Abstract

Low C/S ratios have been generally observed in pyritiferous black shales of the lower Paleozoic and late Proterozoic (Berner and Raiswell, 1983). In addition to factors such as high rates of bacterial sulfate reduction which might have existed during these early periods due to the low oxygen content of ocean water, availability and quality of organic matter, or lack ofbioturbation and availability of reactive iron, low C/S ratios may also reflect low sedimentation rates. Three distinct situations within a single vertical stratigraphic column in the mid-Proterozoic Amjhore pyrite deposit reflect the effect of anoxic levels and sedimentation rates on pyrite formation. In the first (lower shale with a mean C/S ratio of 2.03), pyrite formation was diagenetic, whereas in the second (massive pyrite ore with a negligible carbon content) and the third (upper shale with a low C/S ratio of 0.27), pyrite formation took place under euxinic conditions. In environments where anoxic levels are high and the sedimentation rate low, low mean C/S ratios may occur. In such environments variations in total sulfate reduction per unit column of sediment deposited (resulting from variation in sedimentation rates or anoxic levels) are reflected in the slope of the carbon-sulfur plot by a greater slope. This may also result in lower intercepts on the sulfur axis. For upper shale samples the carbon-sulfur regression line has a very high slope of 3.73 and a low intercept on the sulfur axis of 0.21. Consistency of anoxic levels during deposition of upper shales is, however, indicated by the low standard deviation (0.051) of C/S data and a dominant control of the sedimentation rate on the sulfur content of the sediments is inferred. Sulfur isotope data on pyrite samples indicate an environment closed to SO (super -2) 4 which was initially open to H 2 S (or HS (super -) ) and later during deposition of upper shale became closed to it due to a limited supply of reactive iron. This confirms that the pyrite bed and upper shales were laid down in a restricted environment and substantiates the interpretation of the carbon-sulfur relationships observed in them.

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