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 |
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Authors: | Yu Yokoro and Kazuo Nakashima |
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Institution: | Graduate School of Science and Engineering and;Department of Earth and Environmental Sciences, Yamagata University, Yamagata, Japan |
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Abstract: | The 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. |
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Keywords: | calcareous rock Green-Tuff region Ohori deposit skarn Pb-Bi-S minerals tertiary granitoids xenothermal-type deposit |
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