日本地球惑星科学連合2019年大会

講演情報

[J] ポスター発表

セッション記号 H (地球人間圏科学) » H-RE 応用地質学・資源エネルギー利用

[H-RE16] 資源地質学

2019年5月29日(水) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:大竹 翼(北海道大学大学院工学研究院 環境循環システム部門)、荒岡 大輔(産業技術総合研究所地圏資源環境研究部門)、高橋 亮平(秋田大学大学院国際資源学研究科)、野崎 達生(海洋研究開発機構海底資源研究開発センター)

[HRE16-P07] Transformation of ore minerals and their geochemical signatures in submarine hydrothermal ore deposits in Izena Hole, Okinawa Trough

*池島 拓郎1大竹 翼1佐藤 努1伊藤 茜1小野 修司1石橋 純一郎2野崎 達生3熊谷 英憲3前田 玲奈3 (1.北海道大学、2.九州大学、3.海洋研究開発機構)

キーワード:海底熱水鉱床、伊是名海穴、SIP、交代作用、鉄同位体、ちきゅう

It is important for exploration and development of submarine hydrothermal ore deposits to understand zoning of ore bodies and their mineral assemblages, particularly with depth from the seafloor. Transformation of ore minerals via dissolution and re-precipitation may cause such a zoning corresponding to the evolution of a hydrothermal system because geochemical conditions may then vary with the depth from the seafloor. In this study, we aimed (1) to clarify the mineral assemblage changes with increasing depth of a mound sulfide deposit by microscopic observations and SEM-EDS analyses, (2) to understand geochemical evolution of hydrothermal fluids based on Fe contents in sphalerite determined by EPMA, and (3) to understand the formation processes of Fe sulfides by analyzing Fe isotopic composition of the bulk samples with MC-ICP-MS. The samples used for the analyses were obtained from Site C9027, which drilled up to 72.5 meter below seafloor (mbsf) through a mound sulfide deposit in Hakurei site, Izena Hole, middle Okinawa Trough during the CK16-05 cruise operated from November to December, 2016 under the SIP program.
Results of microscopic observations show that sphalerite and galena are comparatively dominant with minor occurrence of pyrrhotite and anglesite in the samples at shallow depths. In contrast, samples from middle and deep depths are more abundant on pyrite and chalcopyrite, and pyrite, respectively. The texture that pyrrhotite was dissolved and replaced by pyrite or marcasite was observed in the samples in the depth of 0.05 – 44.0 mbsf. The Fe content in sphalerite tends to decrease with increasing depth, and also change within single sphalerite grain. These results suggest that hydrothermal fluid geochemistry such as fugacity of S2 and O2, and temperature changes for the depth and time. The δ56Fe values (δ56Fe (‰) = 1000 * [(56Fe/54Fe) sample / (56Fe/54Fe) IRMM-014 -1]) of the bulk samples range from -0.62 to +0.11‰, which tends to increase toward the deeper part. This indicates that Fe sulfides at shallow depths was formed by non-equilibrium precipitation with a rapid cooling, which caused a kinetic isotope effect. In contrast, those at deeper depths were likely formed near the equilibrium conditions.