Geology and Mineralogy of the Kamiokoppe Limestone Mine, Hokkaido

Especially on Cause of the Hard Part of Hidariyama Orebody

Abstract

The Kamiokkope mine is one of the important limestone mines in Hokkaido. The production was begun in 1934. This mine consists of Migiyama, Hidariyama and Hommanosawa orebodies, all of which belong to the Kamui horizon of Jurassic Hidaka Group. These orebodies occur as irregular layer, lens or at some places massive body.
The Migiyama orebody is the first operated one and has been almost mined out. The Hidariyama orebody is the second, initiated in 1941. Recently a very hard part that is unsuitable for mining was discovered at the lower part of the orebody. It is now under investigation by drilling. The writer has collected various samples from this hard part and compared them with the other normal limestone.
Almost all of limestone at this mine show strong recrystallization texture; while recrystallization of the lower part is much stronger than the others. This was previously thought cause of the hardening. Field and microscopic observations indicate evidences of severe crushing at the lower part besides recrystallization, namely many sliding planes observed in field and crushed textures under the microscope. Recrystallization may not be only reason for the hardening.
Unsoluble residue test for rocks from the lower part revealed existence of abundant silica material. Electron microscopic observation also supports this result. No.13 specimen looks entirely pure limestone by naked eyes, yet it belongs to the hard type. Abundant microscopic chert was found in the matrix of the specimen. Very fine particles of chert or cherty sandstone seem to be the main reason for the hardening of limestone at the lower part of Hidariyama orebody.