Structural and Morphological Features of Ore Deposit of the Nishinokawa Mine, Ehime Prefecture

Abstract

The ore deposit of the Nishinokawa Mine is of a bedded cupriferous pyritic type (so-called "Kieslager type"), occurring conformably or concordantly in the green schist formation in the central part of the Sambagawa metamorphic terrain of Shikoku Island. There are two types of ore shoots having different plunges. The one in the upper to middle part of the deposit plunges to S 80° E (25°), while the other in the middle to lower part abruptly changes its trend to S 45° W (25°). Then, two differently orientated goaves overlapping each other can be seen in some places in the underground map. The structural features mentioned-above are remarkably distinctive, because, generally speaking, most Kieslagers in the terrain have simple ore shoots elongated in one direction.
The writer consider that the peculiar structure has been formed by the "junction" of "primary" ore shoot having an E-W trend and later "abnormal folding zone" having a NE-SW trend along which "secondary" ore shoot occurred. Mechanism of the formation of the "secondary" ore shoot or bonanza can be schematically explained by folding up of original ore bed in the folding zone.
It has been described that shape of ore shoot of Kieslager is "controlled" by two or more sets of folding systems, but in most cases the influence of folding system of second or third order is rather slight on formation of ore shoot. In the case of the Nishinokawa Mine, howeuer, the influence of second stage folding was so remarkable that not only the "primary" ore shoot was formed by earlier folding, but also the "secondary" ore shoot along the later "abnormal folding zone".