Abstract
The Motoyama ore deposits of the Kosaka mine belong to the 'Kuroko (Black ore)' type deposits. They are almost conformable with the bedding of the country rocks. The Kuroko (Black ore) consists mainly of sphalerite, galena, and barite. It occurs in the uppermost horizon of the Motoyama tuff breccia formation which is composed of alternation of sandy tuff and rhyolitic volcanic breccia. The Kuroko is classified into two types in the texture, i. e., stratified (or banded) and breccia-like (or blocky) ores. The stratified ore characterized by well-developed colloform texture in pyrite occurs in, or alternating with, sandy tuff. In the mode of occurrence and texture, the breccia-like ore is very similar to volcanic bomb which is found in the former and also in rhyolitic volcanic breccia.
Judging from these evidences, it seems that the condition of the deposition of Kuroko was substantially the same as that of the sedimentation of the Motoyama tuff breccia formation. Then, it is possible that the materials of the Motoyama ore deposits were syngenetically derived from submarine volcanism.
