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Nakamaru Mine

Minami-Nomaki, Shimonita city, Kanra county, Gunma pref., Japan


Moderately low-T hydrothermal deposit

Combination deposit of the moderate-T hydrothermal deposit and the low-T hydrothermal deposit associated with the late Miocene (6.0 Ma) andeste-dacite dikes intruded in andesitic tuff breccia of the Motojyuku Formation of the same age. The Motojyuku Formation is composed of andesitic tuff breccia and andesitic-dacitic dikes filling the Late Miocene-Pliocene Motojyuku Caldera. Approximately 1 km apart from the Ichinokaya Caldera Complex, the caldera complex of the Motojyuku caldera. The deposit is associated with sericitization and silicification alteration. Stibnite and barite were formed at the final stage of the depsit formation process. Ages of volcanic rocks were determined by K-Ar radiometric dating.

Reported Minerals

  • Stibnite
  • Stibiconite
  • Sphalerite
  • Kermesite?
  • Pyrite
  • Tetrahedrite
  • Galena
  • Anglesite
  • Baryte
  • Stilbite

Mineral Assemblages

  • Quartz - Stibnite - Stibiconite
  • Quartz - Sphalerite - Galena - Pyrite
  • Quartz - Sphalerite - Stibnite - Tetrahedrite
  • Quartz - Baryte
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