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Nogami-Shimogo, Nagatoro city, Chichibu county, Saitama pref., Japan




Greenschist facies serpentinite

Serpentinite in the Late Jurassic (150 Ma) to Early Cretaceous (130 Ma) accretionary complex subjected to Greenschist facies metamorphism as a part of the high-P Sambagawa metamorphic belt in the Late Cretaceous (70 Ma). This serpentinite block contains dolomite-talc veins. This serpentinite block is lens-shaped and is concordant with schistosity of the Sambagawa metamorphic rocks. The Sambagawa metamorphic rocks are considered that it has been near the ground surface since early Paleocene (65 Ma). The origin of this serpentinite block is considered as sedimentary serpentinite mélange from serpentinite seamounts at the fore-arc slope. Metamorphic age was determined by K-Ar radiometric dating of muscovite.


Outcrop of a white dolomite-talc vein in a serpentinite block associated with the Sambagawa metamorphic rocks. The massive white parts are dolomite, and the fibrous pale brown parts at the upper left are talc. Talc is at the boundary between dolomite and serpentinite, and its fibers are vertical to the vein wall. Gold has been recovered from this dolomite vein. It is considered that the dolomite-Talc veins were formed by the reaction of CO2 fluid with serpentine in fissures:

2Mg3Si2O5(OH)4 (Serpentine) + 3CO2 >> Mg3Si4O10(OH)2 (Talc) + 3MgCO3 (Magnesite component in Dolomite) + 3H2O

On the other hand, massive talc deposits in serpentinite block can be explained by the reaction of serpentine with SiO2.

Reported Minerals

  • Talc
  • Dolomite
  • Gold
  • Scheelite

Mineral Assemblages

  • Dolomite - Talc
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