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Yokokawa river

Imai, Okaya city, Nagano pref., Japan


Metasomatic magmatic deposit

Hydrothermally altered serpentinite at the boundary between Yokokawagawa Metamorphic Rocks and early Miocene (20 Ma) Tanakoba Mudstone Member of Yokokawagawa Formation. The serpentinite was metasomatised by the middle Miocene (15 Ma) quartz diorite and the andesite dike. The quartz diorite is I-type (Magnetite-series), and the serpentinite is approximately 500 m apart from the quartz diorite and the andesite dike, heat sources. The serpentinite and the mudstone of the Tanakoba Mudstone Member have been subjected to silicification alteration by the metasomatism. This serpentinite boundary between Yokokawagawa Metamorphic Rocks and Yokokawagawa Formation is considered as a part of the Median Tectonic Line. Age of sedimentary rocks was determined by radiolarians.

Yokokawa river

Outcrop of metasomatised serpentinite. It is so yellow that local people call this as "Kinako" stone. "Kinako" is yellow soy bean powder seasoning rice cakes or Japanese sweets. The eastern side of this outcrop is Yokokawagawa Metamorphic Rocks, a part of Sanbagawa Metamorphic Rocks, and the western side is Miocene sediments. This serpentinite block is a part of the Medial Tectonic Line, more than 600 km long fault from Shikoku island to this outcrop. This is a very important outcrop from geological point of view.

Reported Minerals

  • Lizardite
  • Ni bearing pyrrhotite
  • Magnetite
  • Chromite
  • Kaemmererite
  • Aragonite
  • Dolomite
  • Pectolite

Mineral Assemblages

  • Lizardite - Ni bearing pyrrhotite - Magnetite
  • Lizardite - Chromite - Kaemmererite
  • Lizardite - Aragonite - Pectolite
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