[S-CG52] 海洋底地球科学

[SCG52-P22] Dating the gold-rich, low-temperature Higashi-Aogashima Knoll Caldera hydrothermal field (Izu-Bonin Arc, Japan) with barite

*Man-Yin Tsang¹、Jun-ichiro Ishibashi²、Shin Toyoda³、Marin Yamamoto³、Tatsuo Nozaki⁴、Kazuki Kohama⁵、Kazuya Kitada⁴、Yuzuru Yamamoto¹ (1.Kobe University、2.Kobe Ocean-Bottom Exploration Center、3.Okayama University of Science、4.Japan Agency for Marine-Earth Science and Technology (JAMSTEC)、5.Kyushu University)

キーワード：Island arc, Seafloor massive sulfide, Barite, Electron spin resonance dating, ESR dating, Lead-radium disequilibrium dating

The Higashi-Aogashima Knoll Caldera hydrothermal field, discovered in 2015 (The University of Tokyo, 2015; Katase et al., 2016), hosts abundant electrum grains as well as rocks and sediments with more than 100 ppm of gold (Iizasa et al., 2019). As the temperature of this hydrothermal field is below 300 °C (Nozaki et al., 2021), previous studies hypothesize that the gold-formation process here is different from well-documented processes in other island arc settings (Iizasa et al., 2019). The sulfide mounds in the hydrothermal field are rich in barite (BaSO₄). Barite can be a proxy of its formation fluid and is commonly used for dating hydrothermal fields. Here we extract barite minerals from the hydrothermal sulfide mounds for ²¹⁰Pb-²²⁶Ra disequilibrium dating and electron spin resonance (ESR) dating, as well as analyze their ²²⁶Ra activities. Our findings can enhance the understanding of the geological history and the geochemistry of fluid involved in forming the gold-bearing sulfide mounds in the Higashi-Aogashima.

We also compare and contrast the ²¹⁰Pb-²²⁶Ra disequilibrium method and the ESR method with respect to dating hydrothermal fields using barite. We will discuss how having multiple phases of mineral formation and lead-rich samples can cause a discrepancy between the results of the two dating methods.

References:

Katase, F., Iizasa, K., Mizuno, K., & Asada, A. (2016). Seafloor Hydrothermal Deposits Exploration by Bathymetry and Backscattering Data Using Multibeam Echo-Sounder in the Higashi-Aogashima Caldera. The Journal of the Marine Acoustics Society of Japan, 43(4), 208-218. https://doi.org/10.3135/jmasj.43.208

Iizasa, K., Asada, A., Mizuno, K., Katase, F., Lee, S, Jojima, M., & Ogawa, N. (2019). Native gold and gold-rich sulfide deposits in a submarine basaltic caldera, Higashi-Aogashima hydrothermal field, Izu-Ogasawara frontal arc, Japan. Mineralium Deposita, 54, 117-132.

Nozaki, T., Torimoto, J., Tsang, M.-Y., Ishibashi, J., Methou, P.E.B-H., Wakai, S., & Kitada, K. (2021). R/V Shinsei Maru Cruise Report KS-21-20: unraveling an abnormal gold enrichment mechanism at the Higashi-Aogashima Knoll Caldera hydrothermal field. Japan Agency for Marine-Earth Science and Technology, Japan. https://doi.org/10.17596/0002430

The University of Tokyo. (2015). Discovery of the seafloor hydrothermal deposit at the eastern offshore of Aogashima Island, Izu area; Development of a tool that can discover seafloor hydrothermal deposit in a short span of time. Press Release on 7th August 2015.