The Minamishiraoi kuroko-type massive barite deposit is located at Morino, Shiraoi Town, Hokkaido. The deposit occurs in Miocene volcanics, consisting of upper barite ore body, lower barite-bearing silicified breccia and footwall clay zone.
X-ray diffraction analyses, DTA-TG and microscopic observation of the clay samples revealed that the footwall clay zone consists of kaolinite, nacrite, dickite, montmorillonite, illite/montmorillonite mixed layer minerals and chlorite/montmorillonite mixed layer minerals together with sulfide minerals such as pyrite, marcasite, chalcopyrite, sphalerite, tetrahedrite and galena.
The clay mineral-sulfide associations are subdivided into two zones in spatial distribution. That is, nacrite, dickite, illite/montmorillonite mixed layer minerals, which are associated with pyrite and chalcopyrite, are distributed in the central zone below the barite-bearing silicified breccia, while kaolinite and montmorillonite, which are associated with marcasite and sphalerite, occur in the peripheral zone.
Judging from the mode of distribution of the clay minerals and their stability data in literatures, it is estimated that the central clay zone below the barite-bearing silicified breccia formed at a temperature range of 100 to 200°C, and the peripheral zone at temperatures lower than 100°C.