Smectite zone
Chemical Process (Alteration) - Open system under ground (Neutral metasomatism)
Correlated: Low-T zeolite facies
Synonym
Bentonite Deposit, Argillized zone (Smectite)
Index Minerals
Smectite
Required Geological Setting
Occurrence
Metasomatic zone formed by neutral hydrothermal fluid of 50-100 degree associated with Andesitic magma activities. This alteration is called smectization and is a part of argillization. Smectite zone may be associated with Hydrothermal Au-Ag deposits, Hot spring deposits or Hydrothermal deposits along faults. The water related to the alteration reaction of the smectite zone is meteoric water and/or magmatic water, and the water/rock ratio is more than a few tens times. It means that minerals were formed by reactions of the open system. On the other hand, the water related to the diagenesis is interstitial water in sediments, and the water/rock ratio is less than ten times. Minerals by diagenesis were formed by reactions of the closed system.

Distribution of Smectite zones described in this site. Yellow lines are volcanic fronts.
Change of mineral assemblages
- [ Appear ] Smectite
Mineral Assemblages
- [ Felsic Rocks ] Smectite, Montmorillonite, (Calcite)
- [ Mafic Rocks ] Smectite, Saponite, (Calcite)
Localities
- Bentenzaki (Montmorillonite deposit)
- Fukaura (Lava, Celadonite)
Related Occurrences
- Halloysite zone (Alunite zone, Argillized zone)
- Kaolinite zone (Alunite zone, Argillized zone)
- Minamiite-Alunite zone (Alunite zone)
- Pyrophyllite zone
- Smectite zone (Argillized zone)
- Interstratified mineral zone (Argillized zone)
- Illite-Chlorite zone (Propylite zone)
- Epidote-Actinolite zone
- Stilbite-Mordenite zone (Zeolite zone)
- Heulandite-Analcime zone (Zeolite zone)
- Laumontite-Albite zone (Zeolite z., Propylite z.)
- Wairakite-Albite zone (Zeolite zone)