Oxygen and Sulfur Isotope Composition of Barite and Anhydrite from the Fukazawa Deposit, Japan
Minoru Kusakabe, Hitoshi Chiba, 1983. "Oxygen and Sulfur Isotope Composition of Barite and Anhydrite from the Fukazawa Deposit, Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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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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This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.