Clay Veins and Clay Minerals in the Granitic Rocks in Hiroshima and Shimane Prefectures, Southwest Japan: Effect of the hydrothermal activities on the decomposition of the granitic rocks

This paper deals with the clay minerals found in the granitic rocks distributed in Hiroshima and Shimane Prefectures with special reference to the effects of hydro-thermal activities on the decomposition process of the granitic rocks. Many clay veins and hydrothermal clay deposits are commonly developed in the granitic rocks and their mode of occurrences were investigated in detail. The preferred orientations of the clay veins and microcracks found in the constituent minerals of granitic rocks were examined. The constituent clay minerals and their mineralogical characteristics of clay veins, clay deposits and alteration products of plagioclase in granitic rocks were investigated by means of X-ray diffraction, optical microscope, electron microscope (TEM and SEM), hydrogen isotope ratio and so on. The major results obtained are as follows.
1) The preferred orientations of microcracks within granitic rocks are very similar to those of clay veins which have been formed under the regional compression stress field.
2) The constituent clay minerals of clay veins, clay deposits and alteration products from plagioclase and their mineralogical characteristics are almost identical with each other.
3) Mineral sequence found in the vertical direction of a clay vein and altered granitic rocks resemble to that observed in some present geothermal areas.
4) Mineralogical characteristics of the clay minerals indicate that the clay minerals were formed by hydrothermal solution subsequent to the post granitic activity. The temperature of hydrothermal solution ranges about 50° - 300° C and originated from meteoric water. Some clay minerals seem to be directly precipitated from the hydro-themal solution.
Based on the results mentioned above, it is considered that granitic rocks distributed in the investigated areas have been strongly fractured and characterized by remarkable alteration to clay minerals at hydrothermal stage before the weathering stage.