Qian'echong low-F porphyry Mo deposits in the Dabie Mountains, central China

Abstract

The Qian'echong Mo deposit is a large porphyry Mo deposit located in the northwest Dabie Mountains, central China, with proven Mo reserves of 741Mt at 0.081%. Chondrite-normalized REE patterns of most zircon from samples QEC002, 003, 004 and 011 show HREE enrichment with distinctive positive Ce and moderately negative Eu anomalies, which are typical of magmatic zircon in porphyry deposits. Zircon grains from sample QEC001 are all hydrothermal in origin, whereas there are only several hydrothermal zircons in the other samples. The Ti-in-zircon thermometer yields temperatures of 700–740°C for the magmatic zircon. The Ce4+/Ce3+ calculated from the Ce anomaly is a sensitive and robust indicator of magmatic oxidation status. Magmatic zircon have Ce4+/Ce3+ and (Eu/Eu*)N ratios ranging between 10–577 and 0.2–1.3, respectively. Compared to the Shapinggou deposit (Climax type) also in the Dabie Mountains, the Qian'echong zircon has systematically lower Ce4+/Ce3+. Apatite from the Qian'echong deposit belongs to the F-apatite variety, with fluorine concentrations varying between 2.13–4.50% and Cl concentrations between 0.01–0.28%. Whole rock samples of the porphyry from the Qian'echong deposit have F contents of 360–1230ppm. The porphyry intrusion is consistently associated with calc-alkaline granitic to granodioritic porphyries, with lower F, Nb, and Ta concentrations, and systematically lower Mo contents than the Climax-type deposits. All these characteristics are similar to other low-F porphyry Mo deposits along the eastern Pacific margin. Fluorine is mainly hosted in phengite during plate subduction, which may decompose far beneath the surface of the subduction zone. Therefore, high F deposits may be interpreted as locating the farthermost position reached by the subducting slab. Nevertheless, the F contents of the Qian'echong deposit is much higher than in adjacent barren granites, and may have incorporated additional F from the subduction slab. Given that the Qian'echong porphyry Mo deposit is only 77km to the west of the Shapinggou Climax-type high-F porphyry Mo deposit, the dramatic difference in F contents suggests that the westward subduction of the Pacific and Izanagi plates may at least have extended to somewhere between the Shapinggou and Qian'echong deposits.