[S-CG56] Ocean Floor Geosciences

[SCG56-P13] Mineralogical and geochemical study of sediment cores collected from the Gondou hydrothermal field in the Okinawa Trough

*Jun-ichiro Ishibashi^1,2, Shuhei Totsuka², Akihi Tsutsumi², Naru Tsukamoto², Yusuke Kira², Kazuhiko Shimada¹, Toru Yamasaki³, Ken Ikehara³, Toshiro Nagase⁴, Yutaro Takaya^5,9, Kei Ikehata⁶, Takashi Goto⁷, Chitaro Gouzu⁷, Ryuichi Shinjo⁸, Hideaki Machiyama⁹, Koichi Iijima⁹, Hirofumi Yamamoto⁹, Hidenori Kumagai⁹ (1.Department of Earth and Planetary Sciences, Kyushu University, 2.Graduate School of Science, Kyushu University, 3.National Institute of Advanced Industrial Science and Technology, 4.Tohoku University Museum, 5.School of Creative Science and Enginerring, Waseda University, 6.Faculty of Life and Environmental Sciences, University of Tsukuba, 7.Hiruzen Institute for Geology and Chronology, 8.Faculty of Science, University of the Ryukyus, 9.Japan Agency for Marine-Earth Science and Technology)

Keywords:seafloor massive sulfide deposits, seafloor scientific drilling, hydrothermal alteration

Intense hydrothermal activity associated with sulfide mineralization had been observed in the Gondou field, which is located at the seafloor around a water depth of 1330-1470 mbsl (meters below the sea level) at the western flank of the Daisan-Kume Knoll (Minami and Ohara, 2017; Ishikawaet al., 2016). Scientific drilling was performed by J-MARES during the SIP Integrated Ocean Resource Surveying System JM17-04 Cruise from October to November, 2017. Sediment cores were obtained from 5 sites in Southern area of the Gondou field by drilling using BMS (Benthic Multi-coring System) installed on Shincho-maru. Mineralogy and geochemistry of the obtained cores were studied based on VCD (Visual Core Description), microscopic observation, XRD (X-ray diffractometer) analysis, and electron probe microanalyzer (EPMA) analysis.
Most part of the obtained sediment, except for the surface hemipelagic sediment within a few mbsf (meters below the seafloor), was hydrothermally altered intensely. Abundant occurrence of illite (or illite/smectite mixed-layer mineral) was observed, which is common signature among hydrothermal alteration in hydrothermal fields in the Okinawa Trough. Whereas, occurrence of chlorite was limitedly recognized within a layer of 5-15 m thickness in each site. Kaolin minerals were identified in altered pumiceous volcaniclastic sediment in two sites. These specific occurrences may reflect subseafloor fluid migration of distinctive chemistry, based on the idea that chlorite formation would be related to interaction with heated seater-derived fluid while kaolin minerals formation would be related to interaction with acidic fluid. Occurrence of sulfate minerals such as anhydrite and barite is also limited, which may provide another information about subseafloor fluid migration.

Acknowledgements
This study was conducted under the framework of the Next-Generation Technology for Ocean Resources Exploration Project, Cross-ministerial Strategic Innovation Promotion Program (SIP) funded by the Japanese cabinet office. The sediment cores used in this study were provided from J-MARES (Research and Development Partnership for Next Generation Technology of Marine Resources Survey).