Exploration of the Matsumine Ore Deposit in the Hanaoka Mine

Abstract

The largest black ore deposit in Japan was discovered in the Hanaoka mine, Akita Pref., during June 1963 to May 1966. The ore confirmed by drilling amounts to about 30 million tons, and it was named the Matsumine Ore Deposit. The authors describe history of discovery, exploration and development for this deposit. The ore deposit was syngenetically emplaced within the intermediate to acidic volcanic rocks, especially in these pyroclastic rocks of the Miocene age. It consists successively of gypsum body, pyrite ore, copper-pyrite ore (yellow ore), and zinc-lead-copper-pyrite ore (black ore) from the lower to the upper. The deposit extends horizontally up to 500m from east to west and 800m from north to south. The depth is about 200m to 300m below the surface. It shows generally a shape of blanket.
Underneath the blanket ore body occurs siliceous ore, or white altered rhyolite and its breccia-tuff. Above the black ore lies a clay bed or ferruginous, quartz-clay bed which is in turn. overlaid by the mudstone layer (M2). These geological successions are closely similar to those of the main deposits of the Hanaoka and Kosaka mines.
Until 1955 the prospecting in these mines was done with the conception that black ore deposits were formed epigenetically hydrothermal replacement. The detailed geological investigations, however, brought about such a working hypothesis that the deposit might be formed by syngenetic and hydrothermal sedimentary processes. Thus, extensive drillings on the basis of this hypothesis has led to discovery of the "Matsumine Ore Deposit".
It is now expected that more ore bodies may be discovered similarly in this district by refinement of the working hypothesis and exploration technique.