CRITICAL ELEMENT ASSOCIATIONS WITH RARE EARTHS AND ASH YIELDS IN FORT UNION GROUP LIGNITES, WILLISTON BASIN, NORTH DAKOTA

Uncertainties in international supply chains prompted the U.S. Department of the Interior in 2018 to release its list of 35 critical mineral commodities considered vital to the nation’s prosperity. This list highlighted minerals and elements on which the U.S. is highly import-reliant, causing renewed interest in establishing domestic sources to relieve these strategic vulnerabilities. Coal has emerged as one possible alternative source of several of these commodities as it has been found to contain concentrations comparable to some conventional ores and mining infrastructure already exists across many U.S. basins.

Rare Earth Elements (REEs) in U.S. coal have been the focus of many recent studies, including by the North Dakota Geological Survey, which has collected and analyzed over 900 samples of lignite from the Fort Union Group since 2015. While promising REE concentrations have been identified, any future development of these resources may need to co-produce additional valuable mineral commodities to be commercially viable. Non-REE critical elements remain largely uncharacterized, however, especially in low-rank coals. The USGS COALQUAL database contains between 0 and 292 analyses of each of these elements in samples from North Dakota. Unknowns include: 1) the extent to which economic concentrations exist, 2) whether they occur in tandem with elevated REE concentrations, and 3) whether individual elements are associated with the organic or mineral fraction of lignites.

In this work, 327 North Dakota lignite samples with REE analyses (including Y and Sc) were examined for an additional 2 to 28 elements. New high concentrations were observed for As, Ba, Be, Cs, Cr, Ga, Ge, Li, Mg, Mn, Nb, Rb, Sb, Sn, Sr, Te, Th, Ti, U, V, W, and Zr relative to COALQUAL analyses reported for the state. Twenty of these elements (In, U, Co, Th, Bi, V, Rb, Cr, Hf, Sn, Mo, As, Be, Te, Ga, Cs, Li, Zr, Sb, and Ge) show moderate positive correlations to REE concentrations. Short prox data ranged from 5.96% ash by weight in relatively clean lignites to carbonaceous claystones up to 96.35% ash. Ten elements (Sn, Ti, Ta, Cs, Li, Rb, In, Bi, Te, and Ga) showed strong correlations to the ash yield, and another six (Nb, Th, Hf, Cr, V, and Mg) exhibited moderate positive correlations. Sr showed a moderate negative correlation.