ADVANCING THE UNITED STATES’ GEOLOGIC FRAMEWORK IN THE SEARCH FOR CRITICAL MINERALS: STUDY OF REE-ENRICHED PHOSPHORITES YIELDS NEW INSIGHTS INTO CHICAGO SUBSURFACE RESOURCES

The Upper Ordovician succession of the US Midwest contains multiple thick phosphorites that are enriched in heavy REEs (HREE) several times above conventional deposits. Predicting the location of the most enriched occurrences requires a broad knowledge of the enclosing strata, paleogeography, and event stratigraphy. Understanding of this complex interval is being greatly accelerated through our study of the Chicago area Tunnel and Reservoir Plan (TARP) cores.

These cores were originally drilled to engineer storm water tunnels in the Chicago metropolitan area. The 569 cores contain over 55,000 m of stratigraphy spanning the upper Cambrian to upper Silurian and cover an area of ~2400 km². Our study of the REE-phosphate-bearing Upper Ordovician and overlying Silurian carbonates combines elemental and stable isotope analyses with a variety of sedimentological and paleontological techniques to establish facies models and refine chronostratigraphy yielding evidence: 1) for the coeval deposition of REE-enriched phosphate and hematite deposits within flooded incised paleovalleys and their interfluves and 2) that the persistence of this dissected topography played a role in the spatial distribution of Silurian reefs.

The restudy of the TARP cores demonstrates the value of geologic collections in addressing societal needs. This study has advanced our understanding of local controls on the genesis of REE-enriched phosphorites and the siting of Silurian reefs that may provide potential benefits to the region’s aggregate industry.