Relations Between Ore Veins and Dome-like Structures at the Kishu Mine

Abstract

Ore deposits at the Kishu Mine are fault-filling veins in Miocene sandstones and shales which surround the so-called "Kumano acidic rocks". Chalcopyrite and pyrite are the ore minerals. The vein systems strike roughly in three major directions, namely E-W, N45W and N-S. The first two directions have chalcopyrite-pyrite veins while the last has only pyrite veins; fault displacements in the last are far greater than those in the first two.
For several years it has been thought that bonanzas are mostly confined to the sedimentary rock. The existence of bonanzas is very probable in certain sandstones (called the C members at this mine). Also, it was thought that shales, called the B members, play the role of cap-rocks. However, our latest studies have indicated that although control of the ore deposits by certain layers is certainly true, it is not the sole factor. We have been able to find eight centers of mineralization in this mining district by drawing "copper grade contours". The amount of displacement of the E-W and N45W veins is largest at the centers and decreases gradually outward enabling us to draw "equidisplacement contours".
Through detailed investigations, it was found that there are several flat dornes or basins in a layer which seems monoclinial against the Kumano acidic rocks. Basins and plunging syclines are of small scale. Structure contour lines drawn to show the dome structure coincide with the "copper grade contours" and the "equidisplacement curves". It is concluded that shoots at the Kishu Mine are controlled by the dome-like structures, a fact which will be an important indicator for prospecting.