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The application of isotopic techniques, laboratory reactions, and bacteriological studies has made it possible to define more closely the origin of the Gulf Coast salt-dome sulphur deposits.
The available geologic and petrographic data suggest that the salt-dome sulphur originated through reduction of the anhydrite of the cap rock of the salt dome to hydrogen sulphide and subsequent oxidation of the sulphide to native sulphur. The present laboratory study confirms this mechanism and clarifies some of its details. Experiments designed to produce hydrogen sulphide or sulphur through the direct reduction of anhydrite by petroleum under salt-dome conditions gave negative results in time equivalents of 150 million years. On the other hand, sulphate-reducing bacteria, grown on hydrocarbon substrates, were able to produce hydrogen sulphide from dissolved sulphate. Other experiments showed that at salt-dome temperatures sulphate ion is capable of oxidizing hydrogen sulphide at me surable rates. It is concluded, therefore, that sulphate-reducing bacteria are the primary agents in the production of cap-rock sulphur. Dissolved sulphate is reduced to hydrogen sulphide by the bacteria, after which part of this hydrogen sulphide is reoxidized to native sulphur by reaction with sulphate ion.
Isotopic analyses were made on the sulphur and carbon from a series of salt-dome materials. Samples of anhydrite from the salt stocks of eleven domes had the same ratio of sulphur-32 to sulphur-34 (about 21.85), probably indicating derivation of all of the salt domes from a single evaporite sequence. A sample of a Jurassic (?) sedimentary salt bed from Clarke County, Alabama, which is probably part of the stratigraphic unit from which the salt stocks were derived, has a S32/S34 ratio of 21.83. Anhydrite cap rock has the same sulphur isotopic composition as the anhydrite inclusions of the salt, and evidently has been formed by their accumulation on top of the salt stock as the halite has been leached away by ground water.
The hydrogen sulphide found in cap rock waters is enriched in sulphur-32 by several per cent compared with the sulphate of the anhydrite cap rock. Laboratory experiments show that sulphate-reducing bacteria are capable of causing at least 2.7% increase in the S32/S34 ratio in the hydrogen sulphide which they produce relative to the sulphate source. Calcite cap rock was found to be enriched in carbon-12 by as much as 4.5% relative to normal sedimentary limestones. It has C13/C12 ratios similar to those of cap-rock petroleum. This indicates that the calcite cap rock has been formed by precipitation of carbonate produced during the bacterial oxidation of petroleum. The sulphate remaining in the calcite cap rock represents residues which have su fered repeated bacterial attacks. It is not surprising, therefore, that analyses indicated that sulphate in the calcite cap rock is depleted in sulphur-32 and is inhomogeneous in isotopic composition. Some samples have S32/S34 ratios as low as 20.83.
The native sulphur found in calcite cap rock has a rather uniform isotopic composition compared with the wide variations found in the sulphate with which it is associated. The repeated formation of polysulphides by temporary solution of the native sulphur in the sulphide waters of the cap rock probably provides an isotopic exchange mechanism for the elimination of variations in its composition. The native sulphur usually has an S32/S34 ratio a few tenths of a per cent lower than that of the hydrogen sulphide at the same dome. This is consistent with its derivation through oxidation of the sulphide by sulphate.
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