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Okura Mine

Okura, Minami-Uonuma city, Niigata pref., Japan

Occurrence

Metasomatic magmatic deposit

Hydrothermally altered magmatic deposit formed by the early Paleocene (65 Ma) granite intruded in serpentinite block of the Mizunashigawa Metamorphic Rocks in the Jyoetsu belt. Granite is I-type (Magnetite-series), and the mine is approximately 1 km apart from the granite body. The protolish of the Mizunashigawa Metamorphic Rocks is the accretionary complex probably older than Permian. The Mizunashigawa Metamorphic Rocks have been subjected to Greenshist facies metamorphism in the Permian Cisuralian (290 Ma). The Mizunashigawa Metamorphic Rocks correspond to the Sangun-Renge belt in Southern Japan. Age of granite was determined by K-Ar radiometric dating.

Okura Mine

Outcrop of oxidation zone of the pyrrhotite-bearing serpentinite of the Mizunashigawa Metamorphic Rocks. The brown cliff of this photograph is composed of limonite only. There are also Pb-Zn moderate-T hydrothermal deposits. Pyrrhotite and gersdorffite may be formed by combining S in hydrothermal fluid with Fe and Ni released from peridotite during serpentinization process.

Reported Minerals

  • Pyrrhotite
  • Sphalerite
  • Pyrite
  • Gersdorffite
  • Limonite
  • Goethite
  • Hematite
  • Epidote
  • Phlogopite
  • Talc
  • Serpentine
  • Orthopyroxene(Bastite)
  • Hornblende

Mineral Assemblages

  • Serpentine - Orthopyroxene(Bastite) - Pyrrhotite
  • Serpentine - Gersdorffite
  • Goethite - Hematite - Quartz
  • Quartz - Talc - Pyrrhotite
  • Quartz - Sphalerite - Galena
  • Amphibolite(Host) - hornblende

History

  • 1920's: There is a record that this deposit was mined.
  • 1943: Mining limonite was started.
  • 1945-1951: Mining was most active.
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