HYDROTHERMAL ROCK ALTERATION OF THE TAMAGAWA WELDED TUFF (PART 1)

The rock alteration of this area is classified into regional and local ones. The regional alteration consists of montmorillonite and chlorite zones. Considering the distribution and associated mineral paragenesis, it is assumed that the montmorillonite zone is not directly related to geothermal activity in its genesis, but diagenetic condition, however, the chlorite zone was formed under the alkaline hydrothermal condition. The present writers suggest a new method for alteration survey using _??_2θ value indicating the abundance of expandable layer in the interstratified mineral of sericite and montmorillonite. By this method, the chlorite zone is subdivided into five subzones according to the iso-_??_2θ line as 0, 1, 2 and 3 Alteration minerals such as pyrophylite, kaolin and alunite cccur locally under the acidic hydrothermal condition related to recent geothermal activity.