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Shiromaru Mine

Shiromaru, Okutama city, Nishi-Tama county, Tokyo, Japan


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). The main ore is braunite. This mine produces unique Ba and/or Sr bearing minerals. Harmotome veins are in the Early Jurassic (180 Ma) sandstone, a matrix of this accretionary complex near this manganese deposit. Ages of sedimentary rocks were determined by radiolarians, and metamorphic age by K-Ar or Ar-Ar radiometric dating of muscovite. Type locality of Tamaite and Tokyoite.

Shiromaru Mine

Outcrop of hematite- and cymrite-bearing chert of the Ohirayama Unit. The reddish-brown host is chert including fine grains of hematite and cymrite. The orange to yellow part of the white vein is marsturite. The white part of the vein is aggregate of celsian and quartz.

Shiromaru Mine

Outcrop of braunite deposit in the Ohirayama Unit. The black part at the lower half of this photograph is braunite. The reddish-brown part around the coin is chert including fine grained hematite and cymrite. There are a few white calcite veins. The horizontal red vein near the coin is composed of piemontite-Sr.

Reported Minerals

Mineral Assemblages

  • Quartz - Hematite - Cymrite - Celsian
  • Braunite - Celsian - Hyalophane - Banalsite - Marsturite - Ganophyllite - Tamaite
  • Braunite - Piemontite-Sr
  • Celsian - Sérandite - Copper
  • Quartz - Hematite - Celsian - Tokyoite
  • Sandstone(Host) - Harmotome


  • 1942: Started to mine.
  • 1945: Stop mining.
  • 1955: The mine was closed.
  • 1967: Shiromaru Dam was constructed and the mine sank in the Shiromaru lake.
  • SEP, 1974: The dam was drained by a rain storm.
  • 1998: The dam was drained for maintenance.
  • 2003: The dam was drained for maintenance.
  • 2007: The dam was drained for maintenance.
  • 2015: The dam was drained for maintenance.
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