Abstract
The Zijinshan copper-gold deposit, Fujian Province, is situated at the margins of an Early Cretaceous volcanic basin, beside a caldera made up of dacitic pyroclastic rocks and subvolcanic (dacite-porphyry) bodies. Silicification and hydrothermal alteration of quartz-dickite, quartz-alunite and quartz-sericite occur as concentric zones around the caldera covering a wider area.
The orebodies occur as filling-veins in a dense fissure zone and as a stockwork fissure system, accompanied by hydrothermal breccias. Native gold, pyrite, digenite, enargite and covellite are the main ore minerals, while gold and copper are the main economic metals. The gold veins occur within the silicified zone in the shallow portions of the deposit. On the other hand, the copper veins lie within the quartz-alunite alteration zone at the 350-900m interval, showing good primary mineralization zoning.
The fluid inclution data suggest that the hydrothermal fluids were characterized by low temperature, low salinity (0.0-5.0 wt% NaCl equivalent) and were in acidic conditions (pH=3.64-3.98). The fluid inclution filling temperatures ranged from 120°C to 140°C for the gold mineralization and from 180°C to 240°C for the copper mineralization. Many fluid inclusions showing evidences of boiling were observed in the quartz-alunite alteration zone. The salinity of these inclusions is higher and ranges from 7.5 to 21.6 wt% NaCl equivalent, while homogenization temperatures show a narrow range.
The δ18OH2O values of the hydrothermal fluids range from -3.25 to -4.17‰(SMOW) and the δD values from -60 to -76‰, which coincide with the values to oxygen and hydrogen isotopic compositions of Mesozoic meteoric water in this region. Several evidences suggest that the Zijinshan deposit is a quartz-alunite type epithermal deposit and is the first of its kind to be discovered in the continent of China.
