Genesis and ore deposits of the omine mine, Iwate prefecture

The Ômine copper mine operated by Rasa Industrial Co. Ltd. is situated about 20km to east of Tôno City, Iwate Prefecture, and adjoins to the west of western group of the Kamaishi iron-copper deposits. The mining district is composed of the Permian slate, calcareous slate, sandy slate, which are intruded and metamorphosed by the granodiorite and quartz-diorite. The granodiorite and quartz-diorite are considered to have invaded in the early-Cretaceous age. The ore deposit of Ômine mine is genetically related to the granodiorite intrusion, and ore minerals were produced by the contact metasomatic and high temperature hydrothermal processes. The deposit consists of four ore bodies. Three ore bodies ranging from the first one to the third one occur in the breccia skarn zone, and are in network or impregnation in form. The shape of each ore body is cylindrical and inclined about 60°_??_80° to the northwest. These ore bodies produce a large amount of cubanite as well as chalcopyrite and pyrrhotite, accompanying with a small amount of pentlandite. The skarn of this mine is classified into two types, that is, “the banded skarn” and “the breccia skarn”. The banded skarn is composed of biotite hornfels and diopside-fels, very often interposing leucocratic veins. All these are parallel to the bedded plane. The breccia skarn is composed of diopside skarn and/or diopside-garnet skarn filling up the interstices of numerous breccia-like biotite hornfels. A remarkable K-felspathization (chiefly microclinization) is recognized in the breccia-like biotite hornfels scattering in the breccia zone. The present paper purports to clarify the genesis of the ore deposit of the Ômine mine, and to describe the paragenesis of ore minerals. Special attention is given to the discussion of the K-felspathization and the skarnization of the hornfels in the breccia zone. The existence of the fourth ore body has been affirmed by boring. However, its details will be reported later when prospecting has advanced.