Geochronology and magmatic oxygen fugacity of the Tongcun molybdenum deposit, northwest Zhejiang, SE China

Abstract

The Tongcun Mo porphyry deposit in northwest Zhejiang is hosted in three porphyry units: Huangbaikeng, Songjiazhuang, and Tongcun, from southwest to northeast. U–Pb zircon ages of 162±3.0 Ma for the Huangbaikeng porphyry, 159.9±3.0 Ma for the Songjiazhuang porphyry, and 167.6–155.6 Ma for the Tongcun porphyry indicate that these intrusions formed during the Jurassic and are most likely associated with the northwestward subduction of the Izanagi Plate. Trace element compositions of zircons from the Tongcun deposit constrain the oxygen fugacity (ƒO₂) of the magma using zircon Ce anomalies and Ti-in-zircon temperatures. The average magmatic ƒO₂ for the porphyries in the Tongcun deposit is fayalite–magnetite–quartz (FMQ)+2.7, which is similar to the Shapinggou (FMQ+ 3.2) and Dabaoshan (FMQ+3.5) Mo porphyry deposits, but much higher than that of the reduced Cretaceous ore-barren Shangjieshou porphyry (FMQ-1.1) around 8 km away from the Tongcun deposit. The distinct difference in magmatic oxygen fugacity between the Jurassic and Cretaceous porphyries may help to explain the absence of Mo porphyry mineralization in northwest Zhejiang during the Cretaceous.