Strontium Isotope Geochemistry of the Kuroko Deposits
Clifton W. Farrell, Heinrich D. Holland, 1983. "Strontium Isotope Geochemistry of the Kuroko Deposits", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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The isotopic composition of strontium in sulfate minerals from the Fukazawa and Kosaka ore deposits has been measured in order to evaluate the importance of seawater in the development of the Kuroko deposits. The 87 Sr/ 86 Sr values in samples of anhydrite and gypsum from the sekkoko (gypsum) units in both deposits fall in a narrow range (0.7082-0.7087) whose upper limit approaches that estimated for Miocene seawater. The 87 Sr/ 86 Sr values of the analyzed barites are generally slightly lower than those of the anhydrite and gypsum from the sekkoko and cover a wider range (0.7069-0.7079). None of the ratios are higher than that estimated for Miocene seawater. Coarsely crystalline barite specimens from the siliceous ore zones have 87 Sr/ 86 Sr values which are indistinguishable from those of fine-grained barites in the strata-bound sulfide ores.
Anhydrite was probably deposited in shallow convection cells established in the immediate vicinity of rhyolite intrusives. The mixing of a heated, seawater-dominant hydrothermal solution which had acquired a limited amount of isotopically nonradiogenic strontium from the Miocene volcanics with relatively fresh seawater within the porous, poorly consolidated tuff-aceous sediments near the seawater-sediment interface appears to be the most reasonable mechanism of sekkoko formation. The quantity of barite in individual orebodies greatly exceeds that which could be formed from the barium contained in the white rhyolite domes associated with the ore deposits. The measured depletion of strontium and cations in the volcanics underlying the ore horizon, and the alteration of the Paleozoic basement 450 m below the Kuroko orebodies in the Kosaka area strongly suggest that the ore solutions penetrated beneath the Miocene volcanic cover to leach these cations from the basement metamorphics. The 87 Sr/ 86 Sr data for barite are consistent with a model in which seawater is progressively modified through interaction with the Miocene volcanics, the Paleozoic basement, deep granitic plutons, and possibly through the addition of magmatic fluids associated with the granitic plutons.
The most appealing model for the formation of the Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately above the vent exit in the overlying seawater. However, before a full understanding of the mechanism-of sulfide and barite precipitation is possible, much more must be learned about the nature of the vent, the flow rate of hydrothermal solutions, the mixing properties of the ore solution with seawater, and the thermal gradients in the upper part of the feeder channel.
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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.