The ores of the Yanahara Mine, which has hitherto been widely noted for its leading pro-duction of iron sulphide ore in Japan, are deposited only in the ‘Yanahara complex' correlative to a member of the upper Permian including a complicated alternation of acidic tuff, slate, rhyolite, dacite, and so forth.
Modes of their occurrence are classified into two types such as the massive, and the disseminated ores, of which the former involves mainly pyrite together with pyrrhotite and magnetite. The pyrite ore contained conformably in the Yanahara complex seems to have been formed through low-temperature hydro-thermal replacement accompanied subsequently with cataclastic, and then thermal metamorphisms. The magnetite, and pyrrhotite ores are found concentrated around pyrite ore bodies and along the fractures cutting across the latter or along quartz porphyry dike, showing no evidence for metamorphism in their textures. Most of these ores seem to have been derived from interaction of the pyrite ore with hydrothermal solution ascending along the fissures distributed around or in the pre-existing pyrite ore bodies, and the formation of some of the pyrrhotite ore related to dikes might have been ascribed to thermal metamorphism.

Thesis presented for the degree of D. Sc. of the Hiroshima University.