Internal Structure of Black-Ore Deposit of the Yokota Mine

Abstract

The Yokota Mine is located at about 44km west of Aizu-wakamatsu City, Fukushima Prefecture. The Neogene Tertiary formation in this area are divided into four stratigraphic units as follows : 1) Sodeyama Formation, 2) Oshio Formation, 3) Takizawagawa Formation, and 4) Yokota rhyolite. The first three formations are characterized by rocks of the so-called "green tuff" facies. Black ore deposits of the Yokota mine comprise two types from the viewpoint of their internal structure. Banded black ore deposit occurs under black mudstone of the Oshio Formation, and massive disseminated or network deposit is found mainly in the Takizawagawa Formation and Yokota rhyolite. Zonal arrangement of these to types of ore deposits are shown below.
Oshio Formation
Banded black ore deposit
Massive disseminated or network deposit
Takizawagawa Form. and Yokota rhyolite
mudstone
soapstone (roof)
quartz-hematite zone
compact black ore zone
drusy black ore zone
chalcopyrite rich zone
siliceous yellow ore zone
siliceous ore of stockwork type zone
rhyolite and tuff breccia (bottom)
Motoyama mine lot, situated in the southeastern part of the Yokota mine, forms the center of silicified zone and banded black ore deposit developed along an anticlinal axis. Massive disseminated or network ore deposit are associated with many sand dykes in the anticilinal structure. By these facts, the hydrothermal solution might have ascended continuously along these sand dykes during Takizawagawa and earlier Oshio stages.