Zircon U-Pb isotopic geochronology, geochemistry and geological significance of the quartz diorite porphyrite in Yangxie gold ore district, East Qinling

The middle-sized Yangxie gold deposit is a typical quartz-vein type gold deposit in Yangxie-Fengbeihe area, East Qinling. Its mineralization is closely related to quartz diorite porphyrite, which has the characteristics of medium-high temperature alteration and mineral composition. LA-ICP-MS zircon U-Pb dating indicates that the weighted average ²⁰⁶Pb/²³⁸U age and concordance age of zircons from the quartz diorite porphyrite are 149.5±2.7 Ma and 149.5±2.1 Ma respectively, suggesting the Late Jurassic. Petrogeochemical studies show that the quartz diorite porphyrite belongs to high K calc-alkaline metaluminous rocks which are characterized by high contents of SiO₂(65.53%~65.76%), Al₂O₃(15.65%~15.87%) and alkali (Na₂O+K₂O=8.07%~8.30%) but low MgO content (1.37%~1.40%). These rocks are relatively enriched in LILE (Rb, Ba, K, Pb, Sr) and LREE, and depleted in HFSE (Nb, Ta, Ti) and HREE. Meanwhile, their characteristic parameters of high Sr values, high Sr/Y rations, low Y and Yb values as well as positive Eu anomalies are consistent with those of typical adakitic rocks, indicating that the parental magma was probably derived from partial melting of the thickened lower crust. The comprehensive studies suggest that the Yangxie gold deposit is a magmatic-hydrothermal gold deposit, which was formed in the transitional environment of intracontinental subduction to extension of the Qinling orogenic belt from Late Jurassic to Early Cretaceous.