The Tofua Arc at the southwestern Pacific, is formed by the subduction of the Pacific plate beneath the Indo-Australian plate. This volcanic arc is dominated by a large funnel-shaped caldera ∼6.5km in diameter and >1,1㎞ in depth. It is primarily basaltic andesite in composition and hosts two distinct, hydrothermal sites: the ECVF(East Caldera Vent Field) and the WCVF(West Caldera Vent Field). In particular the sulfate and sulfide minerals such as anhydrite, barite, sphalerite, pyrite, galena, and chalcopyrite are predominant in the hydrothermal chimneys. The chalcopyrite-dominant ECVF is rich in Cu, whereas the sphalerite-dominant WCVF is rich in Zn.
In the ECVF, the collected hydrothermal chimneys demonstrated a diverse range of mineralization, which can be classified into three zones : (i) massive Cu sulfides zone, consisting of chalcopyrite, sphalerite, pyrite, and minor amount of tennantite, bornite and chalcocite; (ii) porous zone, consisting of chalcopyrite, sphalerite, pyrite, and minor amount of barite and tennantite ; (iii) thin sulfates zone, consisting of sulfates(barite and anhydrite/gypsum) and sulfides(sphalerite, pyrite, and minor amount of chalcopyrite). In these chimneys, bornite, chalcocite form along the outer margin of the chalcopyrite as a result of oxidation by seawater. Also, the Zn-rich chimneys from the WCVF can be classified into three zones : (i) massive Zn sulfides zone, consisting of sphalerite, chalcopyrite, pyrite, and minor amount of enargite and tennantite; (ii) porous zone, consisting of sphalerite, pyrite, chalcopyrite, galena, arsenopyrite, and minor amount of marcasite and enargite ; (iii) thick sulfates zone, consisting of sulfates(barite and anhydrite/gypsum) and sulfides(pyrite, sphalerite, galena and minor amount of chalcopyrite).
Homogenization temperature and salinities of fluid inclusions in barite from Cu-rich chimneys are estimated at 236∼272℃ and -1.8∼-1.4 equiv. wt.% NaCl, respectively. These values suppose that ore-forming processes occurred at around 250℃ and that the sulfur fugacity changed from ≤10-10.9 bar to ≥10-8.48 bar and the oxygen fugacity from ≤10-34.3 bar to ≥10-38.79 bar during the process. On the other hand, homogenization temperature from the WCVF are estimated at 212℃ for Zn-rich chimney and 148∼341℃ for mound. Salinity of the fluid inclusion from the Zn-rich chimney is 3.6 equiv. wt.% NaCl, whereas those from the mound samples are 0.4∼3.1 equiv. wt.% NaCl. These values suppose that ore forming processes in the Zn-rich chimney were occurred at around 200℃ and that the sulfur fugacity changed from ≥10-12.7 bar to ≤10-13.9 bar and the oxygen fugacity from ≥10-38.7 bar to ≤10-39.9 bar during the process. Also, ore forming processes in the mound samples were occurred at higher temperature than Zn-rih chimney.