Yoshikawa Mine

Occurrence

Oxidation zone of serpentinite

Oxidation zone of the Ni deposit in serpentinite of the Mikabu Greenrocks associated with the Middle Jurassic (170 Ma) Kamiyoshida unit or the Early Cretaceous (120 Ma) Kashiwagi unit, members of the Northern Chichibu accretionary complex. Various hydrocarbonate minerals were formed by oxidation by meteoric water. The Mikabu Greenrocks is probably a part of the Middle Jurassic to Early Cretaceous accretionary complex cropped out between the Nothern Chichibu acretional complex and the Sambagawa metamorphic rocks. The Mikabu Greenrocks are composed of Carboniferous to Triassic Basalt lava, basaltic tuff breccia, basaltic tuff, and gabbro, and has been subjected to Pumpellyite-Actinolite facies metamorphism as a low grade part of the Sambagawa metamorphic belt in the Late Cretaceous (100 Ma). The protolith of the Mikabu Greenrocks is considered as mafic volcanic rocks and ultramafic rocks of oceanic plateaux and seamounts. Serpentinite is possibly serpentinite seamount and/or sedimentary serpentinite at the fore-arc slope. Ages of sedimentary rocks were determined by conodont, and metamorphic age by K-Ar or Ar-Ar radiometric dating of muscovite.

Reported Minerals

• Aragonite
• Artinite
• Hydromagnesite
• Dypingite
• Nesquehonite
• Pyroaurite
• Pentlandite
• Heazlewoodite
• Magnetite

Mineral Assemblages

• Serpentine - Aragonite
• Serpentine - Artinite
• Serpentine - Hydromagnesite - Nesquehonite
• Magnetite - Pentlandite - Heazlewoodite