Volume 9 (1959) Issue 38 Pages 360-368
The Inakuraishi Mine is located in the central part of the Shakotan Peninsula, southwestern Hokkaido, and works manganese ore. A Neogene Tertiary formation is extensively developed around the-mine. The formation is divided into four groups : tuff breccia, propylite, propylitic agglomerate, and tuff interbedded with some thin shale beds.
The ore deposits belong to an epithermal fissure-filling vein. The vein systems are summarized as follows:
1) N55°W, 75°N……Mansei-hi
2) N75°E, 70°S……Kinsei-hi
3) N80°E, 75°N or 75° S……Many branch veins of Mansei-hi and Okuinakuraishi-hi
The Mansei-hi and Kinsei-hi consist of several veins of different magnitude, the former being the main vein of the mine. Judging from the modes of occurrence and natures of veins, it is concluded. that the fissures filled by veins of the Mansei-hi are shear fractures, and those filled by veins of the Kinsei-hi are tension fractures, formed by a N 75°E stress acting on the mining area.
This ore deposits consist mainly of rhodochrosite with some sulphide minerals. The ore is classified into the following two types:
(a) deep rose-colored rhodochrosite ore
(b) light rose-colored rhodochrosite ore
Ore of the type (a) contains about 0.1—0.2% Fe whereas that of type (b) contains 1.5% or more Fe. Such difference in the ores is due to the chemical composition of the contained rhodochrosite. The light rose-colored rhodochrosite is of an earlier stage than the deep rose-colored rhodochrosite.
The wall rock alteration is characterized by chloritization, silicification, argillization, pyritization, sericitization, albitization, and carbonatization. Argillization is observed in the extremity of the vein.
Three altered zones can be distinguished in the wall rock of the Kinsei-hi on the basis of mineralogical assemblages, as follows:
1) Quartz-sericite zone, (2 meters on both sides of the vein)
2) Sericite-quartz zone, (from 2-30 meters on both sides of the vein)
3) Chlorite-quartz-albite zone, (beyond 30 meters on both sides of the vein)