UPPER ORDOVICIAN REE-ENRICHED PHOSPHORITES IN THE CENTRAL U.S. — STATUS AND NEW DEVELOPMENTS

Earth MRI regional geochemical reconnaissance of Ordovician REE-enriched phosphorites across the central U.S. has added knowledge about the age, distribution, and origin of these little known critical mineral resources. Results demonstrate that phosphorites are restricted to the Upper Ordovician, which contains a total of 12 mappable REE-enriched phosphate intervals. One of the most highly prospective is found near the base of the Maquoketa Shale, informally known as the “Graf Phosphorite”.

Preliminary data in the Dubuque area of northeastern Iowa and adjacent portions of Illinois and Wisconsin indicate the Graf Phosphorite is up to 15 m-thick, divisible into three sub-units, and contains horizons with up to 25% phosphate showing REE enrichment. Whole-rock geochemistry of the purest end member suggests francolite in the Graf Phosphorite averages ΣREE of ~1600 ppm with 800 ppm attributable to ΣHREE. This level of ΣHREE enrichment is considerably higher than currently mined Miocene and Permian phosphate deposits in the U.S. (~500 ppm HREE; Emsbo et al., 2015).

Our new Earth MRI project focuses on detailed mapping of REE-enrichment in the Graf Phosphorite of the Dubuque area. Across this area the Graf Phosphorite is covered by unconsolidated sediments and vegetation broken only by a few road cuts and quarries. Strategic coring across the study area is fundamental to generating new geochemical data sets needed for mapping phosphate and REE concentrations and thickness and overburden characteristics. This mapping will be enhanced by aerial electromagnetic survey using a frequency-domain system to provide the best near-surface resolution available. The combination of subsurface study, geochemistry, and geophysics will generate a detailed 3-dimensional foundation from which this unconventional critical minerals resource can be further evaluated.