Abstract

Manus Basin, located in the equatorial western Pacific, is a back arc basin formed by collision between the Indo-Australian and the Pacific Plates. The basin is host to numerous hydrothermal vent fields and ore deposits. The basement rocks of the Manus Basin consist primarily of dacite and basaltic andesite. Some of the minerals that form the hydrothermal chimneys that were dredged on the Manus basin include pyrite, chalcopyrite, marcasite, sphalerite and galena. The chimneys can be classified into chalcopyrite dominant Cu-rich type and sphalerite dominant Zn-rich type. The concentration of Zn shows good positive correlation with that of Sb, Cd and Ag. The content of Cu, on the other hand, positively correlates with that of Mo, Mn and Co. For samples that were taken from Zn-rich chimney, a strong positive correlation is found between Au and Zn contents. The chimney also shows enrichments of Cd, Mn and Sb. On the other hand, the samples from Cu-rich chimney exhibit strong correlation among Au, Zn and Pb, and are enriched in Mo and Co concentration. Average contents of Au in Cu-rich and Znrich chimneys were 15.9 ppm and 29.0 ppm, respectively. Because of high concentration of Au with Ag and Cu, the ore deposit have high economic potential. Homogenization temperatures and salinities of fluid inclusions in anhydrite and amorphous silica from Zn-rich chimney are estimated to be 174~220^oC and 2.7~3.6 equiv. wt. % NaCl, respectively.
These value suggest that ore forming processes were occurred at around 200oC and that the oxygen fugacitychanged from ≥ 10^-39.5 bar to ≤ 10^-40.8 bar and the sulfur fugacity from  ≤ 10^-14.7 bar to ≤ 10^-13.4 bar during the process. It appears that the temperature at which the ores formed on Cu-rich chimney was higher than that on Zn-rich chimney.

Keywords back are basin, hydrothermal chimney, metal chimney, metal contents, fluid inclusion