Geology and Mineralization of the Akagane Mine

Abstract

The rocks in the vicinity of the Akagane mine consist predominantly of pyroclastic rocks (tuff, tuff-breccia and lavas), sedimentary rocks (slate and limestone), and igneous rocks (quartz porphyry, porphyrite, gabbro, granite and granodiorite). These pyroclastic and sedimentary rocks, belonging to the Yonesato and Siba formations of Carboniferous System, have suffered thermal metamorphism by Hitokabe granodiorite. These rocks strike NNS, dip about 30-60° eastward, and are strongly folded. The ore bodies are formed by replacement on limestone and volcanic pyroclastic rocks close to and along the contact of quartz porphyry or gabbro. They are associated with skarn minerals such as brown garnet, epidote, amphyboles, diopside, plagioclase, etc. The deposits are composed predominantly of pyrrhotite, magnetite and chalcopyrite, with subordinate amounts of pyrite, sphalerite, arsenopyrite and cubanite. Geologic history of formation of the ore deposits seems to be as follows: 1. sedimentation of rocks of Carboniferous System, 2. folding and faulting of the System, 3. intrusion of dikes such as quartz porphyry and gabbro, 4. skarnization and metallic mineralization accompanied by minor folding, 5. intrusion of granite, and 6. faulting.