Geology and Ore Deposits of the Shakanai Mine (1)

On the Mode of Occurrence of Kurokô (Black Ore) in the Daiichi Ore Deposit

Abstract

The Daiichi ore deposits of the Shakanai mine are of the socalled "Kurokô" type, having a zonal arrangement of Kurokô (black ore : sphalerite-galena-chalcopyrite-pyrite-barite), Oko (yellow ore : chalcopyrite-pyrite), siliceous ore (pyrite-chalcopyrite-quartz) and gypsum ore, from top to bottom. The hanging wall consists of pumice tuff and mudstone, while the foot wall rocks are rhyolite, rhyolitic tuff, lapilli tuff and tuff breccia. The Kurokô can be divided into several "unit-bodies", most of which are flat and concordant to the sedimentary rocks mentioned above. The Kurokô is classified into "massive Kurokô" and "breccia-like Kurokô" by the mode of occurrence. The breccia-like Kurokô is very similar to turbidite. It has sole marks on the foot wall, imbrication of ore fragments, and vertical sorting of ore fragments. Within each unit-body, the direction of current assumed by the imbrication of ore fragments is from the massive Kurokô toward the margin of the breccia-like Kurokô in agreement with isopach. It is suggested by the shape of ore fragments in breccia-like Kuroko that the fragments were considerably plastic when they were emplaced. Thus, it is reasonable to assume that the massive colloidal Kurokô was formed at the sea bottom. Some parts of the mass was broken down into ore turbidite forming breccia-like Kurokô.
To be more specific, all the unit-bodies are not on the same horizon, but thin mudstones containing fossils of radiolaria, are between them. In a plan view, the unit-bodies are arranged along the NE-SW direction. It is possible that the ore solution passed through the NE-SW fissures of the foot wall rhyolite, and that the mineralization of the Kuroko took place intermittently.