Abstract

Samples of the Middle Pennsylvanian-age Mecca Quarry Shale Member and stratigraphically equivalent beds from Indiana, Illinois, Missouri, and Kansas have been analyzed for both inorganic and organic constituents. Trends for Mo values reported by Coveney and Martin (1983), who found the highest tenors to be associated with the ancient shoreline in Indiana, have been confirmed regionally across the Midwest. Similarly it has been found that Cd, V, U, Se, and Sb are most concentrated near the shoreline, whereas constituents such as Ca and Sr are richest offshore (e.g., Kansas). Organic matter analyses, including pyrolysis-gas chromatography, show the eastern samples to be richest in total organic carbon and also in terrestrial organic matter.Unbuffered aqueous solutions containing powdered organic-rich shales from Indiana equilibrate to acid pH values (pH = 4-6) where Mo is readily retained by organic matter. Laboratory experiments show the shales retain a capacity to fix Mo from dilute solutions. Correlations between organic matter type and metal values, such as those existing between Mo and V and terrestrial organic matter in shale samples, generally fit with the syngenetic model for metal fixation in near-shore, brackish waters, proposed by Coveney and Martin (1983). However, other evidence, including the remnant activity of organic matter-rich samples to fix metals, suggests that nonsyngenetic agents, such as basinal brines or meteoric fluids, have probably augmented the metal contents of the deposits subsequent to sedimentation and diagenesis.

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