Okutama Mine

Hikawa, Okutama city, Nishi-tama county, Tokyo, Japan

Occurrence

High-T zeolite facies bedded manganese deposit

Chert-hosted bedded manganese deposit in a chert block of the Early Jurassic (180 Ma) Ohirayama unit, a member of the Southern Chichibu accretionary complex. The Ohirayama unit has been subjected to weak Prehnite-Pumpellyite to Zeolite facies metamorphism as a low grade part of the Sambagawa metamorphic belt in the Late Cretaceous (100 Ma). The age of chert is Permian Guadalupian (270 Ma) to Early Jurassic (200 Ma). Ages of sedimentary rocks were determined by radiolarians, and metamorphic age by K-Ar or Ar-Ar radiometric dating of muscovite.

Outcrop of bedded chert of the Ohirayama Unit. Chert beds are parallel to the ground in front of the adit, and vertical on the adit wall. The black zone at the left part of this photograph is mudstone, a matrix of the Ohirayama Unit. The deposit is in bedded chert near the boundary between the chert block and the mudstone matrix. As present manganese nodules and crusts cover the ocean floor, the left side of this chert block might be the higher stratigraphic level.

Reported Minerals

Hausmannite
Rhodochrosite
Jacobsite
Manganosite
Caryopilite
Rhodochrosite
Quartz
Tephroite
Rhodonite
Pyroxmangite
Ganophyllite
Pyrolusite
Cryptomelane
Ice/Water

Mineral Assemblages

Hausmannite - Rhodochrosite - Jacobsite - Manganosite
Caryopilite - Quartz - Rhodochrosite
Rhodochrosite - Tephroite
Rhodonite - Pyroxmangite - Rhodochrosite - Ganophyllite

History

1939: Started to mine.
1943: The mine was closed.

Localities

Kusama Mine (Mudstone-hosted, Miocene)
Kiyokawa Mine (Chichibu belt)
Okutama Mine (Chichibu belt)
Shiromaru Mine (Braunite, Chichibu belt)

Related Occurrences

Other Occurrences