The Kosaka Mine tried in vain for a considerable time to discover new ore deposits. By the end of world war II the Motoyama Deposits which had been the only workable deposit in Kosaka were almost exhausted. But, the enthusiastic geological investigation and drillings executed in recent years turned out a success. In the summer of 1959, the Uchinotai Deposits were discovered.
The Uchinotai Deposits are Kuroko or Black Ore deposits like those of the Motoyama, and consist. chiefly of four kinds of ores, that is, black ore (sphalerite-galena-chalcopyrite-tetrahedrite-barite), yellow ore (chalcopyrite-pyrite), siliceous ore (pyrite-chalcopyrite-quartz) and gypsum ore. The geological structure of a NNW-SSE trend, predominant in the vicinity of the Kosaka Mine, is considered. to control ore deposition. The ore deposits concordantly underlie the Akamori pumice tuff formations of Miocene, and their lower parts grade into the Motoyama breccia or the Baramori rhyolite which. are considered to have an intimate relation to the ore genesis.
At the end of 1961, the reserves of the Uchinotai Kuroko Deposits were estimated at about 9, 200, 000 tons of ore (Cu 2.48 %, Pb 1.3 %, Zn 4.2%), as the result of the 116 borings which were drilled at intervals of 50 meters. The fact shows that the Uchinotai has become one of the greatest copper ore bodies in Japan.