Geology and Ore Deposits of the Myoho Mine

Abstract

The Myoho mine is located in the southern part of Kii peninsula, southwestern Japan. Geology of the mine area consists of sandstone, shale and breccia of Miocene age, and of the Kumano acidic igneous, complex. The Miocene formation is divided into K, A, B and C beds in ascending order. The ore deposits of the mine are chalcopyrite-pyrite-calcite-quartz-chlorite veins, partially containing chalcocite, bornite, sphalerite and galena, in the Miocene formation, The ore shoots are mostly bound in sandstone of C bed. The Kumano acidic igneous complex consists of rhyolite and granite porphyry. The rhyolite forms a large ring dike. It runs at the northern part of the mine. where it branches off several minor dikes. Vein fractures are intimately related to the, ring dike and the branches., Granite porphyry covers the Miocene formation and forms a part of the ring dike. The igneous rocks intruded and extruded prior to the vein formation. The ore deposits were formed in three stages; 1 st stage : Deposition of pyrite, calcite, quartz and chlorite with small amount of chalcopyrite ; 2 nd stage : Deposition of chalcopyrite, pyrite, calcite, quartz and chlorite ; and 3rd stage : Deposition of chalcocite, bornite, chalcopyrite and pyrite.