Geology and Vein Formation of the Nan-etsu Mine, Niigata Prefecture

Abstract

The Nan-etsu mine, situated at about 15km west of Muika-machi in the southern part of Niigata prefecture, is characterized by lead, zinc and copper vein deposits. Monthly production of crude ore from the mine is about 5, 000tons, containing 4.0% lead, 8.0% zinc and 0.6% copper.
The mine area is composed of Palaeozoic formation which consist of clayslate, sandstone, chert and pyroclastic basic rocks, intruded by an older intrusive, i. e. hornblende-gabbro. Both Palaeozoic formations and hornblende-gabbro have been converted to hornfels by intrusion of stocks of the younger biotite granite. Dike rocks, such as porphyrite, quartz-porphyry and dacite, are found in the mine area, and are thought to have been derived form a source similar to that of the younger intrusive.
The principal ore deposits of the mine are hypothermal or mesothermal fissure-filling veins, occurring along faults of ENE trend and in fissures around the younger granite stocks. The principal veins are seen to penetrate the dikes, especially those of dacite and quartz-porphyry, thus suggesting their formation during the latest stage of younger igneous activity.