Alteration-mineralization zoning and fluid inclusions of the high sulfidation epithermal Cu-Au mineralization at Zijinshan, Fujian Province, China

The Zijinshan mine in Fujian Province in the southeast of China is the first recognized example of high sulfidation epithermal Cu-Au mineralization of Cretaceous age in mainland China. The Cu-Au mineralization occurs around a dacitic volcanic pipe in the central part of a calc-alkaline, trachydacitic to dacitic volcanic dome of Cretaceous age that has intruded an S-type Jurassic granite. The Cu-Au orebodies are subparallel, northwest-trending breccias, veins, and massively silicified rocks which have developed along extensional fractures in wide hydrothermal alteration zones. At depth the dacite porphyry pipe changes downward into a potassic-altered granodiorite porphyry associated with porphyry copper-style mineralization, indicating a close spatial and temporal association between high sulfidation Cu-Au and porphyry copper mineralization. Zoning of hydrothermal alteration and ore mineralization assemblages at Zijinshan is typically recognized from the deep and central zones toward shallower and outer portions within the epithermal system. A deep sericite + quartz + pyrite assemblage in the phyllic alteration zone gives way upward to a dickite + quartz + pyrite ± zunyite ± sericite assemblage in the dickite alteration zone and successively to the alunite + quartz + pyrite assemblage in the funnel-shaped alunite alteration zone at depths between 350 and 1,200 m from the land surface. High sulfidation copper mineralization is typically developed within the alunite alteration zone and shows a gradual change of ore mineralogy from digenite and enargite preferentially at deeper levels to covellite and gold in shallow levels. The silicic alteration and associated gold mineralization are retricted at shallow depths (less than 350 m). Vertical metal zoning in the sequence Au-Ag-As → Cu → Cu-Pb-Zn is also recognized within the silicic and alunite alteration zones. Paleogeographic reconstruction indicates depths of <850 and 850 to 1,700 m for the silicic alteration and alunite alteration, respectively. Geochemical, mineralogical, and fluid inclusion data are consistent with progressive acid leaching toward shallower portions of the hydrothermal system, accompanying the general decrease of temperature from 300°to 380°C for the deeper phyllic assemblage and associated subeconomic chalcopyrite + tennantite + bornite mineralization, through 250°to 320°C for the alunite alteration and high sulfidation copper mineralization, to 100°to 180°C for the shallow silicic alteration and gold deposition. Hypogene hydrothermal fluids at Zijinshan were derived initially from a deep-level granodiorite porphyry in the southeastern area of the mine. Following the formation of phyllic and dickite alteration assemblages at 180°to 380°C through simple cooling and dilution of hydrothermal fluids, the vapor-rich magmatic fluid containing HCl and SO ₂ was probably formed by phase separation of a deep magmatic fluid and subsequently ascended northwestward. The upwelling magmatic vapors condensed into meteoric water at shallow levels (higher than 1,700 m below the paleosurface), forming the high sulfidation epithermal fluids (<320°C) responsible for the formation of the funnel-shaped, alunite and silicic alteration zones. The Cu and Au mineralization occurred after the acid leaching of rocks by the vapor condensates and the subsequent hydrothermal brecciation. Progressive mixing with meteoric water and the subsequent changes of the physicochemical environment (e.g., decreases of temperature and chloride ion activity and increase of pH) could result in the successive precipitation of copper (within the alunite alteration zone) and gold (in the shallow silicic alteration zone) at temperatures below 250°C.