日本地球惑星科学連合2022年大会

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[J] 口頭発表

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-RE13] 資源地質学

2022年5月25日(水) 10:45 〜 12:15 201B (幕張メッセ国際会議場)

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)、コンビーナ:実松 健造(国立研究開発法人 産業技術総合研究所 地圏資源環境研究部門 鉱物資源研究グループ)、高橋 亮平(秋田大学大学院国際資源学研究科)、コンビーナ:野崎 達生(国立研究開発法人 海洋研究開発機構 海洋機能利用部門 海底資源センター)、座長:高橋 亮平(秋田大学大学院国際資源学研究科)、野崎 達生(国立研究開発法人 海洋研究開発機構 海洋機能利用部門 海底資源センター)

11:00 〜 11:15

[HRE13-07] 東北日本の小坂・尾去沢・細倉鉱床から産出した硫化鉱物のRe-Os同位体組成の特徴とその放射年代決定に対する示唆

*小笠原 瑞姫1大田 隼一郎1,2石田 美月1矢野 萌生2,1藤永 公一郎2,1安川 和孝1中村 謙太郎1,2加藤 泰浩1,2 (1.東京大学、2.千葉工業大学)


The Re-Os isotope system is a powerful tool in Earth science, especially in radiometric dating of sulfide minerals and black shale. Since Re is a chalcophile element expected to be enriched in various sulfide minerals, the Re-Os isotope geochronology has been applied to the direct age determination of sulfide deposits. By combining acid digestion using HClO4 [1] and sparging introduction to MC-ICP-MS [2, 3], we developed a simple analytical method for the Re-Os isotopes [4].
Using this method, we implemented Re-Os isotope analysis on sulfide ore samples from the Kosaka deposit, one of the Kuroko ore deposits in the Hokuroku district, northeastern Japan. The Kuroko and Oko ore samples from the Kosaka deposit showed concentrations of 31.3 - 57.4 ppb for Re and 124 - 251 ppt for Os, although the chalcopyrite sample showed low Re and Os concentrations, 528 ppt for Re and < 2.00 ppt for Os. Based on the results, we obtained the isochron age of 11.698 ± 0.022 Ma, which is consistent with a possible range of the Kuroko formation (16-11 Ma, proposed by [5]) in the Hokuroku district.
We also analyzed ore samples from two epithermal vein-type sulfide deposits in NE Japan, the Osarizawa and Hosokura deposits. Two chalcopyrite samples from the Osarizawa deposit, located in the Hokuroku district, showed extremely low Re and Os concentrations: < 30 ppt for Re and < 0.1 ppt for Os. Thirteen sulfide samples from the Hosokura deposit also showed relatively low Re concentrations (< 500 ppt except one sample with 2,092 ppt) and extremely low Os concentrations (< 3 ppt). These low Re-Os concentrations resulted in large error ranges and thus hampered obtaining reliable isochrons.
Our results imply that the hydrothermal fluids that formed these ore samples were unexpectedly depleted in Re and therefore lacked Os derived from radioactive decay of Re. Although further investigations are required, these two epithermal vein-type sulfide deposits could have been formed with Re-poor source materials and/or involved in processes that removed Re before mineralization.

1: Gao, B. et al. (2019) Microchem. J. 150, 104165.
2: Nozaki, T. et al. (2012) Geostand. Geoanal. Res. 36, 131-148.
3: Kimura, J.-I. et al. (2014) J. Anal. Atom. Spectr. 29, 1483-1490.
4: Ogasawara, M. et al. (2021) JpGU Meeting HRE12-02.
5: Tanimura, S. et al. (1983) Econ. Geol. Mon. 5, 24-39.