| 초록 |
Alteration of deep granitic rock has been recently focused on in order to evaluate the long-term geological and geochemical changes for LPG storage, CO 2 storage and high-level and low-level radioactive waste disposal, etc. In this paper, as an example, alteration in deep granitic rock was investigated using drilling core material of Toki granitic rock distributed in Gifu pref., central Japan. Detailed mineralogical and geochemical analyses show the alteration degree is correlated with the high-density fracture zone and sealed shear-fractures but not the depth. Geochemically, Ca, Fe and Mg are also decreased at the fracture zones. Mineralogically, sericite, chlorite, calcite and iron hydroxide are observed as secondary formed minerals. Biotite is replaced by chlorite along the cleavage. Plagioclase is altered to sericite in Ca-rich central zones of the crystals. Calcite usually forms the veins filling in shear-microfractures. These features can be explained by a series of hydrothermal alteration as follows: 1) alteration of biotite to chlorite yields K-rich water; 2) it enhances plagioclase replacement by sericite; and 3) it produces Ca-rich water leading to the precipitation of calcite in microfractures. Iron hydroxide is precipitated finally in microfractures and grain boundaries along groundwater-conducting fractures near the depth of 140 - 230 m, probably due to the oxidized rainwater penetration. Consequently, it is inferred that different stages of the granite alteration is related to fracturing after the solidification. Such kind of rock -water interaction is assumed to be quite dominant in the deep granite alteration process with fracture-flow system. |
