It is pointed out by KuMITA et al. (1982) in the Shakanai ore deposits and KANBARA et al. (1984) in the Hanawa that the intrusion of post-ore dacites caused the deformation of the ore deposits. The present paper is to make clear the mechanism of the deformation of the Shakanai ore deposits, based mainly on the observation of the mode of occurrence of the ore deposits and their host rocks. The conclusions of this study are as follows;
(1) The deformations, such as folding and displacement, are observed in the ore deposits which overlie the convex surface of the gypsum zone. These deformations may be considered as a result of the upward expansion of the initial thick portion of the gypsum zone due to its volume increase by hydration of anhydrite.
(2) Intrusive dacites and hyaloclastic dacite lavas (MMD), occurring at the hanging wall and/or at the side wall of ore deposits, seem to have the ore deposits deformed in manners as displaced, steeply dipped, inverted, and folded. This activities are assumed to have taken place through the late Shakanai stage and the Sainokami stage.
(3) The size of MMD bodies around the No.11 ore body is maximum about 300m to 500m in diameter in plan. Seeing in cross-section, the shape of MMD body varies from flat one with low aspect ratio to ellipsoid one with high aspect ratio.
(4) MMD shows characteristic structure and texture according to the shape of its body. The flat body has a lamina-like structure concordant with its elongation. On the other hand, the ellipsoid one has a massive and breccia-like structure except on its marginal portion.
