Samples were taken from the drill core, maximum depth 2002 m, in the granodiorite at Ashio, central Japan. The host granodiorite has many microfractures with width of less than 2 mm. The main alteration minerals in the host rock are illite, chlorite, calcite and fluorite.
Plagioclase is altered to illite and biotite to chlorite.
The microfractures are filled with quartz, illite, chlorite, laumontite or calcite. There are rare correlations between the alteration degree and the depth. The alteration degree increases toward the microfracture, so the hydrothermal solution along the fractures altered the nearby host rock. Although the measured present temperature of 2000 m deep ground water is 91 C, the alteration minerals formed at less than 100 C are not observed. Therefore the alteration minerals have not been formed by the present deep ground water.