Abstract
Around the Izu-Bonin back-arc rift, JOGMEC had conducted surveys targeting for deep-sea hydrothermal mineral deposits from 2000 to 2003 as a part of the deep-sea mineral exploration program supported by the Ministry of Economy, Trade and Industry (METI). As a result, a huge hydrothermal sulfide deposit(Hakurei ore deposit)was discovered in the Bayonnaise knoll caldera during the April 2003's survey cruise. It was the first discovery of Kuroko-type ore deposit in the Izu-Bonin back-arc rift. There are many inactive sulfide chimneys inside the caldera wall of Bayonnaise knoll. The water depth of Hakurei ore deposit is about 700-800 m, which is the shallowest sea-floor hydrothermal deposit known in the world. These sulfide chimneys mainly consist of sphalerite associated with chalcopyrite, galena, pyrite and barite. The maximum metal contents of four chimneys from Bayonnaise knoll caldera are 37.1 g/t Au, 1, 275 g/t Ag, 1.36 % Cu, 5.57 % Pb and 51.56 % Zn. The horizontal dimension of sulfide deposit is 700×500m, and is almost equal to that of the Sunrise ore deposit in the Myojin knoll caldera, one of the largest submarine hydrothermal deposits in the Izu-Bonin island arc, situated in the volcanic front. The potential of Kuroko-type ore deposit in the Izu-Bonin back-arc rift seems to be as high as that in the Izu-Bonin volcanic front. The survey results gave not only new knowledge about the potential of Kuroko-type ore deposit in the Izu-Bonin back-arc rift but also an useful guide for how to explore submarine Kuroko-type ore deposits.
