Mineralogy, Geochemistry, and Mineral Chemistry of Siliceous Ore and Altered Footwall Rocks in the Uwamuki 2 and 4 Deposits, Kosaka Mine, Hokuroku District, Japan
L. Taras Bryndzia, Steven D. Scott, J. Elizabeth Farr, 1983. "Mineralogy, Geochemistry, and Mineral Chemistry of Siliceous Ore and Altered Footwall Rocks in the Uwamuki 2 and 4 Deposits, Kosaka Mine, Hokuroku District, Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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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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The Kuroko and Related Volcanogenic Massive Sulfide Deposits
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.