Silver Mineralization of the Karuizawa Mine, Fukushima Prefecture, Japan

Abstract

Silver deposits of the Karuizawa mine are network and vein types developed in fracture zone of Miocene rhyolite lava dome altered hydrothermally. The network ore body consists of veinlets along fracture and spots of ore filled up cavity after spherulite in rhyolite. Its ore is principally composed of barite, sphalerite and galena in association with small amounts of quartz, pyrite, marcasite, chalcopyrite, polybasite, pyrargyrite and argentian tetrahedrite (12 to 18 wt% Ag). On the other hand, the vein along fissure in silicified rhyolite consists mainly of galena, sphalerite, pyrite, marcasite and barite associated with quartz, chalcopyrite and small amounts of bournonite, argentian tetrahedrite, polybasite and pyrargyrite. These mineral assemblages in ores are similar to those of kuroko ore. In high silver ore, fine grained silver minerals such as polybasite, pyrargyrite and argentian tetrahedrite are usually found as inclusion in galena, and often show a pseudo-micrographic texture with galena. These silver minerals were crystallized at late stage of the mineralization as shown in Fig.9. Homogenization temperature and salinity of fluid inclusion in barite are 230°to 320°C and 3.6 to 5.0 wt% NaCl equivalent, respectively. Hydrothermal alterations of silicification and adularization are conspicuously observed in or around the ore deposite. K-Ar age for adularia of the alteration product is 12.8±0.6 Ma which is approximately same as that of alteration rock of the kuroko deposits. The ore deposits of the Karuizawa mine are thought to have been produced by mineralization in the relation to formation of the kuroko deposits.