Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-CG Complex & General

[S-CG56] Ocean Floor Geosciences

Sun. May 26, 2019 9:00 AM - 10:30 AM A05 (TOKYO BAY MAKUHARI HALL)

convener:Kyoko Okino(Atmosphere and Ocean Research Institute, The University of Tokyo), Chairperson:shuhei Totsuka, Ayanori Misawa

9:15 AM - 9:30 AM

[SCG56-02] Mineralogical and geochemical study of hydrothermal deposits beneath the seafloor at the Gondou field in the Okinawa Trough

*Shuhei Totsuka1, Jun-ichiro Ishibashi2, Kazuhiko Shimada2, Kei Ikehata3, Hideaki Machiyama4, Koichi Iijima4, Hirofumi Yamamoto4, Hidenori Kumagai4, Ken Ikehara5, Toru Yamasaki5, Yutaro Takaya6, Toshiro Nagase7, Thomas Tindell8, Kotaro Yonezu8, Yuki Tada8 (1.Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 2.Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 3.Faculty of Life and Environmental Sciences, University of Tsukuba, 4.Research and Development Center for Submarine Resources, JAMSTEC, 5.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 6.Department of Resources and Environmental Engineering School of Creative Science and Enginerring, Waseda University, 7.Tohoku University Museum, 8.Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University)

Keywords:Submarine hydrothermal deposits, Back-arc basin

The Gondou field is located at the western flank of the Daisan-Kume Knoll, at a water depth of 1330-1470 mbsl (meters below sea level). Many hydrothermal spire and mound structures were recognized in an area of ca. 1400 m × 400 m (Minami and Ohara, 2017). Previous study (Ishikawa et al., 2016) reported high Cu content in hydrothermal deposits collected from the Gondou field. 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 using BMS (Benthic Multi-coring System) installed on Shincho-maru. Mineralogical and geochemical studies of sulfide minerals in the obtained cores were conducted by microscopic observations, electron probe microanalyzer (EPMA) analysis, and X-ray diffractometer (XRD) analysis.
Massive and disseminated sulfide layers were recognized at Site GDUC06, which was covered with silty clay sediment and underlaid by silicified volcaniclastics. Two distinctive types of massive sulfide were recognized; the upper one is composed of sphalerite, pyrite, galena and associated with barite, while the lower one is composed of pyrite, chalcopyrite and associated with quartz, illite and chlorite. The underlaid silicified volcaniclastics layer includes pyrite. It is notable that formation of sphalerite-rich massive sulfide and chalcopyrite-rich sulfide is comparable to the black ore and yellow ore found in Kuroko-type deposits.
Similarity to the Kuroko-type deposits can be found in chemical composition of sulfide minerals. Iron content in sphalerite from the Gondou field ranges from 0.22 to 5.72 atm%, which would be comparable to the reported value from 0.07 to 0.35 atm% for the Matsumine and the Shakanai deposits of the Hanaoka mine, one of typical Kuroko-type deposits in the Hokuroku district (Shimazaki and Horikoshi, 1990)). Tetrahedrite-tennantite series occurred in the sphalerite-rich massive sulfide exhibit high As/(As+Sb) and Zn/Fe ratios, which are close to the range found in massive sulfide from the Hanaoka mine (Shikazono and Kouda, 1979). Since chemical composition of sulfide minerals is considered to reflect chemical condition such as fS2 during the formation, these similarities would provide keys to understand the mineralization processes.

This study is conducted under the framework of the Next-Generation Technology for Ocean Resources Exploration Project, Cross-ministerial Strategic Innovation Promotion Program (SIP) by the Japanese cabinet office. We are thankful to J-MARES (Research and Development Partnership for Next Generation Technology of Marine Resources Survey) for providing sediment cores used in this study.