TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 50 (No. 6), pp. 461-475, 2016

Indication of hydrothermal deposits and ore area on caldera floor and shallow sub-seafloor of the Bayonnaise knoll, based on high-resolution acoustic investigation

Miho Asada,1* Takafumi Kasaya,1,2 Keizo Sayanagi3 and Tada-nori Goto4

1Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan
2Strategic Innovation Promotion Programs (SIP), Project Team for Development of New-generation Research Protocol for Submarine Resources, JAMSTEC, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan
3Institute of Oceanic Research and Development, Tokai University, 3-20-1, Orido, Shimuzu-ku, Shizuoka 424-8610, Japan
4Department of Civil and Earth Resources Engineering, Kyoto University Graduate School of Engineering, Kyoto Daigaku-katsura, Nishikyo-ku, Kyoto 615-8540, Japan

(Received February 4, 2015; Accepted October 13, 2015)

Abstract: The Bayonnaise knoll, an active submarine volcano belonging to an actively rifted part of the Izu-Bonin volcanic arc, exhibits hydrothermal ore deposits on its caldera floor in a region known as the Hakurei Sulfide Deposit (HSD) area. We observed the HSD area using high-resolution acoustic observation equipment consisting of multibeam echo sounder (MBES), sidescan sonar (SSS), and sub-bottom profiler (SBP) systems, on the AUV Urashima. We used visual and acoustic results to examine the consistency of the HSD area extent and to consider possibilities of other ore areas within the caldera. The resultant high-resolution acoustic imageries suggest expansion of the HSD area to the northeastern caldera wall and the southwestern sub-seafloor of the caldera floor. The SBP data show a thick sediment layer on the western part of the caldera floor where many high-backscattering signals were observed. Small chimney-like features were acoustically observed in the HSD area and also at the central cone and along the rim of the caldera. However, most are remnant features of ancient volcanic activity of the knoll, and thus may not indicate current hydrothermal deposits. Acoustic investigations such as this, along with appropriate interpretation, are very useful to determine the detailed distribution of ore on the seafloor and at the shallow subsurface, and should be an effective tool for regional site surveying before seabed mineral mining.
Key words: hydrothermal deposits, high-resolution acoustic observation, AUV, sub-seafloor, SBP

*Corresponding author E-mail: asadam@jamstec.go.jp

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