ABSTRACT

The mineralogy of fracture system and the chemistry of groundwater have been studied in deep boreholes drilled in granitic rock to understand the chemical evolution of groundwater and redox conditions within the deep underground which preserved the Tono uranium deposit. Geological studies revealed that fracture system within the granitic rock can be classified into intact zones, moderately fractured zones and intensely fractured zones, using the degree of fracturing. Fracture fillings within the fractured zones consist mainly of clay minerals such as montmorillonite. Hydrogen and oxygen isotopic data show that young, shallow groundwater has been traveled into a depth of at least approximately 186 m. The precipitation of iron hydroxide provides an evidence to support the idea that the oxidizing surface water has traveled down to a depth of approximately 130 m from the surface. Groundwater is rich in Na⁺, Ca²⁺ and HCO₃^- near the surface and rich in Na⁺ and HCO₃^- in depth. Mineral saturation indices suggest that the dissolution of plagioclase and calcite is more significant in the shallower part than in the deeper part of the groundwater system. Na⁺ concentration of the groundwaters increases with depth and Ca²⁺, Mg²⁺ concentrations decrease at a depth of 200 m and over. Below this depth, the increase in Na⁺ concentrations is nearly equivalent to the decrease in combined Ca²⁺ and Mg²⁺ concentrations, suggesting that a Na-(Ca²⁺, Mg²⁺) ion exchange reaction occurs between the montmorillonite and groundwater. The chemical evolution of groundwater in the granitic rock is controlled mainly by the dissolution of plagioclase and calcite, and by the ion exchange reaction between montmorillonite in the fractured zones and groundwater. In-situ Eh-value of groundwater in the fractured zones is approximately 0 mV at a depth of approximately 180 m. This value is on the boundary between the Fe(OH)₃, and Fe²⁺ stability field on the Eh-pH diagram for the Fe-S-H-O system. The redox conditions of deep groundwaters in the fracture system are probably controlled by Fe²⁺ and Fe³⁺ in the granitic rocks. On the other hand, the amount of pyrite in the granite is limited. Pyrite crystals that maintain their original form are observed in the intact zones, whereas the pyrite with dissolution texture is found in the fractured zones. This can be explained that the minerals in the fractured zones have reacted with oxidized groundwater over a long period of time. There is a possibility that the redox condition of groundwater is controlled by aqueous reduced sulfur species and pyrite, and that the Eh value of groundwaters may be around -300 mV.