Structural Features and their Relation to Metal Quantities in Ore Deposits of the Kishû Mine

Abstract

Abundant copper vein deposits of the mine occur in the Itaya formation composed of sandstone and intercalating shalebeds of the Miocene Miyai group. Principal ore mineral is chalcopyrite, and it is associated with pyrite, sphalerite, galena, quartz, calcite, chlorite, sericite, etc. Chloritization, silicification and sericitization of wall rocks are very distinct. Vein fissures of the mining district can be grouped in three sets of strike direction: N-S, E-W and NW-SE groups. Fissures of N-S and E-W group are shear fractures, while those of NW-SE group are tension cracks. The ore-bearing veins of the E-W and NW-SE groups have been mined chiefly for copper ore. Fissures of the N-S group contain a large amount of pyrite but are rather barren of chalcopyrite.
Depth zone that comprises the workable extent and ore shoots of each veins is remarkably limited within the sandstone-rich members of the Itaya formation.
Th0e Miyai group surrounding the main ore-bearing area has general strike of N20-4°E and dips 10-30°eastward. The main ore-bearing area is situated in a, dome within the northwestern part of a large basin of the. Miyai group. The long axis of the dome is orientated parallel to the general strike of the Miyai group. The ore-bearing area is divided into four districts by the difference of local structures: northeastern, northwestern, southeastern and southwestern districts. Structural and mineralogical features of ore-bearing fissures in each district reflect the difference of geologic structure. A notable difference is that the fissures in the northeastern district run at nearly right angles to the strike of the strata, while those in the southwestern district are parallel to them.
Four stages of mineralization can be recognized: the first stage (Cu-Py), the second (Pb-Zn), the third (Au-Ag) and the fourth (calcite).
Distribution of the metals deposited in the area are expressed as assay grade multiplied by the width of veins, and iso-metal quantity contours are illustrated on the maps. It seems possible that mineralization centers of each four stages overlap one after another in the southwestern district, while no evidence of coincidence is observed in the northeastern district. Fissures of the N-S group can be regarded as the channelways of mineralizing solutions.
Cu and Pb-Zn zones can be depicted successively outwards from centers where two sets of fissures of the N-S group and the E-W or NW-SE group cross each other. These features might be useful for further exploration.