Abstract

The consolidated rocks underlying the Walton-Cheverie area are mainly sediments of Lower Carboniferous (Mississippian) and Triassic age. Diabase sills, probably of Triassic age, occur in one place. The Mississippian rocks consist of a lower series of terrestrial fluviatile and lacustrine sediments (Horton Group) and an upper sequence of dominantly marine sediments that include evaporites (Windsor Group). The Triassic sediments are entirely of terrestrial origin. The Mississippian rocks are intricately folded in places, and all are extensively faulted; the Triassic rocks are relatively flat lying and have been faulted in many places. The economic deposits of the area are gypsum and anhydrite in sedimentary beds and epigenetic barite and sulfide bodies.The sulfur in the sulfates and sulfides of the country rocks and epigenetic deposits exhibits a wide range in delta 34 S composition, from +33.6ppm to -- 42.0ppm. All of the sulfates, including anhydrite and gypsum in the evaporites, barite in epigenetic deposits, and sulfate dissolved in deep circulating brines, near-surface ground waters, and sea water are enriched in 34 S(delta 34 S range, +33.6ppm to +5.7ppm)- The pyrite in argillites and shales of the Horton Bluff Formation ranges from +18.3ppm to --10.9ppm in delta 34 S; in the calcareous anhydrite beds of the evaporite formation the pyrite is greatly depleted in 34 S(delta 34 S = -- 18.4ppm.). Pyrite in the Triassic diabase is also depleted in 34 S(delta 34 S = --13.1ppm).The average delta 34 S values for the sulfides and sulfosalts in the Magnet Cove deposit are all negative, ranging from -- 0.9ppm to -- 39.6ppm. There is a progressive decrease in 34 S with respect to the paragenetic sequence of minerals, the last sulfosalts and sulfides to be crystallized being significantly depleted in 34 S. Disseminated pyrite in certin coalified plant zones has negative delta 34 S values (average -- 39.6ppm) whereas that in zones devoid of plant remains has positive delta 34 S values (average +11.6ppm).It is concluded that the large massive cryptocrystalline barite body in the Magnet Cove deposit originated mainly by replacement of bedded gypsum and anhydrite (Ba (super 2+) for Ca (super 2+) ) because of similarities in the isotopic composition of the sulfates in the evaporites and in the resultant barite. Other coarsely crystalline barite veins and pods appear to have been deposited from deep circulating brines. The sulfides and sulfosalts in the Magnet Cove deposit appear to have been derived from sulfate in the deep circulating brines by reduction processes, the shift in the isotopic constitution to more negative values being due to these processes which may have been partly aided by bacterial activity.

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