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Mount Obira

Moroyama city, Iruma county, Saitama pref., Japan

Occurrence

Oxidation zone of Pumpellyite-Aclinolite facies bedded manganese deposit

Chert-hosted bedded manganese deposit in a chert block of the Early Cretaceous (130 Ma) Kashiwagi unit, a member of the Northern Chichibu accretionary complex. The kashiwagi unit has been subjected to Pumpellyite-Actinolite 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) or Late Jurassic (145 Ma). Ages of sedimentary rocks were determined by radiolarians, and metamorphic age by K-Ar or Ar-Ar radiometric dating of muscovite.

Mount Obira

Outcrop of bedded chert of the Kashiwagi Unit around the adit. The bedding planes of bedded chert are clear, and its surface turns black by manganese dioxide. There is no clear record on this mine but it is said that this deposit was mined approximately 100 years ago and closed because of high Fe content.

Reported Minerals

  • Mn dioxide
  • Caryopilite
  • Greenalite
  • Rhodochrosite

Mineral Assemblages

  • Caryopilite - Greenalite - Rhodochrosite

History

  • late 19 century to early 20 century: Mined.
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