Redox buffer capacity of sedimentary rocks around uranium deposit –Study of water-mineral-microbe system at the Tono uranium deposit, Japan–

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  • ウラン鉱床の長期保存に関わる岩盤の酸化還元緩衝能力 –東濃地域における天然環境の水–鉱物–微生物システムの研究例–
  • ウラン コウショウ ノ チョウキ ホゾン ニ カカワル ガンバン ノ サンカ カンゲン カンショウ ノウリョク トウノウ チイキ ニ オケル テンネン カンキョウ ノ ミズ コウブツ ビセイブツ システム ノ ケンキュウレイ

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The redox buffer capacity of the water-mineral-microbe system in and around the Tono uranium deposit, Japan, was studied to evaluate the long-term stability of the redox system. The uranium deposit occurs in the lower part of a Miocene sedimentary rock sequence that unconformably overlies granite. In the groundwater, profiles of redox sensitive solutes such as sulphate and sulphide ions, abundance and viability of microbes, and sulphur isotope ratios of sulphate ions suggest that microbial sulphate reduction involving organic matter and subsequent pyrite precipitation are dominant redox reactions at the depths of the uranium ore bodies. Concentrations of both the sulphate and chloride increase with increasing depth. The dissolved sulphate is surmised to have originated from dissolution of sulphate and sulphide minerals in a geologic marine formation precipitated in marine environments, in the upper part of the sedimentary rocks. Such a redox process in the water-mineral-microbe system is inferred to have continued from the time when the marine formation underwent uplift above sea-level, because sulphate-reducing bacteria can use sulphate ions dissolved in fresh water that infiltrates from the marine formation and organic matter located in the deeper sedimentary rocks. The chloride ion also has a linear relationship with groundwater residence time. From the relationship between sulphate and chloride ion concentrations, the sulphate ion dissolution rates from the marine formation are estimated at about 5.9×10-5mM/yr. Calculations by using the sulphate-S contents of the rocks and the sulphate dissolution rate suggest that microbial sulphate reduction alone could maintain sufficiently reducing conditions to preserve the uranium ore for several hundred thousand years, in the case where a hydrogeological system continues to exist without much change.


  • Chikyukagaku

    Chikyukagaku 37 (2), 71-82, 2003

    The Geochemical Society of Japan

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