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Abstract Mississippi Valley-type (MVT) lead-zinc ore deposits found in the midcontinent United States have isotopic signatures that indicate the potential involvement of a single highly radiogenic source mobilized by hydrothermal fluids or the mixing of two sources of Pb with differing radiogenic signatures. A genetic link between black shales and major MVT ores has been thought to exist because of (1) the presence of mature hydrocarbon and brine fluid inclusions within many MVT ores, (2) the association of the hydrothermal fluids that are linked to MVT ore formation with organic compounds and oil-like droplets, and (3) the highly radiogenic nature of the ores. The analysis of 69 shale samples from 21 stratigraphic units from across the midcontinent United States revealed variations in Pb concentrations and isotopic compositions across different Cambrian- to Pennsylvanian-age shales. Shales with high organic carbon content, particularly those of Pennsylvanian age, displayed elevated Pb abundances, suggesting their formation under anoxic conditions was conducive to the preservation of organic carbon binding with Pb ions. However, the Pb isotope signatures of most black shales are generally less radiogenic than the ores, suggesting that they were not a source of radiogenic Pb. Several samples of the Devonian-Mississippian Chattanooga Shale and the Ordovician Polk Creek Shale have significant quantities of radiogenic Pb that more closely resemble those of the MVT ores. This suggests that certain shales may have directly interacted with the hydrothermal fluids that precipitated the MVT ores. Some shales may have acted as an aquitard as the hydrothermal fluids were migrating through the subsurface, causing some shales to become more enriched in radiogenic Pb than others. 

We conducted research to determine neodymium model dates (TDM), εNd values, and the relative proportions of rare earth elements (REEs) of Paleozoic black shales from the Midcontinent United States to constrain their sediment sources. Twenty-one Cambrian to Pennsylvanian shale formations of the Illinois, Cherokee, Forest City, and Arkoma basins, the Ozark Dome, and the Ouachita Mountains were examined. Findings reveal that these midcontinental shales consist primarily of felsic detrital minerals that originated from the craton. The Cambrian Mt. Simon, Eau Claire, and Tunnel City shales of the northern Illinois Basin exhibit REE patterns and Nd isotopic signatures similar to those of the Ordovician Mazarn and Womble shales of the Ouachita Mountains, which indicates derivation from a similar sediment source. Sediment was likely derived from the Superior and/or Trans-Hudson cratonic provinces. The dominant sediment source shifted during deposition of the Middle Ordovician Womble Shale due to the uplift of the Appalachian Mountains during the Taconic orogeny, as suggested by the younger TDM dates, less negative εNd values, and similar REE patterns compared to those of the older Cambrian and Early Ordovician shales. The Grenville province and Appalachian Basin remained the primary sediment source into the Pennsylvanian. 

This study analyzes Pb isotopes combining biological (ancient human and prehistoric animal teeth) and geological (soil leachate, whole rock, and rock leachate) samples to determine the origins of prehistoric skeletal elements. It exemplifies how the biologically available Pb method assesses the early lifetime locations of ancient human populations using prehistoric animal teeth and the multivariate/linear nature of Pb isotope data. Lead isotopes provide a valuable technique, in part due to the correlation between their six stable isotope ratios. Other studies have used Pb isotopes for similar purposes, but no clear method for determining a local range has yet been formally defined and tested. The biologically available Pb method uses many prehistoric animal tooth enamel samples to establish a baseline for local ratios in the region, then compares their ratios’ linear patterning to human remains to test if they are non-local. The case study compares Pb isotopes from prehistoric animal teeth, human teeth, and whole rocks from southwest Arkansas. These results are compared to animal samples from Louisiana and Mississippi and human data from Illinois and New Mexico. Soil leachates, Pb concentrations of tooth enamel, and trace element analysis are used to assess contamination. Comparisons to southwest Arkansas whole rock Pb isotope ratios suggest they are too variable to be used for direct comparison to ancient human remains, illustrating that prehistoric animal teeth are more appropriate for direct comparison to prehistoric human teeth. The biologically available Pb method provides a key analysis tool needed for studies of ancient human sourcing.