Geology and Vein Formation of the Yaso Mine in Northeastern

(On the Genetic Relation with Rhyolite Dikes)

Abstract

The Yaso mine is a vein-type copper deposit in the Miocene tuffaceous and the Palaeozoic argillaceous rocks, belonging. to subvolcanic type. Monthly production of crude ore is about 15, 000 tons of 1.45% copper.
This region is geologically composed of Palaeozoic formation, granite, Miocene pyroclastic and sedimentary rocks, rhyolitic and andesitic dike rocks, and dacite lava flow. The Palaeozoic slate, sandstone and quartzite formation and the granite that belongs to the Mesozoic era form the basement of this region. The Miocene formation consisting of basal conglomerate, tuffaceous sandstone and rhyolitic tuff is widely developed. These rcoks are intruded by many dikes of lithoidite, rhyolite, and nevaditic plagioclase rhyolite and by a few smaller dikes of propylitized andesite. Though in the northeastern part of the area the above-stated rocks are covered by a later thick lava flow of dacite, the occurrence of the rhyolitic dikes shows a U-shape. As the inside blocks surrounded by the U shape dikes show a slight subsidence, it can be said that the occurrence of the dikes corresponds to the ring-dike in the sense interpreted by M.P. BILLINGS. The mineralization took place mainly along the western wing of the U-shaped ring-dike. Consequently, it is inferred that the ore solution originated from the underground acidic magma that made the rhyolitic ring-dike.
The deposits of the mine consist of four groups of typical chlorite-quartz-copper veins. They are Akakura, Kido, Maekura and Ooba deposits. The largest vein is the 905m-Vein in the Akakura area, and is developed as long as 1200m in strike side and as deep as 460m.
The ore minerals are chalcopyrite, pyrite, sphalerite, galena, bismuthinite, etc., and the gangue minerals are chlorite, quartz and calcite. Though the zonal distribution of minerals is not so conspicuous, the upper parts of the veins are rich in sphalerite and galena. Bismuthinite has an intimate relation with chalcopyrite and it is presumed that the mineral has been deposited mainly in the early stage of the. sulphide mineralization. The alteration of wall rocks comprises silicification and chloritization. The former is stronger in the upper parts of the deposits and the later is striking in the lower.