[B-PT23_30PM2] 地球史解読：冥王代から現代まで

[BPT23-26] 熱水性鉱床生成過程におけるスラブ起源流体の寄与の解明

*藤永 公一郎¹、加藤 泰浩¹、稗田 裕樹¹、荒木 修平¹、谷水 雅治²、角野 浩史³、清水 徹⁴、中村 仁美⁵、岩森 光² (1.東京大学大学院工学系研究科システム創成学専攻、2.独立行政法人海洋研究開発機構、3.東京大学大学院理学系研究科附属地殻化学実験施設、4.独立行政法人産業技術総合研究所、5.東京工業大学大学院理工学研究科地球惑星科学専攻)

キーワード：Pb isotopic composition, hydrothermal deposit, slab-fluid

It has been recently pointed out that "geofluids" released from the subducting plates are involved in various products in subduction zones, such as arc magmas, deep-seated hot springs and hydrothermal vein-type deposits. Systematic investigations of these various materials are needed for identifying the geochemical characteristics of the geofluids. Nakamura et al. (2008) revealed the heavy isotopic compositions of slab-fluids derived from two subducted plates (the Pacific plate and the Philippine Sea plate) which contribute largely to the genesis of arc magmas in Central Japan.In this study, we focus on the hydrothermal vein-type deposits in Japan. It has been previously considered that hydrothermal fluids that form sul-phide mineral (pyrite, chalcopyrite, sphalerite, galena etc.) deposits were originated from magmatic and/or meteoric waters [2]. However, we reported that Pb isotopic compositions of the sulphide ore samples were plotted between Philippine Sea plate (PHS)-fluid and Pacific plate (PAC)-fluid, suggesting that ore fluids responsible for the hydrothermal deposits are directly derived from deep slab-fluids. Here we report multi-isotopic compositions (Pb-Nd-He) of sulphide ores, associated volcanic rocks, and the surrounding country rocks from the Toyoha polymetallic (Zn-Pb-Ag-Cu-Sn-In) vein-type deposit (one of the largest hydrothermal vein-type deposits in Japan) in order to understand the rela-tionship between slab-fluid and formation of vein-type deposit in more detail.Results and Discussion: We collected twenty-six sulphide ore samples, and fifteen associated volcanic and country rocks from the Toyoha Mine. The 206Pb/204Pb values of sulfide ore samples are significantly larger than those of the Muine volcanic rocks which have been long thought to be genetically related to the formation of Toyoha deposit. In addi-tion, the 3He/4He values of Toyoha galena samples range between 5 and 6 times the atmospheric ratio, implying the significant contribution of the mantle component, and strongly suggest that there is a contribution from deep-derived fluid to the Toyoha ore fluid. The correlation between 207Pb/204Pb and 143Nd/144Nd shows that the relative contribution of PAC fluid component in the Toyoha ores is significantly higher than that involved in the Muine volcanic rocks. It can be estimated that more than ~80% of Pb of the Toyoha ore deposit is derived from slab-fluids. Based on the present measurements and mass balance calculations, it is very likely that the slab-fluids supplied the major part of Pb and other metals concentrated in the Toyoha district.