Topographical examination and genesis of clay veins found in the Kumogi granite mass in Shimane Prefecture,southwest Japan.

Abstract

On clay minerals found in veins developed in the Kumogi granite mass in the vicinity of Hamada city, Shimane Prefecture, the modes of occurrence and mineralogy such as polytypes and crystal forms were investigated. The clay veins are distinctly divided into three types (types I, II and III) based on the mode of occurrence. The clay veins of type I are formed along nearly vertical faults or fissures, type II occurs as replacement products in vein-form and type III are formed along small fissures or joints. The clay veins of types I and II are composed of mainly mica clay mineral and type III are composed of mainly kaolin minerals. The polytypes of mica clay mineral of type I are identified to be 1Md, IM and 2M, while that of type II is only 2M. The modes of occurrence and mineralogy of the clay minerals of types I and III show almost the same mineralogical characteristics to those of clay veins found in Hiroshima granitic rocks (Kitagawa and Kakitani, 1978a, b, c), while the type II is characteristic in the Kumogi granite mass. Topographical examinations of mica clay (type I) and kaolin (type III) veins show that mica clay veins are mainly distributed in the margin of the granite mass which is situated at the lower level, while kaolin veins in the middle part of the mass is situated at relatively higher level. More detail examination of type I indicats that 2M mica clay minerals are found in the margin of the mass and relatively in the lower than lMd mica clay minerals. It may be indicated from these results and study of Imaoka et al. (1977) and Ishihara et al. (1980), mica clay minerals in type I veins were formed by hydrothermal solution which interacted with meteoric ground water which intruded from the margin of the granite mass.