Geology and Ore Deposits of the Shirataki Mine, Kochi Prefecture (II)

Mode of occurrence of ore deposits and characteristics of ores

Abstract

In the Shirataki mine the main ore deposit follows the plunge of an asymmetrical overturned anticline and is over 4, 000 m long. Several ore shoots are highly folded, and are nearly concordant with the structure of the enclosing schists. The various kinds of ores in the ore bodies include a massive or banded pyritic ore, impregnated ore, chalcopyrite-rich ore and apophyses. Many massive or banded pyritic ores show intense intraformational folding. The constituent minerals are pyrite, chalcopyrite, sphalerite, bornite, chalcocite, covellite, native silver, stromeyerite, tetrahedrite (?), an undetermined mineral (Rosagrau Kuferglanz?), hematite, magnetite, ilmenite, guartz, a carbonate mineral, green hornblende, albite, chlorite, garnet and epidote.
Chalcopyrite, sphalerite, quartz, a carbonate mineral and chlorite show "pressure shadow" features around some pyrite and magnetite crystals in impregnated ores. In a few apophyses ores stromeyerite is found among chalcocite and native silver, and a microtexture of bornite and chalcocite shows lamellar intergrowth and mutual boundaries that are probably of simultaneous origin in metamorphic differentiation process.
The field and microscopic features of this deposit are reasonably explained by the theory of sulphide syngenesis, later modified by regional metamorphism.