Ag‐Cu‐Pb‐Bi‐S Minerals Newly Discovered from the Ohori Base Metal Deposit, Yamagata Prefecture, NE Japan: Implications for Bi‐metallogenesis in the Green‐Tuff Region

Abstract

AbstractThe Ohori deposit, one of the base metal deposits in the Green‐Tuff region, NE Japan, is composed of two types of mineralization; a skarn‐type (Kaninomata orebody) made by the replacement of the Miocene calcareous layer, and a vein‐type (Nakanomata orebody). While the ore mineral assemblage of the deposit (chalcopyrite, pyrite, sphalerite and galena) has been known for being rather simple, some Pb‐Bi‐S minerals have been discovered for the first time in the present study. The minerals mainly occur in the chalcopyrite‐rich ores of both orebodies. They essentially belong to the Pb‐Bi‐S system and contain Cu and Ag in minor amounts, which correspond to the lillianite–gustavite solid solution series (phases Z and X), cosalite, neyite, felbertalite, krupkaite and Bi‐bearing galena. The chalcopyrite‐rich (Bi‐bearing) ores from both orebodies are richer in chalcopyrite, pyrite and chlorite, and have higher homogenization temperatures (>300°C) of fluid inclusions, and higher FeS contents in sphalerite compared to the Bi‐free ores. In the Green‐Tuff region, Bi‐minerals have been reported from many base metal deposits. Most of these Bi‐bearing ore deposits are referred to as xenothermal‐type deposits, and are characterized by the following common features; composite mineralization of high‐ and low‐temperatures in the shallower environments, and close relationships with the Tertiary granitic rocks. The whole mineralization at the Ohori deposit also has a similar xenothermal character because of the coexistence of high‐temperature chalcopyrite‐rich ores with Pb‐Bi‐S minerals, which were formed by the influence of the Tertiary granitic rocks at a shallow depth.