Geological characteristics, metallogenic background, and genesis of the Tongyu VHMS copper deposit in the west part of the North Qinling, Shaanxi Province

The Tongyu volcanic-hosted massive sulfide (VHMS) copper deposit is located in the west part of the North Qinling (NQL), Shaanxi Province. Here we report geological features and trace elemental characteristics of ores, mineralized rocks, and volcanic host rocks of this deposit. We also conducted in situ zircon U-Pb and Lu-Hf LA-ICP-MS dating and lithogeochemical study of the Meigou granodiorite, which is intruded in the south of the Tongyu mining district. It is inferred that the volcanic host rocks of the Tongyu Cu deposit belong to calc-alkaline island arc volcanic rocks, and the volcanic host rocks and the ore-forming materials share common material sources. During subduction of the Paleo-Qinling oceanic slab at around 472 Ma, high oxygen-fugacity aqueous fluid released by dehydration reaction of the subducting slab rose into the mantle wedge, leached Cu and other metal elements, and triggered partial melting of the mantle wedge. Associated with the ore-forming materials, the arc calc-alkaline magmas erupted and formed the Tongyu VHMS deposit. The weighted average zircon U-Pb age of the Meigou granodiorite is 439.3 +/- 3 Ma, suggesting its genesis is associated with the subduction event of the Paleo-Qinling oceanic plate around 440 Ma. In terms of the granitoid's geochemical characteristics, the Mg(#), Ni, and Cr contents are high. REE show strongly fractionated patterns with unobvious Eu anomalies. Sr contents are high while Y contents are low, resulting in high Sr/Y ratios. Two-stage zircon Hf model ages mostly range between 908 and 723 Ma, with epsilon(Hf)(t)> 0. Therefore, we classify Meigou granodiorite as island arc granite with adakitic geochemical features. It was generated by interaction between mantle wedge and melt produced by partial melting of the subducted basaltic slab of the Paleo-Qinling Ocean. The Tongyu VHMS deposit was formed in the island arc tectonic setting during Ordovician-Silurian between 439.3 +/- 3 Ma and 472 +/- 11 Ma. The volcanic exhalation-sedimentation, which was contemporaneous with the submarine volcanics, produced the primary mineralization of the Tongyu Cu deposit. Afterward, the ore bodies and host rocks were reworked and overprinted intensively by structures corresponding with the overall collisional orogenic process of the Qinling Orogenic Belt during the Indosinian period. In contrast to classic VHMS deposits such as the Kuroko deposit in Japan, the Tongyu Cu deposit possesses the dual characteristics of island arc volcanic exhalation and sedimentation as well as structural reworking and overprinting. Therefore, the Tongyu VHMS Cu deposit is a volcanic exhalation and sedimentation-structural reworking and overprinting type deposit.