The Yano Shôkôzan mine has been known for production of pyrophyllite as one of the important deposits in Japan.
The geology appeared in the environs of the mine composes, from the basement to the upper horizon, of the Paleozoic formation, andesitic rocks, dacitic rocks (mainly lava flow), pyroclastic dacitic rocks (containing spherulitic lava flows), shale and crystalline liparitic rocks, among which the volcanic series might have been of the late Mesozoic in age, accompanied with the granite-porphyry, diorite, and dikes of quartz porphyry and of andesite, intruding all of the former formations.
The deposits are inferred to have been produced restrictively in some horizon, that is, especially in the part, intercalated with spherulitic lava flow, of the uppermost pyroclastic dacitic formation. The related formations are recognized broadly subjected to hydrothermal alterations such as silicification, chloritization, pyritization and hematitization.
The pyrophyllite deposits included in the formation concerned bear a nearly lenticular form with dimension of 150～250m in length and 20～60m in width as well as a trend, parallel to the formation, of N45°～70°E in strike with dip of S10°～40° coinciding with that of the mother rocks, and, however, are observed often cut by faults after mineralization.
Both Takinotani and Nishiyama No. 3 pits are now being developed. Genetically, the ore deposits in question are believed to have been produced through a sort of metasomatism accentuated by the iron leaching as a result of the action of epithermal solution. In the ore bodies here dealt with, mineralic constituents are found arranged symmetrically and zonally from the central part towards the upper and lower sides between both walls in the following order:
Mainly pyrophyllitized zone (central part),
Pyrophyllitized zone bearing diaspore, corundum, boehmite and sericite,
Zone composing mainly of kaolinite and alunite,
Zone impregnated with iron minerals, and
Silicified zone and chloritized zone (outermost part).