ASSESSMENT OF CRITICAL MINERAL POTENTIAL OF PENNSYLVANIAN METALLIFEROUS BLACK SHALES ACROSS ILLINOIS

Pennsylvanian (Desmoinesian to Missourian) metalliferous black shales in the U.S. midcontinent are the focus of a multi-state Earth MRI geochemical reconnaissance to evaluate critical minerals enrichment. Some of these shales have previous documentation of elevated critical minerals vanadium (V), zinc (Zn), and nickel (Ni). They also contain high concentrations of molybdenum (Mo), uranium (U), cadmium (Cd), copper (Cu), and selenium (Se). Our preliminary high-resolution geochemical study features 11 different black shales spanning this interval in Illinois. Here we focus on the widespread Excello Shale which provides a case study of critical mineral enrichment in this interval.

Study of the Excello Shale features centimeter-scale analysis utilizing multiple techniques. The Excello Shale is 0.7 m to 2 m-thick, spans both the Illinois and Forest City basins, and was previously mapped in the subsurface using a combination of drill cores and gamma ray logs. We conducted over 5,000 EDXRF analyses, supported by 100 ICP-OES-MS and WDXRF analyses via the USGS geochemistry project, on the Excello within the Illinois Basin. Northern sections have peak concentrations near 0.5% Ni, 1% V, 1% Mo, and 5% Zn. We estimate average concentrations across the basin at 0.04% Ni, 0.16% V, 0.05% Mo and 0.19% Zn. The Excello shows clear, high-frequency, cm-scale oscillations in Ni, V, and Zn (as well as Cd, Cu, Mo, U, and Se) which can be traced across at least 65,000 km² of Illinois and appear to extend, though at slightly lower concentrations, into the Forest City Basin.

These preliminary results confirm that Pennsylvanian black shales have some of the highest concentrations of critical minerals reported globally for metalliferous shales, including those currently being mined, and are comparable to conventional sources. Our study highlights the significant critical minerals resource potential of the Pennsylvanian black shales distributed across much of the midcontinent. Ongoing research aims to define the sedimentary controls on extreme metal enrichment and distribution, which will enhance the mapping of concentration and thickness gradients. This framework will improve our understanding of the source and depositional behaviors of critical minerals in black shales and assist in identifying areas with the highest resource potential.