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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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