Isotopic and Trace Element Studies of Sulfates
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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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.