Ancient subtropical soils may hold the key to critical mineral enrichment in the Williston Basin of North Dakota.

Lynn Helms, director of the North Dakota Department of Mineral Resources and Ed Murphy, North Dakota state geologist have announced that the North Dakota Geological Survey has developed a comprehensive exploration model for lignite coals and organic-rich mudstones enriched in critical minerals.

The model is based on the recent discovery of a thirty-foot-thick interval of rocks containing elevated concentrations of many critical minerals, some of which can be significantly enriched. This brightly colored rock unit is called the Bear Den Member of the Golden Valley Formation that is typically present in upland areas covering 340 square miles (547 kilometers) spread across west-central North Dakota. This important discovery is documented in a just-released 90-page report: ND Geological Survey Report of Investigation no. 133, available for free download on the DMR – Geological Survey website.

Some coal seams are known to contain elevated concentrations of valuable elements and have been proposed as a possible alternative source of critical minerals in the United States. These deposits are thought to be relatively uncommon but have been poorly researched. Questions remain about the exact geologic processes that enrich coal, as a result, it has been difficult to develop reliable exploration models for economic deposits.

Hydrothermal activity or volcanic eruptions have been identified as mechanisms that can transport critical minerals into coal in other basins, but evidence for neither pathway currently exists in North Dakota.

However, sampling by the ND Geological Survey since 2015 has identified elevated critical mineral concentrations scattered throughout the Williston Basin. This sampling project has produced one of the most detailed datasets of coal enrichment in North America through the analysis of over 1,700 samples from more than 300 sites across western and south-central North Dakota. Although these samples represent only a tiny fraction of the state’s estimated 25 billion tons of lignite reserves, the geochemical results from them have already offered new insights into the occurrence and origins of promising critical minerals, especially the rare earth elements.

The U.S. Department of Energy has estimated that a coal-related feedstock could be an economic source of rare earths in concentrations as low as 300 parts per million. Lignite, a low-rank coal, may be an especially promising source because of its ability to easily uptake and release rare earths, potentially making extraction comparatively low-cost and environmentally friendly.

The ND Geological Survey has already identified a handful of other weathering zones in the Williston Basin that will be the subject of future reports. One of those, located 1,000 feet stratigraphically below the Bear Den Member, also contains elevated concentrations of critical minerals and has been used to supply enriched lignite to the University of North Dakota’s Institute for Energy Studies for research on critical mineral extraction technologies.

The discovery and description of these deposits are important steps in the development of a comprehensive exploration model for the coal and mineral industry, with the potential to one day reduce the necessity of critical mineral imports, a strategic vulnerability of the United States.