The Nan-etsu mine is situated at about 11km west of Muikamachi in the southern part of Niigata Prefecture. The prevailing rocks in the environs of the mine are Palaeozoic sediments, such as clayslate, sandstone, chert and basic pyroclastic rocks, and they are intruded by hornblende-gabbro.
The Palaeozoic sediments and hornblende-gabbro were converted to hornfelses by the intrusion of hornblende-biotite-granite stock. Various kinds of dyke rocks are found in the district, and some of them, such as dacite, rhyolite and quartz-porphyry, are closely located near the ore deposit of the mine.
The principal ore deposit is a hypo- or mesothermal fissure-filling vein, and is developed along NE-fault. The ore consists of arsenopyrite, pyrrhotite, magnetite, chalcopyrite, cubanite, sphalerite, galena, tetrahedrite and marcasite. The unmixing textures, namely, star-shaped sphalerite in chalcopyrite and chalcopyrite emulsion in sphalerite, are observed in some ores. Many veins, such as Daikura, Hinata, Mikagura, Arayama, Matagoro, Kogai, Hinokigura and Dozawa veins, are widely distributed in the district, and formation of veins has been recognized as being related to the activity of acidic dyke rocks following the granite intrusion.