Magmatic and Hydrothermal Evolution at Qian’echong, Central-Eastern China: Insights into Dabie-Type Porphyry Mo Mineralization
Authors: Zhang Daohan , Wei Junhao , Nadeau Olivier , Shi Wenjie , Dan Kaibo 
Published: February 2022

Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian’echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian’echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian’echong, all melt inclusions have low concentrations of Mo (2–8 ppm), &gt;5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen.