Investigation of hydrothermal ore deposits on the East Pacific Rise (EPR) and on its off-axis volcanoes was carried out from 1985 to 1994 by the RN Hakurei-maru No.2. More than one and a-half hundred hydrothermal activity sites were observed in the axial rift and on the off-axis volcanoes along the courses of the investigation. The hydrothermal sites were clustered between 12°47" N and 12°51" N, and between 11°20" N and 11°36" N. Though some percentages of the sites were concentrated on the top of ridge and in the spreading center of the rift, the off-axis volcanoes are considered as the most important area in the EPR from the view point of resource potential
In 1979, the first active hydrothermal site in the open ocean with massive sulfide orcs and vent biological colony was found at 21° N in the East Pacific Rise (EPR) by the US and French group (Francherau et aI., 1979; Spiess et aI., 1980; Hekinian et aI., 1980). DORD was trusted to conduct the investigation by the Metal Mining Agency of Japan which was commissioned to plan the investigation from the Agency of Natural Resources and Energy of the Ministry of International Trade and Industry. Block surveys in the axial rift of the EPR as well as some off-axis volcanoes between go N and 14° N were carried out from north to south by ten cruises from 1985 to 1994. Systematic geophysical surveys such as bathymetry, magnetics, and side-scan sonar survey in each block were followed by water sampling. Several sites were selected for seafloor observation by the geophysical and chemical data. Seafloor observation was conducted by a deep-towed camera system (Finder TV camera mounted Deep-sea Camera system: FDC) to find ore indications, which were identified by seafloor color anomaly, biological communities and water temperature anomaly.
