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Nebgene mineralization throughout the Green Tuff region of Japan, closely related to geosynclinal evolution, is classified mainly into two types: the polymetallic, strata-bound Kuroko-type and hydrothermal vein-type mineralization occurring mostly in submarine volcanogenic sediments of Neogene age. In order to examine their genetic relation, especially the possible origins of their ore fluids, we have analyzed δ34S and δ34O values of sulfates in more than 100 samples of barite, anhydrite, and gypsum in the Neogene ore deposits, along with some δ34O values of quartz and adularia associated with these deposits.

Most of the isotopic compositions of Kuroko sulfates collected mainly in the Kuroko (black) and keiko (siliceous) ore zones fall within a relatively narrow range: 19.5 to 25.5 per mil for δ34S and 6.4 to 10.9 per mil for δ18O. These compositions, combined with fluid inclusion data, indicate that the ore fluids were predominantly Miocene seawater. In contrast, the hydrothermal vein sulfates are characterized by more scattered distributions of δ34S (ranging mostly from 16 to 29‰) and δ18O (mostly 0 to 14‰) values and were deposited from ore fluids of predominantly meteoric origin. The large spread in the isotopic compositions of these sulfates can be attributed to varying degrees of mixing or meteoric waters with other fluids such as modified seawater or connate water.

Barite samples from a single Kuroko deposit, the Ainai-Daikoku deposit, display a tendency for the δ18O values of sulfates in the keiko zone to be slightly higher than those in the Kuroko zone. No clear trend exists between the δ34S values from these two zones.

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