Geochemistry of zircon and apatite from the Mo ore-forming granites in the Dabie Mo belt, East China: Implications for petrogenesis and mineralization

Abstract

The Dabie molybdenum (Mo) belt in east China is an important Mo belt in both China and the world. All of the Mo deposits in the Dabie area are porphyry (-skarn) type. The ore-related granite samples from the Qian’echong, Tangjiaping, and Doupo Mo deposits in the Mo belt have been collected for zircon and apatite trace elements analysis. Additionally, the Doupo ore-related granite has also been chosen for zircon U-Pb dating. LA-ICP-MS zircon U-Pb dating of the ore-related granite in the Doupo Mo deposit yields a crystallization age of 127 ± 1 Ma, which is consistent with the molybdenite Re-Os age of the deposit. The chondrite-normalized patterns of all zircons are characterized by positive Ce anomalies with variable negative Eu anomalies. The ore-related granites in the Qian’echong and Tangjiaping Mo deposits show relatively high magmatic oxygen fugacity (ΔFMQ ≥ +1.5), whereas the Doupo ore-related granite has a relatively low magmatic oxygen fugacity (ΔFMQ − 0.3), which might have caused it to have relatively small Mo reserve compared to the Qian’echong and Tangjiaping deposits. All of the apatites from the three Mo deposits are characterized by LREE-enriched and HREE-depleted chondrite-normalized patterns, with various degrees of Eu depletion and Ce enrichment. Not only redox condition but also other factors could have influenced the Eu/Eu* or/and Ce/Ce* of apatite in this study, and the anomalies of Eu and Ce of apatites from the ore-related granites in this study could not be used to evaluate the magma redox states. The geochemical characteristics of apatite indicate that the ore-forming magmas of the Mo deposits in this study are oxidized and metaluminous I or A type magmas. During the Early Cretaceous regional extension and asthenospheric upwelling in the Dabie orogen, partial melting of the crustal rocks generated oxidized Mo-bearing magma. Relatively oxidized condition prevents Mo from incorporating into sulfides or silicates in the magma, and finally leads to Mo precipitation from the ore-forming fluids, which released from the magma.