Mineralogy, Geochemistry, and Ore Genesis of Hydrothermal Sediments from the Atlantis II Deep, Red Sea
R. J. Pottorf, H. L. Barnes, 1983. "Mineralogy, Geochemistry, and Ore Genesis of Hydrothermal Sediments from the Atlantis II Deep, Red Sea", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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The mineralogies of hydrothermal sediments in cores and other samples from the Atlantis II Deep of the Red Sea were examined by optical and scanning electron microscopy, and by X-ray diffraction and electron microprobe methods. The bottom 20 percent of the studied section consists of 1 to 5 wt percent sulfides, dominantly as pyrrhotite, cubic cubanite, chal-copyrite, and pyrite, plus other hydrothermal minerals including vermiculite, anhydrite, hematite, chamosite, and. ilvaite: Above this zone is a sulfide-enriched unit about 4 m thick, of 8 to 66 wt percent sulfides. Chalcopyrite, pyrite, and sphalerite with 3.5 to 4.5 mole percent FeS accompany iron-rich smectite, anhydrite, manganosiderite, amorphous silica, and amorphous ferric oxyhydroxides. Mineralogic breaks between these units indicate variations in tectonic activity which affect both detrital accumulation and locations of hydrothermal vents.
Veins crosscutting the sediments consist mainly of anhydrite, silicates, pyrrhotite, pyrite, and chalcopyrite, with minor valleriite, cubic cubanite, and sphalerite. Early vein sphalerite contains 14.0 to 23.3 and averages 17.3 mole percent FeS, but later vein sphalerite averages 3.6 mole percent FeS. The observed vein assemblage—-pyrrhotite, cubic cubanite, high iron sphalerite, and anhydrite—indicates disequilibrium between H 2 S and > in the depositing fluid. Apparently, mixing between two circulating hydrothermal fluids, one shallow with SO4 > H2S at <250°C, and a deeper, hotter, fluid with H2S > SO4, produced disequilibrium mineral assemblages before discharging onto the sea floor.
Precipitation at 200° to 250°C is implied by the assemblage cubic cubanite + chalcopyrite + monoclinic pyrrhotite. However, temperatures beneath the sea floor >334° ± 17°C are indicated by 100 µm grains of cubic cubanite + chalcopyrite + pyrite that were apparently carried upward by the hydrothermal fluid. Vertical transport of these grains to the sea floor required rapid flow and coojing to preserve the high-temperature cubic form of cubanite. The rmodynamic evaluation shows the hydrothermal fluid at 250°C to haver a H2s = 0.001, log a S2, = -11.8 to -13.7, log aO2 = -36.5 to -38.5, and pH = 4.56 ± 0.5. At 200°C: a H2S - 0.001, log a S2 = -12.9 to -13.0, log a O2, = —41.7 to -41.9, and pH = 4.64 ± 0.5.
Comparisons show that the Atlantis II sediments resemble volcanogenic massive sulfides in most characteristics, including associated volcanism, tectonic setting, pyrrhotite-chalcopyrite-sphalerite zoning, metal grade, sedimentary textures, and most fluid properties. The few differences are related to a flanking evaporite-shale sequence and to the immature state of the Atlantis II deposit.
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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.