Geoscientific studies in the Tono Mine. Annual report 2001

The current geosientific studies in the Tono Mine have been carried out since 1998. The main aims of the studies are to assess solute transport/retardation in Tertiary sedimentary rocks that host uranium ore bodies and to develop comprehensive techniques for the characterisation of faults, near-field geological environments around tunnels, etc in the sedimentary formation. A variety of results were obtained in the 2001 financial year in four different tasks involved in the studies. The long-term time-dependent behaviour of the rock mass was characterised by a couple of experimental studies involving in situ rock mass displacement measurements for over a decade and laboratory creep tests on sedimentary rock samples. In addition, temperature-dependent rock creep behaviour and a stress relaxation process were described. In the tunnel near-field geological environments, 3D distribution of pore water pressure and penetration of oxidised zone into the sedimentary formation were characterised. For the in situ characterisation of an unsaturated zone surrounding the tunnel, the applicability of a packer-type TDR (Time Domain Reflectometry) probe developed was confirmed. As part of studies on solute transport/retardation in the rock mass, the stability of reducing environment and uranium geochemistry in the host sedimentary formation were evaluated based upon the hydrogeochemical data sets developed in this task. Information on microbial diversity and abundance in groundwater and activity of sulphate-reducer was obtained. With respect to the Tsukiyoshi fault observed in the sedimentary formation, all information obtained were synthesised and a spatial distribution, geometrical features, the past movement and its function for groundwater flow were described in detail. Limitations and future approaches to the dating of a fault activity were discussed in this study. (author)