271-13 Delimiting Stratigraphic Intervals of Rare Earth Element and Other Critical Mineral Enrichment in Fort Union Lignites, Williston Basin, North Dakota

Session: Geologic Energy Resources and Storage for Now and the Future (Posters)

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Abstract:

Lignite is a potential source of several critical minerals vital to the production of permanent magnets (Pr, Nd, Dy, Tb), semiconductors (Ga, Ge), and aerospace components (Sc). Coal typically contains low concentrations of these elements, but lignite can become highly enriched via bonding to the carboxylic acid groups that are present in lower ranks of coal. Much of this organically associated critical mineral content is easily extractable with dilute acids at mild temperatures, circumventing the costly extraction techniques from primary minerals that make many other potential sources subeconomic. The Williston Basin of North Dakota contains an estimated 1.3 trillion tons of lignite, but high concentrations of rare earth elements (the lanthanides, Y, and Sc) are limited to seams in unique geologic settings that were subjected to flows of enriched fluids. The Bear Den Member of the Golden Valley Formation (Paleocene-Eocene) and Rhame bed of the Slope Formation (mid-Paleocene) are acidic paleosols that were leached of rare earth and other mobile elements during periods of intense paleo-weathering. Lignites occurring just below these 5 to 10 meter-thick kaolinized horizons preferentially incorporated descending rare earth elements into their organic matrix and can be enriched as high as 4,443 ppm REYSc on a dry coal basis or 1.57% REO (including Y and Sc) on an ash basis. Although these concentrations are an order of magnitude higher than those proposed as promising for coal feedstocks, enrichment in the basin had previously only been documented in thin seams or restricted to the upper margins of thicker seams. New exploratory drilling shows that much thicker lignites are locally present within these two narrow stratigraphic windows where significant enrichment occurs. A 1.48-meter-thick interval of lignite and carbonaceous shale below the Bear Den Member in Mercer County contained an average of 716 ppm REYSc (dry coal basis), and a pair of lignites totaling 1.32 meters in thickness averaged 639 ppm below the Rhame bed in Adams County. Lignites at both sites included intervals exceeding 1% REO (ash basis). These cores and several others from the basin document that intervals of rare earth element enrichment are commonly greater than one meter thick, approaching seam thicknesses utilized in conventional lignite mining.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11146

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