Geology and Ore Deposits of the Ishizu Mine, Gunma Prefecture

Abstract

The geology and ore deposits of the Ishizu mining district are described with special reference to the form and structural control of the ore deposits.
The rocks occurring in the district are chiefly andestic lavas and pyroclastic rocks ejected from the Kusatsu-Shirane volcano. These rocks can be generally classified into five groups, namely, from older to younger, Matsuozawa, Aoba, Komenasi, Motoshirane, and Sessyo groups. The first two are found in the older caldera and the others are in the younger caldera which formed above the former.
The ore deposits of the mine are found mainly in the Matsuozawa group and in the lower part of the Aoba group. The mineralization of sulfur ore is closely associated with the ejection of these lava groups, i.e., after the formation of the older caldera the ejection of lavas of the Matsuozawa group took place and the ore solution ascended through the weak part of the bottom of the caldera, and the mineralization became active on account of the ejection of the lavas of the Aoba group, and then became weak rapidly but not simultaneously with the formation of the younger caldera. And, after the ejec-tion of the lavas of the Motoshirane group which flowed into the younger caldera, there occurred weak fumarolic activities which, however, ceased before the ejection of the lavas of the Sessyo group.
The ore deposits show a bedded form and belong to the so-called Ishizu-Numajiri type. In the upper part of the ore deposits, the ore gradates into wall rocks with dissemination, but in the lower part the ore forms pipes concentrated in the small centers along the weak zone.
The ore deposits are controlled by the following structural factors ; a) fissure groups trending N 10°W and N 40°W, b) distribution and form of the Middle lava, which is a member lying in the center of the succession of the Matsuozawa group and seemingly acting as a cap rock, c) two beds of tuff-breccia trending N 40-50°E, which are the members of the same group and apparently playing an important role in diffusion of the ore solution.
Prospecting of the ore deposit was carried out based on the above-mentioned structral factors. Great attention was paid to the results of the investigations by means of boring, with special interest in the above factors, because of the charactor of the ore deposits under cover.