Mantle contribution to the generation of the giant Jinduicheng porphyry Mo deposit, Central China: New insights from combined in-situ element and isotope compositions of zircon and apatite

Abstract

Identifying crust–mantle interaction is important for understanding the genesis of granite-hosted mineral deposits in post-collisional or intracontinental settings. We have used combined zircon-apatite trace element compositions, zircon HfO isotopes and apatite SrNd isotopes, which were all acquired by in-situ techniques, to evaluate the mantle contribution to the formation of a giant porphyry Mo deposit, the Jinduicheng deposit in the East Qinling–Dabie orogenic belt, Central China. Zircons from the host granites yielded a UPb age of ∼139 Ma, suggesting a post-collisional setting for the ore-related intrusion. Most magmatic zircons from the granites have Ti-in-zircon temperatures lower than 830 °C, moderate Zr/Hf ratios (25–55), and mantle-like δ18O values (peaking at ∼5.6‰), similar to typical I-type granites worldwide. The magmatic zircons have variable εHf(t) values (−26.7 to −7.2) and Ti-in-zircon temperatures (646–894 °C, peaking at ∼740 °C). The coexisting igneous apatites have initial 87Sr/86Sr ratios of 0.70694–0.71027, and variable εNd(t) values (−16.0 to −9.3), Sr contents (132–663 ppm), and Eu/Eu* values (0.12–0.47). The Sr–Nd–Hf isotopes of the apatites and zircons, along with the positive correlations between apatite Sr contents, Eu/Eu* values, and εNd(t) values, a positive correlation between Ti-in-zircon temperatures and zircon εHf(t) values, as well as mixing modeling using SrNd isotopes, suggest that at least two end-member melts were involved in the formation of the host granites: one is the ancient lower crust-derived melt, and the other is the metasomatized lithospheric mantle-derived melt, which have relatively enriched and depleted Sr–Nd–Hf isotopic compositions, respectively. We suggest that the post-collisional lithospheric thinning, asthenospheric upwelling and underplating of the mantle-derived mafic magmas may have triggered the crustal melting, magma mixing, and generation of the host granites of the Jinduicheng deposit. We postulate that, besides the heat, the metasomatized lithospheric mantle-derived magmas likely supplied additional volatiles (e.g., Cl, S), facilitating the porphyry Mo formation. Our study highlights that, combined in-situ element and isotope compositions of accessory minerals can provide multidimensional insights into petrogenetic tracing, and enhance the genetic interpretations of granites and related mineral deposits.