the low-K calc-alkaline rock series.The A/CNK and A/NK values are 0.92~0.99 and 1.1~1.14, respectively, indicating that the rock is metaluminuous.The ΣREE concentration ranges from 132.4×10^-6 to 183.95×10^-6, with the LREE/HREE ratios varying between 8.04 and 8.78.The primitive mantle-normalized trace element spidergrams are characterized by enrichment in Ba, Th, and Pb and depletion in Nb, Ta, and Ti, similar to I-type granites in volcanic arc settings.The granite porphyry exhibits high I_Sr(0.7133~0.7158), (²⁰⁷Pb/²⁰⁴Pb)_i but low ε_Nd(t)(-4.45~-5.15)and ε_Hf(t)(-11.12~+0.13), similar to magmas derived from the lower crust.It is believed that the granite porphyry was formed in a continental margin arc setting due to the northward subduction of the Proto-Tethys Ocean in the Late Ordovician, and that the magma was originated from the partial melting of the lower crust.