The Minamishiraoi deposits, one of the kuroko-type barite deposits located in the southwestern Hokkaido, are embedded in volcanic and pyroclastic rocks of Tertiary age, and are composed of hanging wall, barite are body, silicified zone, argillized zone and weakly argillized zone. The kaolin clay occurring widely and abundantly in the argillized zone, is being exploited. This paper describes the clay mineral composition of the deposits, and some features of clay minerals.
The clay mineral occurring in the hanging wall, is smectite, and halloysite. In the foot wall as silicified zone, argillized zone and weakly argillized zone, it is dickite, nacrite, kaolinite, sericite mineral (sericite and interstratified sericite/smectite), smectite, Al-chlorite and interstratified chlorite/smectite.
Kaolin and sericite minerals, principal constituent clay minerals, show characteristic spatial distributions in the foot wall. Dickite and nacrite occur in the marginal part of the silicified zone and in the argillized zone. On the other hand, kaolinite occurs appoximately in the central or marginal part of the argillized zone. Sericite mineral containing a small amount of expandable layers, occurs in and around the silicified zone. On the other hand, the mineral with a large amount of them, occurs in the argillized zone, approximately.
Smectite, one of the principal constituent minerals, is dioctahedral type, and show compositional difference between the mineral of the hanging wall and that of the foot wall. The former is Fe-rich, and the latter Al-rich. Chlorite minerals are also dioctahedral type, suggesting to be sudoite and tosudite.
Considering the clay mineral composition of the deposits and the kaolin-sericite-solution equilibrium, it may be inferred that the formation of kaolin clay is attributable to the acidic hydrothermal process of low-temperature, which was dominant during the reletively later mineralization of the Minamishiraoi deposits.