40Ar/39Ar and U-Pb constraints on the age of the Zaozigou gold deposit, Xiahe-Hezuo district, West Qinling orogen, China: Relation to early Triassic reduced intrusions emplaced during slab rollback

Abstract

The West Qinling orogen in central China contains dozens of world-class sediment-hosted gold deposits, but the ages of these deposits and their relation to regional tectonism, magmatism, and metamorphism are poorly understood. Zaozigou, in the western zone of the Xiahe-Hezuo district, is one of the larger deposits in the orogen with 142 t Au at an average grade of 2.69 g/t. It is hosted in a middle Triassic turbidite sequence and swarm of crosscutting ilmenite-series intermediate to felsic dikes. Ore bodies are localized along subvertical, NE- or NS-striking faults. Gold occurs in fine-grained arsenian pyrite and arsenopyrite disseminated in hydrothermally altered sedimentary and igneous rocks and in subordinate sulfide-quartz-ankerite veinlets. Such mineralization is locally crosscut and overprinted by NW-striking, gently-dipping, auriferous stibnite-quartz veins.Zircon grains from gold mineralized granodiorite and quartz diorite porphyry dikes have U-Pb ages of 248.9 ± 1.4 to 244.8 ± 1.4 Ma, whereas a post-ore diorite porphyry dike has a U-Pb age of 237.5 ± 1.4 Ma. Two samples of sericite associated with disseminated arsenian pyrite and arsenopyrite in sedimentary and igneous host rocks yield plateau ages of 245.6 ± 1.0 Ma and 242.1 ± 1.0 Ma, that are bracketed by the zircon U-Pb ages on pre- and post-ore dikes. Field relationships and geochronological results, thus, constrain magmatism and gold mineralization at the early Triassic.Previous work has shown that the Au-Cu skarns and intrusion-related gold deposits exposed at a deeper level in the eastern zone of the Xiahe-Hezuo district are genetically related to early-middle Triassic ilmenite series intrusions generated in an extensional back-arc setting during rollback of the subducted paleo-Tethyan slab. Our U-Pb and 40Ar/39Ar data show that the large Zaozigou gold deposit in the western zone of the Xiahe-Hezuo district is broadly coeval with the Au-Cu skarn and intrusion-related gold deposits in the eastern zone. This temporal consistency provides additional support for a genetic link between gold mineralization and magmatism in the Xiahe-Hezuo district. As such, this district may represent the only reduced intrusion related gold system in the West Qinling Orogen.