A Preliminary Oxygen Isotope Study of Tetsusekiei Ores Associated with the Kuroko Deposits in the Hokuroku District, Japan
Makoto Tsutsumi, Hiroshi Ohmoto, 1983. "A Preliminary Oxygen Isotope Study of Tetsusekiei Ores Associated with the Kuroko Deposits in the Hokuroku District, Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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Tetsusekiei beds or chert-hematite layers stratigraphically overlie massive sulfide ores in many Kuroko deposits. Because of the small size of individual grains (less than 1 µm) in these samples, fluid inclusion studies or mechanical separation of individual minerals for isotopic or chemical analysis is difficult. A chemical method, which involves treatment of the powdered samples with an HCl-HI-H 3 PO 2 mixture and with HF, was developed to attain quartz- and hematite-enriched fractions. Oxygen isotope analyses of these fractions, together with the determination of Fe and Si contents of each fraction, allow estimates to be made of the oxygen isotope compositions of "pure quartz" and hematite in the samples.
The δ 18 O values of these "pure" minerals were estimated for eight tetsusekiei samples from four Kuroko mines in the Hokuroku district of Japan: a range of 6 to 21 per mil for the quartz and —19 to +2 per mil for the hematite. The application of the oxygen isotope fractionation factors of quartz-water and magnetite-water of Becker and Clayton (1976) to the δ 18 O values of coexisting quartz and hematite in these samples yields the following values for the temperature and δ 18 O values of hydrothermal fluids responsible for the formation of the tetsusekiei ores: Fukazawa mine: T = 110° ± 30°C, δ 18 O water = -9 ± 5 per mil, and Furutobe mine: T = 90° ± 30°C, δ 18 O water = —3 ± 5 per mil. These data, together with the occurrence of the tetsusekiei bed and the temperature and δ 18 O history of hydrothermal fluids during sulfide mineralization obtained by Pisutha-Arnond and Ohmoto (1983), suggest that the tetsusekiei ores formed during the waning stage of a hydrothermal activity and that a positive correlation exists between the temperature and the δ 18 O values of fluids: negative δ 18 O values at T < 200°C and near zero to positive δ 18 O values at higher temperatures.
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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.