RARE EARTH AND OTHER CRITICAL MINERAL ENRICHMENT IN FORT UNION LIGNITES BELOW UPLAND SURFACES IN SOUTHWESTERN NORTH DAKOTA

Coal has received considerable interest over the last decade as a potential alternative source of the rare earth elements (REE), but its competitiveness with traditional ores hinges on the identification of sufficiently enriched feedstocks. The North Dakota Geological Survey has collected and analyzed over 1,350 samples of lignite and carbonaceous mudstone for rare earth element concentrations since 2015. The samples were sourced from outcrops across southwestern North Dakota, a region of known coal mineralization where uraniferous lignites were mined in the 1950’s and 60’s. Concentrations from the tops of lignite beds have been identified as high as 1,598 ppm REE (including Y and Sc) on a dry coal basis and 5,642 ppm on an ash basis. REE concentrations over 800 ppm (dry coal basis) have now been identified in each coal-bearing unit of the Fort Union Group in North Dakota. These REE-enriched lignites are relatively infrequent, however, and the NDGS has examined multiple possible controls to explain their occurrence. Lignites are not especially enriched where in close proximity to volcanogenic sediments, nor do lignites below permeable lithologies like sandstone appear to receive statistically significant contributions from descending REE-enriched waters. Coals are known to be locally uraniferous in some of these settings, and REE results from lignites with elevated levels of radioactivity did not suggest a strong correlation.

One setting where REE enrichment does appear to commonly occur is the topographically highest carbonaceous beds of a given outcrop, especially where they lie immediately below a level upland surface. Southwestern North Dakota contains many flat-topped buttes and mesas which preserve remnant long-lived upland landscapes, some of which may have been stable and weathering since the Pliocene. Tracy Mountain, Mud Buttes, and many other isolated topographic prominences are characterized by 300 feet of section containing up to a dozen lignite beds which rarely exceed 300 ppm REE except in the upper 60 feet, where as many as seven thin lignites and carbonaceous mudstones exceed 400 ppm REE, up to 1,089 ppm (dry coal basis). Some of these lignites also contain concentrations of As, Ba, Be, Bi, Co, Ge, In, Li, Mo, Sb, U, and W that are 10 to 2500 times that of the upper continental crust.