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Mineralogy, Geochemistry, and Ore Genesis of Hydrothermal Sediments from the Atlantis II Deep, Red Sea

By
R. J. Pottorf
R. J. Pottorf
Ore Deposits Research Section, The Pennsylvania State University, University Park, Pennsylvania 16802
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H. L. Barnes
H. L. Barnes
Ore Deposits Research Section, The Pennsylvania State University, University Park, Pennsylvania 16802
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Published:
January 01, 1983

Abstract

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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Contents

Economic Geology Monograph Series

The Kuroko and Related Volcanogenic Massive Sulfide Deposits

Hiroshi Ohmoto
Hiroshi Ohmoto
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Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
5
ISBN electronic:
9781629490007
Publication date:
January 01, 1983

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