Geologic Structure of the Kurokô (Black Ore) Metalliferous Province in the Western Aizu District, Fukushima Prefecture

Abstract

The Western Aizu district is one of the important Kurokô (Black ore) metalliferous provinces in the green tuff region of Northeast Japan, and is widely covered by acidic volcanic rocks and its pyroclastics which are correlated to the Nishikurosawa and Onnagawa stages of the Middle Miocene.
The stratigraphic succession of the Miocene formations in this district can be divided into the following five formations in ascending order; Takisawagawa, Ôshio, Ogawazawa, Fuzawa and Matsusakatôge formations. The chief black ore deposits in this district, including some working mines such as Yokota, Tashiro and Kurosawa, occur syngenetically in pyroclastic rocks between the upper part of the Takisawagawa formation and the lower part of the Ôshio formation.
The general geologic structure of this district is characterized by the tectonic zone of uplifting and sinking belts with a trend of NW-SE. The ore deposits mentioned above, are distributed always along the boundaries between the two tectonic, belts. It is considered that the structure of NW-SE direction is closely related to fracture zones of the pre-Tertiary basement rocks and also with the Kurokô mineralization. Some considerations on the relation between altered zones due to the mineralization and the tectonic features were mainly presented in this paper.