Submarine hydrothermal mineralization on the Izu-Bonin Arc, south of Japan: An overview

Considerable effort has been expended in studying the Izu-Bonin Are over th e past 15 years, in particular, 43 dives of the Shinkai 2000 have been unde rtaken there to discover and evaluate the extent of submarine hydrothermal activity and mineralization. Most effort has been focused on Myojin Knoll ( 23 dives), Suiyo Seamount (6 dives), and Kaikata Caldera (10 dives). The Izu-Bonin Are is divided in two by the Sofugan Tectonic Line. Eight sub marine caldera are located north of this line but only one is south of it. The physiography of the northern sector of the are is quite different from that of the southern sector. Volcanic rocks from the northern sec:tor are m ore acidic than those from the southern sector. Evidence for submarine hydrothermal mineralization has been observed at fou r seamounts along the Izu-Bonin Arc (Myojin Knoll, Myojinsho Suiyo Seamount , and Kaikata Caldera), and submarine hydrothermal activity is evident at a nother three seamounts along the are (Kurose Hole, Mokuyo Seamount, and Doy o Seamount). The most extensive submarine hydrothermal mineral deposit so far. located a n the Izu-Bonin Are is the Sunrise deposit at Myojin Knoll. This deposit, a t least 400 m in diameter and 30 m high, is associated with black smoker ve nting, inactive sulfide chimneys, massive sulfides, hydrothermal Mn crusts, and a hydrothermal vent fauna. The maximum recorded temperature of the hyd rothermal vents there was 278 degrees C. Some of the sulfide chimneys conta ined as much as 49 mu g /g Au and 3,400 mu g/g Ag. The sunrise deposit is o ne of the largest submarine volcanic massive sulfide deposits so far discov ered in midocean ridge backarc, or are settings and has an estimated mass o f 9 x 10(6) t. This deposit may be of the Kuroko-type. The discovery of the Sunrise deposit in 1997 gives hope that other, similarly large, sulfide de posits may be found in other caldera along the Izu-Bonin Arc. The geological variability along the are, the high seismicity the occurrenc e of active volcanism and submarine hydrothermal venting, and a proven subm arine hydrothermal mineral potential coupled with the proximity of the regi on to Japan suggest that the Izu-Bonin Are could profitability serve as a n atural laboratory for the long-term monitoring of the seafloor.