Exploration for a hydrothermal sulfide deposit in a submarine caldera, Izu-Ogasawara arc

The hydrothermal ore deposit (named "Hakurei deposit") has been found in the Bayonnaise knoll caldera, Izu-Ogasawara arc by our short-term survey in April 2003. Since there are a lot of reports regarding expected sites for sulfide mineralization in submarine calderas and intersections of ridges and faults in the active regions, the survey area about 10,000 km² around the Myojin knoll caldera was selected for the exploration area for hydrothermal sulfide deposit based on the data compilation of previous reports. The detailed bathymetric survey (MBES) outlined precise figure of the Bayonnaise knoll caldera in the eastern edge of the rift zone where was not investigated well. The concurrent magnetic survey (PGM) showed a magnetic lineament crossing the caldera. Following those geophysical results, the geological sampling was carried out by large gravity corer (LC) which detected the showing of hydrothermal activities with sulfide minerals in the caldera. Based on the geophysical and the geological results, the finder-installed deep-sea camera (FDC) survey was carried out to observe the seafloor of the caldera as a sense of route mapping onshore survey. Resultingly, total 21.8 km long, 16 survey lines confirmed the huge hydrothermal field associated with Kuroko-type polymetallic sulfide deposit at the southeastern slope of the caldera. The field is 600 m by 500 m in plane view with many sulfide chimneys about several meters height and hydrothermal alteration zone and temperature anomalies were detected in the water column. Moreover, some in situ sulfide chimneys composed of mainly sphalerite, galena, and chalcopyrite were sampled by finder-installed power grab (FPG). It would appear that our survey methods are one of the effective survey systems to discover rapidly possible hydrothermal sulfide deposits in a submarine caldera. In the near future, no doubt more detailed field observation and three dimensional sampling will be carried out to assess the potentiality of the deposit.