A first extensive hydrothermal field associated with Kuroko-type deposit in a Silicic Submarine caldera in a Nascent Rift Zone, Izu-Ogasawara (Bonin) arc, Japan

A huge hydrothermal field of 500 by 700 m across associated with a modern analogue of Kuroko-type deposit was first found in the Bayonnaise knoll caldera in a nascent rift zone westward the volcanic front of the Izu-Ogasawara (Bonin) island arc, Japan during the April 2003 Hakurei-maru No.2 cruise since two major Kuroko-type deposits had been reported on the volcanic front. The dacite knoll caldera associated with a post-caldera dome has the slightly ellipsoidal caldera rim of 3 by 2.5 km wide with about 200 m relief, and its floor is about 840-920 m below sea level. The hydrothermal field, which occurs between water depths of about 800 m and 700 m around the southeastern caldera floor, exists near the intersection of caldera boundary fault and an inferred N-S trending normal fault in a back-arc rift zone. In the field many sulfide chimneys with different sizes less than 10 m high on sulfide mounds are distributed on the marginal area between the caldera floor and sloping talus deposits at the foot of the caldera wall, and are buried in part by talus composed of pumice and dacite fragments and their breakdown materials. They are still active and some of them are oxidized to reddish materials. Sulfide chimneys recovered by finder-installed power grab are mainly composed of sphalerite associated with chalcopyrite, galena, pyrite and barite, and chemically enriched in Au up to 14 ppm and Ag up to 1400 ppm with major Zn, moderate Ba, and minor Pb, Hg and Fe. Since the caldera is located exactly near the intersection of the caldera boundary fault and the N-S back-arc rift fault, the ambient area including the caldera is tectonically active and in an extensional field. The finding of Kuroko-type deposit implies that possible areas of Kuroko-type mineralization may still exist in the N-S rift zones along the volcanic front.