Geology and Ore Deposits of the Fukufune Copper Mine in Yamagata Prefecture, Japan

Abstract

The Fukufune mine is located in the southern part of the Shinjo Basin, Yamagata Prefecture, which is known as so-called "green tuff" area. It produces monthly 250 tons of copper concentrates. The ore bodies of the mine are network vein-type copper deposits.
The geology of this area is composed mainly of Miocene pyroclastic and sedimentary rocks and rhyolitic lava flow. They generally strik NW-SE and dip 20 to 30° SW. Chloritization, sericitization, silicification, kaolinization and pyritization are closely associated with copper mineralization, and, in addition to these, montmorillonitization, carbonatization and zeolitization are found in altered rhyolitic tuffs outside the copper orebodies.
The ores is classified into two types, one of them being massive ore intensely and secondarily enriched, and the other being network vein. Some differences of the nature of the ores, wall rock alteration and others between the both type are recognized.
The prmary ores contains chalcopyrite, sphalerite, galena, pyrite, small amounts of tetrahedrite and wittichenite, quartz and clay minerals which are chlorite, sericite, etc., and the enriched ones consist of chalcopyrite replaced by chalcocite, covellite, secondary sphalerite and bornite. The paragenesis shows a sequence of three of phases mineralization ; the first one altering country rocks, the second carrying chalcopyrite and other sulphides, and the last carrying secondary minerals.
The ore minerals were examined under the microscope, and the E. P. M. A. was useful for the textural study of veinlets consisting of secondary sphalerite and the determination of small grains of wittichenite.