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Abstract

Oxygen and sulfur isotope ratios were determined for barite and anhydrite samples from the Fukazawa deposit, a typical Kuroko deposit ih the Hokuroku basin of northeastern Japan. There is no systematic variation in the δ18O and δ34S values of barite with the mode of occurrence and locality within an Orebody. Barite, has lower δ18O values (6 ~ 9‰) than anhydrite (11 ~ 14‰), whereas the δ34S values of the two minerals are similar to each other and close to the seawater value. The oxygen isotope systematics suggest that barite and anhydrite were precipitated in oxygen isotope equilibrium with a solution which was a mixture of seawater and hydrothermal fluids.

The lower 87Sr/86Sr ratios of barite relative to anhydrite at the Fukazawa deposit obtained by Farrell (1979) may indicate a larger contribution of hydrothermal fluids to barite precipitation, although a lower water/rock ratio in the hydrothermal system could also explain this feature. The δ18O vs. 87Sr/86Sr relationship of barite may be explained by a mixing model with a seawater contribution of less than 20 percent at temperatures around 210° ± 30°C, while the oxygen and Sr isotope relationship of anhydrite indicates that anhydrite may have precipitated at temperatures around 180° ± 20°C either from a mixed solution of seawater and a hydrothermal fluid which was more diluted in ore metals or from heated seawater which had circulated through volcanic rocks. Strontium and sulfur isotope data support the idea that the sulfates in the hydrothermal fluid were directly derived from the Miocene seawater.

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