<Original Papers>Braunite from the Ainai Mine, Akita Prefecture

The Daikoku and Kanayamazawa ore deposits of the Ainai mine, Akita Prefecture, belong to the black ore type deposit, which are accompanied partially with a small quantity of manganese ores. In the Daikoku ore deposit, the manganese ores occur as lenses or veins in the sericite-montmorillonite-quartz zone which has developed in the upper most horizon of the Okuzu formation of Miocene age. In the latter, however, the occurrence of the manganese ores cannot be observed owing to the collapse of the gallery. The manganese ore minerals consist of braunite, manganite and pyrolusite. It is considered that manganite and pyrolusite have resulted from the alteration of braunite originally formed from hydrothermal solution. The manganese dioxide ores of this mine are conspicuously different from the bedded manganese dioxide ores of this district in the point of the mineral composition of ore, wall rock alteration and the stratigraphic horizon in which ore deposits occur, so that it is necessary to consider the difference between the both manganese ores in their genesis