Molybdenite Re/Os dating, zircon U–Pb age and geochemistry of granitoids in the Yangchuling porphyry W–Mo deposit (Jiangnan tungsten ore belt), China: Implications for petrogenesis, mineralization and geodynamic setting

Abstract

The Yangchuling W–Mo deposit, located in the Jiangnan porphyry–skarn (JNB) tungsten ore belt, is the first recognized typical porphyry W–Mo deposit in China in the 1980's. Stockworks and disseminated W–Mo mineralization occur in the roof pendant of a 0.3km2 monzogranitic porphyry stock that intruded into a granodiorite stock, hosted by Neoproterozoic phyllite and slate. LA-ICPMS zircon U–Pb analyses suggest that of the monzogranitic porphyry and granodiorite were formed at 143.8±0.5Ma and 149.8±0.6Ma, respectively. Six molybdenite samples yielded a Re–Os weighted mean age of 146.4±1.0Ma. Geochemical data show that both granodiorite and monzogranitic porphyry are characterized by enrichment of large ion lithophile elements (LILE) relative to high field strength elements (HFSE), indicating a peraluminous nature (A/CNK=1.01–1.08). Two granitoids are characterized by a negative slope with significant light REE/heavy REE fractionation [(La/Yb)N=8.38–23.20] and negative Eu anomalies (Eu/Eu*=0.69–0.76). The P2O5 contents of the Yangchuling granitoids range from 0.12% to 0.17% and exhibit a negative correlation with SiO2, reflecting that they are highly fractionated I-type. They have high initial 87Sr/86Sr ratios (0.7104–0.7116), low negative εNd(t) (−5.05 to −5.67), and homogeneous εHf(t) between −1.39 and −2.17, indicating similar sources. Additionally, two-stage Nd model ages (TDM2) of ~1.3–1.4Ga and two-stage Hf model ages (TDM2) of ~1.2–1.3Ga are consistent, indicating that Neoproterozoic crustal rocks of the Shuangqiaoshan Group could have contributed to form the Yangchuling magmas.Considering the two groups of parallel Late Mesozoic ore belts, namely the Jiangnan porphyry–skarn tungsten belt (JNB) in the south and the Middle–Lower Yangtze River porphyry–skarn Cu–Au–Mo–Fe ore belt (YRB) in the north, the Nanling granite-related W–Sn ore belt (NLB) in the south, the neighboring Qin–Hang porphyry–skarn Cu–Mo–hydrothermal Pb–Zn–Ag ore belt (QHB) in the north, as well as the Southeastern Coast porphyry–skarn Cu–Mo–Au ore belt (SCB) recognized in South China in this paper, we propose that the latest Jurassic to earliest Cretaceous granitoids and associated ores were formed during a tearing of the subducting Izanagi slab. This tearing of the subduction slab caused the upwelling of asthenosphere and the resulting mantle–crust interaction. The granitoid-related W ore systems in JNB resulted from the remelting of the Proterozoic crust. The mafic–ultramafic volcanic rocks of the Shuangqiaoshan Group intercalated with phyllite and slate, ophiolitic mélange and magmatic arc rocks, mainly comprising I-type granite, basalt, andesite, rhyolite, pyroclastics, together with subduction-related metasomatized lithospheric mantle, would have provided additional mantle material. In this case, the partial melting of rocks of the Shuangqiaoshan Group can produce S-, I- and transitional type granitoids. After strong differentiation it formed tungsten-bearing granitoids characterized by enrichment of high alkali, silicon and volatile components. In the Yangchuling mine area the small monzogranitic porphyry stock has stronger fractionation, volatile content and ore-forming components than the older granodiorite, resulting in the development of the porphyry W–Mo ore system.