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Stockwork siliceous ores in the Uwamuki 2 and 4 deposits are developed as anastomozing quartz veins in autobrecciated white rhyolite lava domes and adjacent tuff breccias. At Uwamuki 2, two concentrically zoned stock works have yellow siliceous ore (chalcopyrite + pyrite) in the center surrounded by black siliceous ore (sphalerite + pyrite + galena). At Uwamuki 4, stockworks of black and yellow siliceous ores and barite, form discrete zones adjacent to one another. Alteration trends of footwall rocks from both deposits appear to be the product of two superimposed events: MgO enrichment followed by relative increase in Al 2 O 3 and decrease in CaO and Na 2 O; K 2 O was only slightly depleted. At Uwamuki 2, the highest ore grades are found in the most intensely altered rocks.

Pressure-corrected filling temperatures of primary fluid inclusions in quartz euhedra inter-grown with sulfides in yellow siliceous ore on the —10-m level from Uwamuki 2 range from 268° to 376°C and salinities from 3.9 to 6.7 equivalent wt percent NaCl. Corresponding data from Uwamuki 4 are 270° to 345°C and 2 to 5 equivalent wt percent NaCl (Marutani and Takenouchi, 1978). Sulfur isotopic temperatures estimated for sulfide-sulfate pairs are uniformly higher than fluid inclusion filling temperatures by about 108°C at Uwamuki 2 and 155°C at Uwamuki 4. These discrepancies are attributed to disequilibrium isotopic processes during deposition of sulfides and sulfate.

On the — 100-m level at Uwamuki 4, δ 34 S of pyrite decreases across the stockworks from 8 per mil in yellow siliceous ore to 5:5 per mil in black siliceous ore. Barite from both stockwork deposits has a mean δ 34 S of 22.6 ± 0.6 per mil (1 σ, n = 4) and is identical to the mean δ 34 S of syngenetic bedded sulfates (barite, anhydrite, and gypsum) from five other Kuroko deposits (22.5 ± 0.8‰, 1 σ, n = 21) from which we conclude that contemporaneous seawater was the probable source of sulfate sulfur.

Mole percent FeS in sphalerite coexisting with pyrite from the black siliceous ore at Uwamuki 4 decreases from 0.62 ± 0.10 on the -100-m level to 0.22 ± 0.02 on the -55-m level, paralleling a decrease of fluid inclusion filling temperatures in sphalerite from 287° ± 18°C to 255° ± 13°C. The vertical decrease of FeS in sphalerite was in response to declining temperature under conditions of nearly constant sulfur fugacity. The area of highest fS2 in black siliceous ore coincides with the locus of maximum Zn + Pb deposition.

The important variations in the ore-forming fluid with time as deduced for Uwamuki 4 are simultaneous decrease in temperature, increase in total dissolved sulfur, increase in fO2, and decrease in pH. Mineralization appears to have involved progressive domination of the ore fluid by seawater, giving rise to the decrease in δ 34 S of pyrite as well as seawater characteristics for δ 34 S of barite in the stockworks. Mass balance calculations demonstrate that the observed enhanced salinity of the ore fluid over seawater is not the result of normal seawater participating in rock-water hydration reactions. An admixture to seawater of a more saline fluid is required, possibly a magmatic fluid on the basis of existing H and O isotope data.

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