Zircon U–Pb and Hf–O isotopes trace the architecture of polymetallic deposits: A case study of the Jurassic ore-forming porphyries in the Qin–Hang metallogenic belt, China

Abstract

The Qin–Hang intra-continental porphyry–skarn Cu polymetallic belt (QHMB) is among the economically important metallogenic belts in South China. The significant differences in the size and metal assemblage of the Jurassic magmatic–hydrothermal ore deposits in this belt remain as an enigma. Here we employ zircon U–Pb and Hf–O isotopes of the Tongshanling and Baoshan Cu–Pb–Zn deposits in the central part of the QHMB to investigate the contrasting metallogenic architecture. Our SIMS zircon U–Pb data indicate that the Tongshanling and Baoshan granodiorite formed at ~160Ma. These rocks show high Mg# values, and negative zircon εHf(t) and high δ18O values suggesting that the magmas of the granodiorite porphyries were mainly generated through the anatexis of older crustal components triggered by the input of mantle-derived magma. The minor content of amphibole phenocrysts, low Sr/Y ratios, negative Eu anomaly, and low zircon Ce4+/Ce3+ ratios indicate that the porphyries are relatively less oxidized with less water content compared with the ore-bearing porphyries in the Dexing and Yuanzhuding porphyry Cu deposits in the northern and southern part of the QHMB, suggesting that high magmatic water content and oxidation state are important prerequisites for the formation of large size porphyry–skarn copper deposits in the QHMB. The positive correlation between zircon εHf(t) values with the Cu reserves, as well as zircon δ18O values with the Cu/(Cu+Pb+Zn) ratios of the deposits indicate that the magmatic sources exerted a first-order control on the volume and metal assemblage of deposits in the QHMB. The Hf and Nd isotope contour maps indicate that the central part of the QHMB has high potential for Pb–Zn-dominated magmatic–hydrothermal deposits, whereas the northern and southern part of the QHMB are prospective for large Cu deposits. Our results have important implications in formulating regional exploration strategies for Jurassic porphyry–skarn Cu–Pb–Zn deposits in the Qin–Hang belt.