Abstract

Black shale beds in the uppermost part of the Clegg Creek Member of the New Albany Shale (Devonian-Mississippian) in southern Indiana have organic C contents from approximately 12 to 25 wt percent and are enriched in Cu, Pb, Zn, Ni, Cd, Mo, and V. Brownish-black to olive-black shale located lower in the formation has normal metal contents and organic C abundances up to 12 wt percent. Shale units lower in the section were deposited under anaerobic conditions, but C-S-Fe relationships and sulfur isotope values suggest that the H 2 S-O 2 boundary occurred near or at the sediment-water interface. Conversely, the carbon- and metal-rich beds near the top of the formation were deposited under strongly euxinic conditions with the chemocline located within the water column. Metal sulfide formation was limited by the availability of metals. A sharp increase in slope at 12 to 14 wt percent C organic is observed in organic C-metal plots and is related to organic carbon preservation and metal adsorption due to the presence of an H 2 S-bearing water column.Sequential chemical analyses and electron microprobe studies indicate that Zn, Cu, and Cd are held in sulfide minerals, Pb as a selenide, Mo both as a sulfide ( approximately 56-85%) and within kerogen (20%), Ni within kerogen (50%) and as a sulfide (45%), and V primarily within clay minerals. Petrographic studies also indicate that the beds in the upper part of the Clegg Creek Member are characterized by the highest proportion of inertinite and vitrinite found in the New Albany Shale.A conceptual model is developed whereby trace metals adsorbed on surfaces of organic material were preserved from release by oxidative degradation in the water column due to the high elevation of the H 2 S-O 2 boundary. Transition metal contents of the Clegg Creek Member may have been locally high due to an increase in the influx of terrestrial organic matter. Once delivered to the sea floor, metals took part in diagenetic reactions that produced metal sulfides and vanadiferous clays. Only Mo and Ni remained significantly enriched in kerogen. Although Ni and V contents are high in the soluble organic material from the shale, their abundance constitutes less than 1 percent each of the whole-rock metal content.

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