Mineralization and K-Ar ages of the Ohmori (Iwami) Au-Cu-Ag vein-type deposits, Shimane Prefecture, southwest Japan

摘要

The Ohmori (Iwami) mine in the southwestern Japan is composed of two types of deposit. One is disseminated Ag deposit called the Fukuishi deposit (the Iwami Ag deposit), and the other is vein type auriferous Cu deposit called the Eikyu deposit (the Ohmori deposit). The two deposits occur in dacites of the Plio-Pleistocene Oe-Takayama volcano group, which are divided into the S-type, the Y-type and the I-type in the Ohmori mining area.K-Ar age data and geological information show that the S-type dacite erupted at 2.2 to 1 .8Ma,the Y-type dacite solidified at 1.8 to 1 .6Ma, and the I-type dacite intruded into the Y-type dacite at 1.5 to 1 .2Ma. It is suggested that the Fukuishi deposit in the S-type dacite formed in association with the intrusion of the I-type dacite, and the Eikyu deposit formed at 1.1Ma along ENE-WSW faults in the I-type dacite.