Studies on the Sulphur and Iron-Sulphides Deposits of the Matsuo Mine (2)

On the Montmorillonite-Zone and Mixed Layer Clay Minerals.

Abstract

A zone of montmorillonitic clay is newly found in the alteration zones underlying the Matsuo sulphur deposits. Thus the zonal distribution of minerals from the shallower to the deeper is as follows: an iron-sulphide zone, a sulphur zone, an opal-alunite zone, a montmorillonite zone, and chloritized andesite.
Pale green clays, characteristic to the montmorillonite zone, occur in the form of disseminated particles, veinlets, and irregular masses in the altered rocks derived from the chloritized andesite. From microscopic, electron-microscopic, differential thermal, X-ray and chemical studies, it is con-cluded that these clays consist of a large amount of montmorillonite, with some halloysite, kaolinite, hydrated halloysite and an unusual 23-24Å mixed layer mineral which is probably composed of regular interstratified lattices of montmorillonite and a 7Å kaolin mineral.
The ore-forming fluid might have been an ascending hydrothermal solution that was weakly alkaline or very weakly acid at the beginning of the mineralization and contained a large amount of gases. From the arrangement of alteration zones and ore bodies as well as from the supposed geologic environment surrounding the deposits at the time of mineralization, it may be concluded that the alteration zones and ore-bodies were formed by the interaction of ascending solutions and wadose water at an approximate depth of 200 to 300 meters from the surface.