[H-RE13] Resource Geology

[HRE13-07] Re-Os isotope characteristics of sulfide minerals from Kosaka, Osarizawa, and Hosokura deposits in northeastern Japan and implications for their radiometric dating

*Mizuki Ogasawara¹, Junichiro Ohta^1,2, Mizuki Ishida¹, Moei Yano^2,1, Koichiro Fujinaga^2,1, Kazutaka Yasukawa¹, Kentaro Nakamura^1,2, Yasuhiro Kato^1,2 (1.The University of Tokyo, 2.Chiba Institute of Technology)

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.