TrekGEO > Mineral Trail > Kanto region > Gunma pref. > Tatarazawa

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Shimo-Hino, Fujioka city, Gunma pref., Japan

"No Trespassing" sign was erected. A permission is needed to observe minerals.


Hydrothermal deposit along fault

Minerals formed by low temperature hydrothermal fluid associated with the boundary fault between the Late Cretaceous (70 Ma) high-P Sambagawa metamorphic rocks and the early Miocene (17 Ma) Ushibuse Formation of the Tomioka Group. This fault is possibly a part of the Median Techtonic Line. The protolith of the Sambagawa metamorphic rocks is the Late Jurassic (150 Ma ) to the Early Cretaceous (130 Ma) accretionary complex. The Sambagawa metamorphic rocks is considered that it has been near the ground surface since early Paleocene (65 Ma). The Ushibuse Formation is composed of arkose sandstone, and conformably underlies the turbidite dominant Obata Formation. Ages of sedimentary rocks were determined by microfossils, and metamorphic age by K-Ar radiometric dating of muscovite.


Outcrop of arkose sandstone of the Ushibuse Formation. The grayish-white host is arkose sandstone subjected to low temperature metasomatism. The white veins are dolomite associated with dawsonite, alumohydrocalcite, and nordstrandite. The translucent white parts at the center of the vein is dolomite, and the opaque white parts at the boundary between sandstone and the vein are dawsonite. The brown grains in the arkose sandstone are siderite.

Reported Minerals

  • Dawsonite
  • Alumohydrocalcite
  • Nordstrandite
  • Dolomite
  • Siderite
  • Pyrite

Mineral Assemblages

  • Dolomite - Dawsonite - Alumohydrocalcite
  • Dolomite - Siderite
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