Geochemical Environments of the Neogene Ore Formation in the Green Tuff Region, Japan

According to the field and laboratory observations, mineral assemblages found, composition of sphalerite coexisting with pyrite, fluid inclusion data, and thermochemical calculations, the overall geo-chemical environments of the Neogene mineralization in the Green Tuff region of Japan are evaluated in the present article. The depositional environments prevailing at the time of the Neogene ore formation can be roughly classified into two types, those of high to intermediate oxygen and sulfur fugacity, and those of intermediate to low oxygen and sulfur fugacity, respectively. The former, including the Kuroko deposits, gold-silver veins, copper-lead-zinc-manganese veins, and pyrrhotite-free lead-zinc veins, is a region bounded by the following mineral assemblages: alabandite-rhodochrosite; pyrite-magnetite (or pyrite-hematite) ; magnetite-hematite; galena-anglesite; pyrite-bornite-chalcopyrite; oldahmite-anhydrite; and fixed iron content of sphalerite (3 mol. % FeS). On the other hand, the latter, including lead-zinc veins with or without pyrrhotite, is a region bounded by the assemblages: alabandite-rhodochrosite; graphite-carbon di-oxide; pyrite-pyrrhotite; pyrite-magnetite; and oldahmite-anhydrite. Among them, the mineral assemblage, pyrite-bornite-chalcopyrite is characteristic of the Kuroko deposits. Then, assuming minerals-fluid equili-brium, the chemical features of the ore fluids responsible for the Neogene ore formation are estimated. The relative concentrations of major metal species present in the ore fluids are as follows: in the low oxygen fugacity region, generally, Pb < Cu< Zn ≶ Fe and Pb < Fe< Cu ≶ Zn at 200°C and 250°C, respectively. In the high oxygen fugacity region, generally, Pb < Fe < Cu ≶ Zn at 200°C and 250°C. The calculated concentrations, based on the molal concentrations in log scale, are compared with those of the active geothermal brines. The results are as follows: the geothermal fluids from New Zealand (e.g. Wairakei, Champagne Pool, and Broad]ands) and possibly sea water have a similar composition to those of the Neogene ore fluids of high to intermediate oxidation state at 200°C, while the fluids from the Discovery Deep, Red Sea and Matupi, T. P. N. G. have a similarity in chemical composition to those of the Neogene ore fluids of low to intermediate oxidation state at 250°C.