Mineralogical and Fluid Inclusion Study on Seafloor Hydrothermal Vents at TA25 Subsea Caldera in Tongan Waters

The extensive hydrothermal deposits have been found, for the first time, on the western TA25 seamount caldera in the Tonga arc. The seafloor hydrothermal vents are active and immature, emitting the transparent fluids of which temperatures range from 150 degrees C to 242 degrees C (average=203 degrees C). The recovered hydrothermal sulfides are mainly composed of sphalerite, pyrite, marcasite, galena, chalcopyrite, covellite, tennantite, enargite and sulfates such as barite, gypsum/anhydrite. Predominant sphalerite categorize it into Zn-rich hydrothermal ore body. Zn-rich sulfide ores have minor enargite, indicating that mineralization occurred in high sulfidation environment. The proportion and FeS content of sphalerite increase from outside to inside of the hydrothermal ores, respectively. In particular, sphalerite has a great silver content (up to similar to 10 wt.%). Chalcopyrite is more frequently observed in mound than in the chimney, implying mineralization temperature in the mound is higher than in the chimney. Homogenization temperatures and salinities from fluid inclusions in barite at the mound range from 148 degrees C to 341 degrees C (average=213 degrees C) and 0.4 to 3.6 equiv. wt.% NaCl, respectively. Homogenization temperatures suggest that sulfides in the mound mineralized at a higher temperature (>= 200 degrees C) than in the chimney.