Geology and Geologic Structure of the Katsuraoka Mining Ditrict, Hokkaido

Abstract

The Katsuraoka mine is situated in the southwestern part of Oshima Peninsula, Hokkaido. Pyrometasomatic magnetite ore deposits occur in the Palaeozoic formation (Katsuraoka formation).
The Katsuraoka formation is classified into 'five members based chiefly upon the remarkable change of the rock facies and yielded fossils such as Fusulinella bocki (MÖLLER) and Chaetetes sp. From these fossils the age of the formation is determined as Middle Carboniferous. The Katsuraoka formation is cut by many faults of NNW or NNE trend which are arranged in a fan shape.
The Miocene strata, covering the Paleozoic formations unconformably, are classified into two formations, the Asshinai formation (green tuff and andesite) and the Nakasuda formation (mud tone).
The ore deposits were formed in the lower chert member, especially in the area where limestone comes near contact with igneous rocks. It is clear that the localization of ore deposits is controlled by the faults and shear zones and intrusive masses.