Tono Uranium Deposit

Itoigawa [3], Yusa et al. [4], Shikazono and Utada [5] and Sasao et al. [6] described the regional geology of the Tono District, which includes the Tono Uranium Deposit. Basement granitic rocks overlain by essentially flat-lying sedimentary formations are distinguishing features of the region (Fig. 1). 

Fig. 1. Overview of the geology of the Tono Uranium Deposit showing locations of selected boreholes, shafts and drifts in the Tono Mine.

Uranium geochemistry is controlled by the ambient redox environment, pH and concentrations of inorganic and organic complexing ligands [17, 18]. Constraints on Eh, pH and carbonate concentrations are interpreted below based on hydrochemical and mineralogical data obtained in field studies of the Tono Uranium Deposit and nearby vicinity. Iwatsuki et al. [19] and... 

Fig. 3. Conceptual model of redox environments in and around the Tono Uranium Deposit [19].

The aqueous-speciation and solubility behaviour of U in groundwaters associated with the Tono Uranium Deposit is evaluated in this section using the geochemical constraints discussed above. Supporting calculations were carried out using the Geochemist s Workbench software package [24] and a thermodynamic database described by Arthur et al. [20, 25]. 

This conclusion seems inconsistent with mineralogical observations indicating that uraninite is in fact present in the Tono Uranium Deposit (Section 2). A possible explanation for this apparent dichotomy is the experimental observation of Neck and Kim [28] that uraninite surfaces in contact with an aqueous phase at pH >3 may be coated with a thin layer of U02(am). Under such conditions, uraninite dissolution is effectively irreversible, and solubility is controlled by the amorphous surface layer. Additional experimental studies and observations of relevant natural systems are needed to test this hypothesis. 

Berke [14] attempted to confirm the presence of U02(am) in rock samples from the Lower Toki Lignite-bearing Formation using a variety of mineralogical techniques. The selected samples were not from the Tono Uranium Deposit, but they did have U concentrations above background levels. Berke [14] found that U deposition is associated with fine-grained, poorly crystalline matrix minerals, either as poorly crystalline or amorphous U minerals or sorbed onto other non-U phases such as Ti-oxides. A definite conclusion regarding the crystalline or amorphous nature of any U minerals in these samples could not be made due to the poor crystallinity of the matrix. 

Tono Uranium Deposit. Local variations in these parameters in time and space could lead to a cycle of dissolution and re-precipitation of U02(am), which may be consistent with the isotopic evidence among natural decay-series nuclides noted in Section 2.1 suggesting that U has been locally remobiUzed in the deposit during the past several hundred thousand years. Such mobilization/ re-precipitation of U would be most sensitive to local variations in Pco gy... 

Natural systems that may be analogous in certain respects to a geologic repository for nuclear wastes provide a useful basis for testing assumptions, models and concepts used in repository performance assessments. The present study demonstrates that maximum U concentrations in groundwaters of the Tono Uranium Deposit appear to be limited by the solubility of the amorphous, hydrous oxide, U02(am). This conclusion, drawn from observations of a geologic system that has evolved over long periods of time, supports the... 

An example of such a system is the sedimentary uranium deposit in Tono, Japan, studied by Nohara et al. The ground water moves through the area with a velocity of 0.001 to 1... 

Katayama N. Kubo K. and Hirono S. Genesis of uranium deposits of the Tono mine, Japan. Reference 1, 437-52. 

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