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Ryushima Mine

Azumi, Matsumoto city, Nagano pref., Japan


Moderate-T hydrothermal deposit

Mn-dominant moderate temperature hydrothermal deposit formed by the Late Cretaceous (69 Ma) Nagawa Granite in the Early Cretaceous (140 Ma) Shimashima Unit. The Shimashima Unit corresponds to the Kanayama Unit of the Tamba-Mino-Ashio accretionary complex. The Nagawa Granite is S-type (Ilmenite-series). Approximately 2.5 km apart from the Nagawa Granite, the heat source. Ages of sedimentary rocks were determined by radiolarians, and granite by K-Ar radiometric dating.

Ryushima Mine

Outcrop of a kutnahorite vein network in slate of the Shimashima Unit. The pale pink network is veins of massive kutnahorite, and euhedral crystals are in open spaces of the vein. Black host is silicified slate. Brownish-black crusts on the outcrop surface at the upper left and the upper right are pyrolusite formed by weathering of kutnahorite.

Reported Minerals

  • Kutnohorite
  • Pyrolusite
  • Pyrite
  • Sphalerite
  • Galena

Mineral Assemblages

  • Kutnohorite - Pyrolusite
  • Kutnohorite - Sphalerite - Galena - Pyrite


  • 1899: Mr. Fukazawa, a local person, found the deposit.
  • 1921: Mr. Amano from Osaka started to mine manganese ore.
  • 1927: Restarted to mine 10-50 t/month ore.
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