2014 年 43 巻 5 号 p. 174-183
Many Bi-bearing ore deposits are distributed in and around the Wasada granodiorite, located in the Tsuruoka district, Yamagata Prefecture, Japan. We examined Bi-minerals from the Kuromori, Hongo, Obari, Yakuwa, Honko, Asashi and Obiro mines. Most of the Bi-minerals are sulfides and sulfosalts, and the occurrence of native Bi is very rare. Cu-Bi-S minerals such as wittichenite occur and are closely associated with bornite and tetrahedrite-tennantite in the Cu-Bi mineralization of the Kuromori, Hongo Obari, Yakuwa and Obiro mines. The Aikinite-bismuthinite series occurs within chalcopyrite in the Obari, Honko and Obiro mines. Pb-Bi-S minerals such as cosalite occur and are intergrown with galena in the Cu-Bi-Pb-Zn mineralization of the Obari, Asashi and Obiro mines. Therefore, the formations of Bi-mineral phases were affected by coexisting major Cu and/or Pb minerals. A mineral assemblage of coexisting iron sulfides indicates a lack of native Bi in this region, attributed to higher fS2 in the studied deposits than other Bi-deposits in the Inner Zone of SW Japan.
