Geology and Ore Deposits in the Area of the Kohnomai Mine (I):On the Relation of Geological Structures and Mineralization

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The Kohnomai mine, the greatest gold deposits in Japan is situated in the northeast part of Hokkaido island. The ore deposits are of epithermal fissure-filling Au-Ag veins occurring in the sedimentary rocks (tuff, mudstone and shale) and volcanic rocks (propylite and rhyolite) of Miocene age. The area is divided into three blocks by two shearing fractures trending N35°-45°E direction. The volcanic activities occurred in these blocks, being controlled by the fractures of second order which originated from the master fractures, and the deposits are closely related to the andesitic activities. The fractures of second order standing at 45 degrees with the master shearing fractures are predominate in the northern parts and some radial arrangements of fractures caused by doming in the southern parts. The representative deposits filling the former fractures are the No. 5 and the Sanno deposits while the Sumiyoshi, Motoyama and Fujishima deposits are of the latter. The No. 8 and Toyoshima deposits are formed along the shearing fractures.
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