Mineralogical and Chemical Studies of Clay Veins in Granitic Rocks of Chugoku District, Southwest Japan

This paper deals with the chemical composition of clay veins found in granitic rocks distributed in Chugoku district with special reference to constituent clay mineral and chemical composition. Clay minerals were collected from clay veins developed in granitic rocks in Rokko, Hiroshima, Mitoya, Izuha and Kumogi districts. Granitic rocks distributed in Rokko and Hiroshima districts belong to the ilmenite-series, whereas those in Mitoya, Izuha and Kumogi districts belong to magnetite-series. The constituent clay minerals of clay veins were identified by means of X-ray powder diffraction. The element contents of clay minerals were investigated by means of atomic absorption spectro-photometry, and the values obtained were examined from mineralogical view points. The results obtained are as follows.
1) Clay veins are generally composed of mainly mica clay mineral, smectite and kaolin minerals with a small amount of interstratified mineral of mica clay mineral and smectite. A few veins are composed of zeolite, especially laumontite.
2) Clay veins of Rokko and Hiroshima districts are rich in kaolin minerals and smectite, and those of Mitoya, Izuha and Kumogi districts are rich in mica clay mineral.
3) Among the major elements, content of Mn in clay veins is high in Izuha and Kumogi districts.
4) Na,Ca and K contents depend on the constituent clay minerals. That is, content of K depends on the amount of mica clay mineral, whereas content of Ca depends on that of smectite.
5) Contents of minor elements such as Li, Cu and Pb, vary with respective districts. Li content is high in clay veins of Rokko and Hiroshima districts. In Mitoya, Izuha and Kumogi districts, Li content is very low. This fact may represent the difference of the host granite, ilmenite-series and magnetite-series. Cu and Pb are abundant in clay veins of Hiroshima and Izuha districts.
Based on the above results, chemical characteristics of clay veins in respective district and mineralogical and chemical characteristics of constituent clay minerals were discussed in relation to the origin of hydrothermal activities of the respective post granitic activities.