Formation of the giant Jinduicheng porphyry Mo deposit in East Qinling, Central China: Evidence from geochemical and isotopic compositions of magmatic accessory minerals

Abstract

The East Qinling Mo belt is one of the most important Mo provinces in the world where there are abundant collision-related porphyry deposits. As a representative Mesozoic porphyry Mo deposit in this region, Jinduicheng provides an excellent opportunity to investigate the physical-chemical processes of the ore-forming magmas leading to the significant Mo endowment. In this study, magmatic apatite and zircon from the causative granite porphyry have been selected for in situ chemical and Sr-Nd-Hf isotopic analyses to investigate the key factors controlling the magmatic Mo fertility. The Jinduicheng granite porphyry yields zircon U-Pb age of 143.9 ± 1.7 Ma, which coincides with the previously published molybdenite Re-Os ages. Magmatic apatite grains display relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7076–0.7106) and negative ε_Nd(t) values (−18.0 to −13.3), revealing that the causative magma was derived dominantly from ancient lower crust. The interpretation is also supported by the negative zircon ε_Hf(t) values (−26.2 to −4.6) with T_DM2 ages ranging between 2844 and 1488 Ma. The high zircon Ce and Eu anomalies (Ce_N/Ce_N* = 3–323 and Eu_N/Eu_N* = 0.32–0.65), magma oxygen fugacities (ƒ_O2 = −16.53 to −12.63 and ΔFMQ = +1.0 to +3.6), together with the relatively high apatite Eu contents (43.7 ppm in average), all suggest a high magma oxidation state. The magmatic S concentrations have been further estimated, while the results (22–57 ppm) are relatively low. These results highlight the importance of high oxygen fugacities, rather than the enrichments of magmatic S and/or Mo, in the formation of the giant Jinduicheng Mo deposit.