The massive gypsum deposits of the Yonaihata and the Iwasawa mines occure in the sedimentary rocks of middle Miocene age. At the Yonaihata mine, the mudstone of the wall rocks are partly altered into Mg-chlorite zones by hydrothermal solutions. In this paper, the dolomite from the both mines are examined by chemical analysis, and the relation between dolomitization and Mg-chloritization has been stressed. And then the characteristics of hydrothermal dolomie and typical sedimentary dolomite are ascertained from the results of differential thermal and spectrographic analyses.