Geology and Ore Deposit of the Shirataki Mine, Kochi Prefecture (I)

Geologic structure around the main ore bodies

Abstract

The main ore deposit of the Shirataki Mine is a cupriferous pyritic bedded ore deposit. The mine is in the Sanbagawa crystalline schists of Central Shikoku.
The geology of this district consists mainly of alternating layers of spotted black schist and spotted epidote-hornblende schist accompanied by spotted piedmontite-quartz schist and spotted sandy schist. These rocks are characterized by the presence of albite porphyroblasts.
Usually the "phyllitic green schists" are developed around cupriferous pyritic ore bodies in this mine, and it has been said that these highly schistose rocks were altered from spotted epidote hornblende schists at the stage of fissure filling mineralization after the recrystallization of country rocks was nearly completed. The main constituent minerals of the "phyllitic green schists", however, are green hornblende, chlorite, sericite, epidote, albite, quartz and a carbonate mineral. Chloritization of green hornblende at the mineralization stage is not observed in these rocks, and green hornblende and chlorite show an equilibrium relationship during the regional metamorphism.
As a result of fabric analysis of the S-surface and lineation, it seems possible that the layers in this district were controlled by two main stages of foldidg during one cycle of kinematic movement, and the layer of the "phyllitic green schists" within ore bodies chiefly influenced by folding of the later stage.
It appears that the metamorphic facies of the rocks that include the "phyllitic green schists" corresponds to the epidote amphibolite facies.