Zircon Trace Elements as Indicators of Physicochemical Conditions and Mineralisation Potential in the Nannihu Porphyry Mo–W Deposit, East Qinling, China

Physical and chemical (physicochemical) conditions are crucial for the genesis of a mineralised porphyry. For instance, a hydrothermal system with high oxygen fugacity facilitates Mo migration and precipitation. Zircon, the most common mineral enriched in rare earth elements (REEs), is widely used to determine the oxygen fugacity, magma evolution and mineralisation potential of a porphyry body. The Nannihu is a giant Mo–W porphyry deposit in the Qinling Orogen of central China and consists of porphyritic monzogranite. Previous studies have primarily focused on the age and petrogenesis of the deposit, but the physicochemical conditions remain poorly understood. Additionally, the mineralisation potential of the porphyry has not been evaluated, hampering the discovery of concealed Mo–W ores. In this study, we present in situ trace element analysis of zircon from the Nannihu porphyry to decipher the physicochemical conditions, magma evolution and evaluate the mineralisation potential of the area. The REE contents of the zircons range from 357 to 4768 ppm, characterised by depleted light REEs (LREEs) and enriched heavy REEs (HREEs). Using the zircon–Ti-thermometer and Ce/Ce* ratio, the crystallisation temperature of the Nannihu porphyry is estimated to be ~651°C–769°C, with an oxygen fugacity ranging from −30 to −1.7. By contrast, the crystallisation temperatures of the Nannihu porphyry are similar to those of nearby Shibaogou, Huangbeiling and Shangfanggou plutons in the Luanchuan region; its oxygen fugacity is moderate compared to these plutons. The porphyry formed in an intraplate setting and is derived from the crust. The crust in the Nannihu region underwent double thickening during ~148–138 Ma, reaching a depth of ~60 km. The zircons in the porphyry represent early products of a magma rich in H₂O with a high oxygen fugacity, potentially influenced by F-rich hydrothermal input, which indicates a high prospectivity for Mo–W mineralisation in the vicinity of the deposit.