Geologic Features and Classification of the Pyrophyllite Deposits in the Hokushin District, Central Japan

Abstract

The pyrophyllite deposits in the Hokushin district, Nagano Prefecture, can be divided into the next two groups. The first group deposits are distributed in the eastern area of the Chikuma river. Since they occur in or adjacent to diorite-porphyrite of the Miocene age, it is probable that they were formed by hydrothermal alteration closely related with the intrusion of the porphyrite. Usually, the deposits that occur in the porphyrite are chimney-like in shape and rather small in scale. Pyrophyllite, zunyite, diaspore, colundum, nacrite, dickite, etc. were found in the deposits of this group. But exceptionally, the pyrophyllite deposit of the Shinyo mine occurs in the tuffaceous sediments intruded by porphyrite branch dikes. And it was largely developed replacing mainly the permeable beds of the sediments. Pyrophyllite, sericite, kaolinite and diaspore were found in this deposit.
The second group deposits are distributed in the western area of the same river. They occur in volcanic rocks and pyroclastics of the Miocene age and occasionally in basaltic andesite lava of post-Miocene age. Because of the extensive distribution of altered rocks, it is reasonably considered that they were formed by exhalative hydrothermal alteration relating with areal volcanic activity. Some deposits of this group occur being accompanied by remarkably silicified rocks. Main constituent minerals of these deposits are pyrophyllite, quartz, diaspore, sericite and alunite.
As summarized in the table, there are distinct differences in geologic occurrence and mineral association between these two groups. Especially, the zonal arrangement from the pyrophyllitized zone to the weakly altered zone through the intermediate one is characteristic of the first group deposits, which belong to the higher temperature hydrothermal type. On the other hand, some of the second group deposits that belong to the exhalative hydrothermal type show the zonal arrangement from the silicified zone to the weakly altered zone through the pyrophyllitized one. These facts suggest that there were differences in chemical property of hydrothermal solution which took part in the formation of pyrophyllite deposits.