SIGNIFICANT ENRICHMENT OF RARE EARTH ELEMENTS AND OTHER CRITICAL MINERALS IN LIGNITES BENEATH PALEOCENE AND EOCENE WEATHERING PROFILES, WILLISTON BASIN, NORTH DAKOTA

The United States has prioritized the identification of domestic sources of critical minerals, increasingly vital components in the manufacturing of modern products, especially those needed for electrified energy infrastructure and advanced defense technologies. Lignite has received interest as a potential source, as low-rank coals can accumulate significant critical mineral contents from infiltrating fluids, and much of this organically associated trace element content can be easily remobilized and potentially recovered economically. The rare earth elements and other high-value critical elements like gallium and germanium are especially promising candidates to one day be commercially produced from pre-combustion U.S. lignite feedstocks. The North Dakota portion of the Williston Basin contains an estimated 1.2 trillion metric tons of Paleocene to Eocene age lignite in the Fort Union Group and Golden Valley Formation, which underlies roughly 83,000 square kilometers over the western half of the state.

The North Dakota Geological Survey has analyzed over 1,700 samples since 2015 to characterize the stratigraphic and geographic distribution of the rare earth element contents of these lignites, identifying two intervals that can exhibit significant enrichment. Both of these intervals occur below prominent ancient weathering profiles. The Bear Den Member of the Golden Valley Formation formed under the intense climate of the Paleocene-Eocene thermal maximum, and the Rhame bed formed during an apparent extended pause in deposition earlier in the Paleocene. Both intervals are characterized by 5 to 10 meters of kaolinized sediment, which is proposed to have weathered as acidic meteoric waters infiltrated soil profiles under warm and humid conditions. Rare earth elements leached from siliciclastic sediment into underlying lignites, where concentrations can reach 2,570 ppm (dry coal basis, including Y and Sc) beneath the Bear Den Member and 2,792 ppm beneath the Rhame bed.