Genesis of the Shangjinshan W–Mo polymetallic deposit in the Eastern Jiangnan tungsten belt: Evidences from geochemistry, geochronology and zircon Hf isotope analyses

Abstract

The newly discovered Shangjinshan W–Mo polymetallic deposit, located at the southern Anhui, southeast China, is a large-scale skarn-type tungsten polymetallic deposit. According to mineral assemblages, ore textures/structures and superposition relationships of the mineralization, three ore types have been recognized: veinlet-disseminated skarn (the primary type), quartz vein type and porphyry type. The mineralized event of this deposit could be divided into four stages: (I) the prograde skarn stage, (II) retrograde skarn stage, (III) quartz-sulfide vein stage, and (IV) quartz-calcite vein stage. The intrusion in the tungsten-rich district of the Shangjinshan is mainly composed of the granodiorite with zircon U–Pb ages of 138.4 ± 1.2 Ma to 136.7 ± 1.3 Ma. Meanwhile, Re–Os isotopic dating of five molybdenite samples yields an isochron age of 141.9 ± 3.1 Ma and a weighted average age of 139.3 ± 1.7 Ma, respectively. The ore-forming ages are consistent with the rock-forming ages within errors, indicating that the W-Mo mineralization is genetically related to the Shangjinshan granodiorite. The granodiorite is I-type granites, characterized by metaluminous to slightly peraluminous and high-K cal-alkaline compositions. They are also enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs) and heavy rare earth element (HREEs), with obviously negative Eu anomalies (Eu/Eu* = 0.50–0.84). The zircon εHf (t) values and two-stage Hf model ages (TDM2) fall into the range −6.18 to −2.04 and 1.5–1.3 Ga, respectively. The relatively low rhenium contents in molybdenite and high radioactive Pb isotopes of the pyrites from stage II indicate that the ore-forming material was derived from Shangjinshan granitic magmatism. Together with previously published data, we propose that the W-related granitoids in the JNB were likely generated by partial melting of the Shangxi Group with some additional mantle materials. The granitic magmatism was caused by the asthenosphere upwelling, triggered by the roll-back of subduced Paleo-Pacific plate during Late Jurassic to Early Cretaceous. These granitic stocks predominately contributed to the production of skarn alteration and mineralization, resulting in the giant tungsten polymetallic district in the Jiangnan tungsten belt.