On the Exploration of the Oshio Ore Deposit, Tashiro Mine

Abstract

The Tashiro Mine is situated in the so-called "Nishiaizu Kuroko Area", approximately 45km southwest of Aizu-wakamatsu city, Fukushima Prefecture. The ore deposits are divided into the Tashiro group and the Oshio group. The former consists of five orebodies, namely, Takamori No.1, No.2, Manakura North, Manakura and Shibozawa.
In gathering the previous geological data, the Oshio group were then chosen as the most promising exploration target and the following was established as the ore indicator;
the presence of
(1) Kuroko ore horizon,
(2) Structual weak zone with NW-SE trend,
(3) Ferruginous quartz zone, and
(4) Network and impregnated zone
Thus, the Oshio orebody (350m×150m×12m) and the Lower Oshio orebody (150m×100m×8m) have been discovered since 1960.
The ore deposits occur syngenetically in the pyroclastic rocks of the upper part of the Takizawa Formation. They underlie the alternation of perlitic tuff and mudstone of the Oshio Formation, which corresponds to the Nishikurosawa stage. Generally speaking, the ore deposit shows the zonal arrangement in the following order from the hangingwall toward the footwall; ferruginous quartz-, barite-, black ore-, pyrite-and network zone. However, it is characteristic that the gypsum occurs predominantly in the lower part of the Lower Oshio orebody.
With progress of the exploration, the following efforts have been made in addition to looking for the above ore indicators;
(1) To locate the altered zone (sericitisation-chloritisation zone)
(2) To clarify the sedimetation mechanism of the country rock.
(3) To study the distribution of the minor element.
(4) To explore the area of the mudstone hangingwall (Globorotalia-Hopkinsina zone).
It is expected that as the underground working is developed, the geological occurrence of the ore may be clarified and the more concrete indicators for the future exploration becomes available.