Colloform Textures of Cupriferous Pyrite Ores in the Okuki and Nebutoyama Mines

Abstract

Cupriferous pyrite deposits of the Okuki and the Nebutoyama Mines are in a very weakly metamorphosed region of Shikoku, Japan, The ore-deposits of bedded or lenticular type are nearly concordant to the country rocks which are mostly volcanic and pyroclastic rocks.
The ores consist of pyrite and chalcopyrite with minor amounts of sphalerite, and galena. Magnetite, hematite and ilmenite are found locally near sulfide ores of the bedded type and also separately in other horizons. A layered texture of "lamina" type is well shown by the ores.
Various kinds of colloform texture of pyrite are commonly found in the ores. On the basis of the mineral assemblage, special well-preserved colloform textures in the ores include (1) angular colloform banding in some quartz-pyrite ores and (2) rounded colloform-pyrite in chalcopyrite-pyrite ores.
Most pyrite. crystals include very small grains of other minerals. This texture is called the "bleb" type. Such "blebs" are found also in euhedral crystals of pyrite. There are many intermediate textures between typical colloform and "bleb" textures. The "bleb" texture of pyrites from the two mines, therefore, seems to have been formed by the recrystallization of original fine-grained pyrite of colloform type.
The presence of such ores in a weakly metamorphosed region suggests formation of the original iron sulfide deposits of the two mines under low temperature conditions and later metamorphosism of the deposits to banded, cupriferous pyritic ores of "Kieslager-type".