Kosaka Mine

Synonym

Uchinotai Mine, Toki Mine

Occurrence

Kuroko massive sulfide deposit

Kuroko Massive Sulfide deposit in dacite and dacitic pyroclastics in the early Miocene (17 Ma) Nabekozawa Formation. The Nabekozawa Formation corresponds to the Hotakizawa Formation in Odate region. This deposit was formed in the early Miocene (16 Ma) Uchinotai Caldera, a part of the Hokuroku Caldera Complex. The Hokuroku Caldera is 20 km in diameter, and is a caldera complex composed of Miocene submarine volcanos and Miocene-Pliocene terrestrial volcanos. There are at least 13 small calderas and craters in the Hokuroku Caldera. Ages of volcanic rocks were determined by K-Ar radiometric dating.

Reported Minerals

• Copper
• Pyrite
• Chalcopyrite
• Sphalerite
• Galena
• Chalcocite
• Bornite
• Covellite
• Azurite
• Tetrahedrite
• Baryte
• Gypsum
• Calcite
• Quartz var. Amethyst

Mineral Assemblages

• Sphalerite - Galena - Pyrite - Chalcopyrite
• Baryte - Amethyst - Sphalerite

Details of the mine

Mainly silver and copper were mined. This mine became one of the biggest mines in Japan by the development of a new smelting method for Kuroko ore which had been impossible to use as a resource of copper. This mine hired 4,500 workers at the best days.

Produced metal

• Copper: 3,700 tons per year

History

• 1683: The deposit was found. Mined for copper.
• 1861: A farmer, Yosaku Kobayashi, mined silver.
• 1866: Nambu local government aquired this mine.
• 1870: Nambu local government began to run the mine.
• 1881: Western style smelter and Ag separator were built, and became the best Ag mine in Japan.
• 1884: Fujita group bought the mine.
• 1898: Engineers invented pyritic smelting method, and began to mine Kuroko ore as Cu resource.
• Present: Quit mining and dedicated to smelting of imported Cu ore.