Geochemistry, geochronology and fluid inclusion studies of the Dongji epithermal gold deposit in Fujian province, south‐east China

Abstract

The Dongji epithermal Au deposit formed in the Early Cretaceous is located in the Dongkeng volcanic Basin of the South China Block. The orebodies of the deposit are mainly hosted in the Jurassic volcanic–subvolcanic rocks. Here, detailed zircon U–Pb geochronology, major‐trace elements, and fluid inclusion data of the Dongji deposit are reported. At least two epochs of magmatic activity were identified at Dongji, including the Late Jurassic rhyolitic crystal tuff lava (165.3 ± 1.8 Ma to 162.8 ± 2.6 Ma) and quartz porphyry (156.0 ± 1.9 Ma), and the Early Cretaceous dacitic crystal tuff lava (108.4 ± 5.7 Ma) and purplish‐red crystal tuff lava (98.1 ± 0.8 Ma). The Late Jurassic volcanic–subvolcanic rocks are characterized by high SiO2 content, enriched LILE (large ion lithophile element) elements, depleted HFSE (high field strength element) elements, and obvious P, Ti and Eu anomalies. These geochemical characteristics suggest that the volcanic–subvolcanic rocks were generated by partial melting of the lower crust and controlled by fractional crystallization of Ti‐bearing phases, apatite, and plagioclase. In comparison, the Early Cretaceous volcanic–subvolcanic rocks are characterized by high K contents, and arc‐like signatures (e.g., enriched Rb, Th, K, and depleted Nb, Ta, Ti), indicating a partial melting of crust–mantle interaction under an extensional tectonic setting. The ore‐bearing dacitic crystal tuff lava and the caprock of purplish‐red tuff lava constrain the timing of gold mineralization in the range of 108–98 Ma. The Au mineralization at Dongji is divided into the early stage of quartz‐pyrite association and the late main mineralization stage of quartz‐polymetallic sulfide association. The fluid inclusion microthermometry results show that the ore‐forming fluids at the early mineralization are characterized by enrichment of liquid‐rich two‐phase inclusions and CO2‐bearing aqueous inclusions. The fluids with moderate homogenization temperatures (292–382°C) and low salinities (5.4–10.0 wt% NaClequiv) were possibly exsolved from magmatic rocks. Nevertheless, fluid inclusions in the late‐stage contain liquid‐rich two‐phase inclusions and pure liquid phase inclusions. The homogenization temperatures (222–309°C) and salinities (2.4–8.3 wt% NaClequiv) of the late‐stage fluid inclusions are relatively lower than those of the early stage, probably indicating a fluid mixing of magmatic and meteoric fluids. Based on the above, we propose that fluid mixing is the key factor facilitating Au precipitation in the Dongji deposit, probably applicable to other Au deposits in the Dongkeng volcanic Basin.