Ore Deposits of the Ohori Mine, Yamagata Prefecture, Japan

Abstract

The Ohori mine is located about 15 kilometers east of the city of Shinjo, Yamagata Prefecture, in a steep mountain, about 700 meters above sea level. This mountain is mainly formed of Miocene pyroclastic rocks and their lava flows, such as altered andesite and its tuff, green tuff breccia, rhyolitic tuff, rhyolitic breccia and muddy tuff, all of which are intruded by granitic rocks, plagiorhyolite and dolerite. Two types of ore deposits, a bedded type and a vein type, are found in the mine.
The bedded type, named the Kaninomata deposit, has been formed by metasomatic replacement of a calcareous bed inserted in rhyolitic breccia. The ore consists of sphalerite, chalcopyrite, pyrite and. galena intimately accompanied by wollastonite, bustamite, garnet, diopside, rhodonite, epidote, quartz, calcite and rhodochrosite. The chemical composition of the garnet is given as (Ca2.81 Mn0.26 Mg0.12 Fe''0.02)3.21 (Al.50 Fe'''0.45)1.95 (Si2.89 Ti0.04)2.93 O12.00, with a unit cell lattice constant of 11.885A. Also, the molecular formula of bustamite is (Ca0.44 Mn0.50 Fe0.03 Mg0.08)1.05 Si0.98 O3.00. The appearance of wollastonite, diopside, garnet and bustamite indicates that the early stage of mineralization of the Kaninomata deposit was under high temperature. Metasomatism under such high temperature is very rare in the Tertiary formations in Japan. The ore texture of the sulphide minerals produced in the late stages of mineralization, however, is simple, and unmixing textures are not observed.
The vein type deposit named the Nakanomata deposit, mainly fills fractures in dolerite that has intruded muddy tuff and rhyolitic tuff. The ore consists of chalcopyrite, sphalerite and pyrite with gangue minerals such as quartz, epidote, calcite, chlorite and sericite. It seems to belong to the common type of shallow-seated veins deposited in the Miocene formation in northeastern Japan.
The bedded Kaninomata deposit is often penetrated by veinlets composed of chalcopyrite, sphalerite, pyrite and quartz, which are thought to have been formed at same time as the Nakanomata vein deposit. Thus, the mineralization of the Kaninomata deposit seems to be earlier than that of the vein type, and probably was emplaced by ore-forming fluid derived from subvolcanic magma that formed the Tertiary granite and diorite in this area.