CRITICAL MINERALS IN ILLINOIS BASIN PENNSYLVANIAN BLACK SHALES

Metalliferous black shales in the Pennsylvanian strata of the midcontinent are highly enriched in critical minerals (CM). The objective of our ongoing collaborative research, funded under the USGS Earth MRI initiative, is to collect new geochemical data on Pennsylvanian black shales across the midcontinent to evaluate them as potential CM resources. We are currently characterizing CM concentrations by age of the shales, documenting geochemical gradients, sedimentology, and characteristics of black shale-enclosing strata. Together, these data will be integrated into ore system models to identify areas that have higher grade and thicker accumulation of CM across the vast area that might be economic resources.

To advance understanding of CM distribution in the Pennsylvanian Illinois Basin we analyzed five Desmoinesian black shales (Will Scarlett, Veale, Carrier Mills, Mecca Quarry, and Excello) in six drill cores. Elemental concentrations were analyzed using a portable X-ray fluorescence analyzer (pXRF) at 1 cm stratigraphic spacing through each shale for a total of ~2000 analyses verified by 100 samples submitted to USGS for conventional geochemical analyses. Metal-rich intervals have profiles that show high-frequency cm-scale oscillations in Cr, Ni, V, and Zn along with other redox sensitive elements Cd, Cu, Mo, U, and Se. The number of oscillations varies between each of the shales analyzed, creating a unique geochemical signature that serves to fingerprint each shale. For example, the Excello Shale contains seven oscillatory bundles that can be correlated across >1000 km² in the southern portion of the basin. Overall, concentrations and thicknesses increase toward the northern portion of the basin, with V, Zn, Mo, Ni, Cu, Cr values approaching percent-level concentrations in significant intervals.

These preliminary results indicate Pennsylvanian black shales in Illinois contain some of the highest CM concentrations reported for metalliferous shales, including some currently being mined. Initial results of this study support the economic potential of these resources and is helping to build an exploration model to predict where the thickest and most enriched metalliferous intervals occur.