MINERALOGY AND GEOCHEMISTRY OF HAKUREI SULFIDE DEPOSIT, BAYONNAISE KNOLL CALDERA, IZU-BONIN ARC

The Izu-Bonin arc extends over ~2,800 km south from Tokyo, Japan, and includes several volcanic islands and submarine volcanoes. Several seafloor hydrothermal localities have been discovered for past 26 years in Izu-Bonin arc, and these have similar structure and geological settings to northeast Japan in the time of middle Miocene; the area can be recognized as modern analog of Kuroko belt. The Hakurei sulfide deposit is a typical example of hydrothermal deposits of the area, which locates on the Bayonnaise knoll caldera, and distributes 500 m x 700 m in area at the depth of 680-820 m. Caldera wall is composed of dacitic lava and its clastic materials, and floor is filled with sandy sediments containing volcanic rocks. The samples of this study are BMS cores of ~6 m in length collected during the cruise by JOGMEC in 2009.

The Hakurei deposit consists of upper massive sulfide zone of ~1 m thick from seafloor, middle altered dacitic volcanic rocks with disseminated sulfide of ~1.5 m thick, and bottom anhydrite zone of >1.5 m thick. Sulfide minerals observed in upper and middle zones are dominated by sphalerite and lesser amounts of pyrite, chalcopyrite, tennantite, galena, enargite and covellite are accompanied. Grain size of sphalerite in upper massive sulfide zone is fine, whether sphalerite of middle zone is coarser in size with euhedral crystal morphology showing color zoning. Chalcopyrite and/or tennantite disease texture is common in sphalerite. Anhydrite occurs in dacitic tuff, or it occurs as massive crystal that accompanies small amount of sulfides.

Homogenization temperature and salinity of fluid inclusion hosted in sphalerite and anhydrite was examined. Homogenization temperatures vary between 100° and 300°C, and they generally increases with depth from seafloor. Salinities of fluid inclusion are in the range of 1 to 12 wt. % eq. NaCl,. Data of microthermometry for anhydrite　occurs in tuff and massive anhydrite are remarkably different. Former has high homogenization temperature and high salinity, while latter has variable homogenization temperatures with low salinity. The d³⁴S values for sulfide minerals fall in narrow range between +3.1 and +5.2 per mil, while those of anhydrite fall between +21.3 and +21.8 per mil, slightly heavier values than that of seawater.