On the Dai-ichi Ore Body of the Yaguki Copper Mine

with Special Reference to the Chemical Composition of Pyrrhotite

Abstract

The copper ore bodies of the Yaguki mine, Fukushima Prefecture, are of high-temperature replacement type, occurring in limestone along its contact with underlying slate. The Dai-ichi ore body, which is the largest one in the mine, is localized in the main synclinal portion of the system of gently plunging folds. The skarn minerals are zonally arranged from the limestone side to the slate side as follows: limestone -"green skarn" (clinopyroxene-epidote-amphibole skarn)- garnet skarn -"banded skarn" (epidote-carbonate-quartz skarn)- slate. Chalcopyrite, magnetite and pyrrhotite are the main constituents of ore body, with minor amounts of sphalerife, pyrite, arsenopyrite, molybdenite, bismu-thinite, cobaltite, cubanite, galena and hematite. The zonal distribution of ore minerals around quartz diorite mass is remarkable in the upper part of the ore body. The crystallization of pyrrhotites may be divided into two stages, judging from their modes of occurrence and chemical compositions. The earlier pyrrhotite has commonly a lower atomic ratio of metal content than that of the later. There is a variation of atomic ratio of metals in the earlier pyrrhotites from top levels to the bottom, and the tendency is reverse in the later pyrrhotites.