Smectite‐Zeolite Envelope Surrounding the Tsukiyoshi Uranium Deposit, Central Japan: A Natural Analogue Study

Abstract

. The Tsukiyoshi uranium deposit in Gifu Prefecture is the largest one in Japan. It is embedded in lower part of the Mizunami Group of Miocene age. Relating to the existence of this uranium deposit, the constituent minerals in sediments were studied by XRD and SEM, using many drilling cores. The most abundant authigenic mineral is smectite. The amount of smectite increases generally from upper to lower horizons, and a highly smectitized zone is situated around the uranium deposit. Smectitization predominated in mafic glassy grains of sediments, which was probably formed in early burial diagenesis. Zeolites including clinoptilolite‐heulandite, mordenite, analcime, chabazite and philipsite are secondly abundant authigenic minerals. They seem to have been formed at early to late diagenetic stages. Opaline silica is rather rare. Carbonate minerals, including cal‐cite, dolomite, siderite and rhodochrosite are common. They may be formed by diagenesis as well. Gypsum and pyrite occur in upper horizons and lower horizons, respectively. In particular, a highly smectitized zone including pyrite probably played an important role for retarding the migration of uranium and as a result keeping the uranium deposit for past one million years. This smectite‐zeolite envelope surrounding the Tsukiyoshi uranium deposit is regarded as a natural analogue of the buffer materials surrounding the high‐level radioactive waste repository.

Petrographic study of the Miocene Mizunami Group, Central Japan: Detection of unrecognized volcanic activity in the Setouchi Province

“…The most abundant authigenic mineral is smectite. Zeolites, mainly clinoptilolite–heulandite, are the next most abundant authigenic minerals (Utada ). Smectite group minerals, mostly saponite, occur throughout the Mizunami Group, and its content generally increases with depth.…”

“…Smectite group minerals, mostly saponite, occur throughout the Mizunami Group, and its content generally increases with depth. This downhole variation is interpreted to be either the result of progressive burial diagenesis or the increased content of precursor mafic glassy materials (Utada ).…”

“…Though the Mizunami Group has been extensively studied, its provenance remains controversial. For example, Utada () noted that the sediments comprising the Mizunami Group were derived from basement rocks such as granites, acidic intrusive rocks and andesitic rocks, based on their mineral compositions, including authigenic minerals. In contrast, Sato () investigated the ‘non‐opaque’ heavy mineral composition of the sandstones in the Mizunami Group.…”

Calcium and Magnesium‐bearing Sabugalite from the Tono Uranium Deposit, Central Japan

“…Kamiyama et al, 1966). The alteration around the deposits is quite peculiar, characterized by abundant zeolites of heulandite-clinoptilolite group (Utada, 1970(Utada, , 2003. Uranium occurs mainly as uraninite and coffinite in the primary ores (Komuro & Horikawa, 2004).…”

Hydrochemical disturbances measured in groundwater during the construction and operation of a large-scale underground facility in deep crystalline rock in Japan

“…Precipitation of Fe-oxy-hydroxides and dissolution textures of carbonate minerals are observed up to 60 m below ground level, suggesting low pH and oxidizing conditions in these shallow sedimentary rocks. The diagenetic clay mineral assemblage in the Mizunami Group is dominated by the smectite group (Utada 2003;Sasao et al 2006).…”