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 mineral ization of Cretaceous age in mainland China. The Cu-Au mineralization occurs around a dacitic volcanic pipe in the central part of a calc-al kaline, trachydacitic to dacitic volcanic dome of Cretaceous age that has intruded an S-type Jurassic granite. The Cu-Au orebodies are subpa rallel, northwest-trending breccias, veins, and massively silicified r ocks which have developed along extensional fractures in wide hydrothe rmal alteration zones. At depth the dacite porphyry pipe changes downw ard into a potassic-altered granodiorite porphyry associated with porp hyry copper-style mineralization, indicating a close spatial and tempo ral association between high sulfidation Cu-Au and porphyry copper min eralization. Zoning of hydrothermal alteration and ore mineralization assemblages at Zijinshan is typically recognized from the deep and cen tral zones toward shallower and outer portions within the epithermal s ystem. A deep sericite + quartz + pyrite assemblage in the phyllic alt eration zone gives way upward to a dickite + quartz + pyrite + zunyite +/- sericite assemblage in the dickite alteration zone and successive ly to the alunite + quartz + pyrite assemblage in the funnel-shaped al unite 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 as sociated gold mineralization are restricted at shallow depths (less th an 350 m). Vertical metal zoning in the sequence Au-Ag-As --> Cu --> C u-Pb-Zn is also recognized within the silicic and alunite alteration z ones. Paleogeographic reconstruction indicates depths of <850 and 850 to 1,700 m for the silicic alteration and alunite alteration, respecti vely. Geochemical, mineralogical, and fluid inclusion data are consist ent with progressive acid leaching toward shallower portions of the hy drothermal system, accompanying the general decrease of temperature fr om 300 degrees to 380 degrees C for the deeper phyllic assemblage and associated subeconomic chalcopyrite + tennantite + bornite mineralizat ion, through 250 degrees to 320 degrees C for the alunite alteration a nd high sulfidation copper mineralization, to 100 degrees to 180 degre es C for the shallow silicic alteration and gold deposition. Hypogene hydrothermal fluids at Zijinshan were derived initially from a deep-le vel granodiorite porphyry in the southeastern area of the mine. Follow ing the formation of phyllic and dickite alteration assemblages at 180 degrees to 380 degrees C through simple cooling and dilution of hydro thermal fluids, the vapor-rich magmatic fluid containing HCl and SO2 w as probably formed by phase separation of a deep magmatic fluid and su bsequently ascended northwestward. The upwelling magmatic vapors conde nsed into meteoric water at shallow levels (higher than 1,700 m below the paleosurface), forming the high sulfidation epithermal fluids (<32 0 degrees C) responsible for the formation of the funnel-shaped, aluni te and silicic alteration zones. The Cu and Au mineralization occurred after the acid leaching of rocks by the vapor condensates and the sub sequent hydrothermal brecciation. Progressive mixing with meteoric wat er 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 degrees C.