Rodenberg reported in late April, 2023 that the North Dakota Department of Mineral Resources (DMR), in conjunction with State Geologist Ed Murphy, has announced that ND’s Geological Survey has discovered a nearly thirty-foot-thick interval of rocks containing untapped critical minerals.
Critical minerals are defined by the U.S. Government as minerals that are essential to the economic or national security of the states. Typically, these are minerals that make up vital components of modern technologies, especially energy infrastructures and those used in defense applications. However, as important as these minerals are, there is little to no production of them in the United States — and manufacturers in the country are often forced to rely on supplies from foreign countries, some of which are adversaries of our national interests. Much of the global supply of these more valuable rare minerals currently comes from South China.
Although their name suggests that rare minerals are difficult to find, they are relatively common in some types of rocks — however, they do not always concentrate into ores that can be mined. Currently, the US only has one such deposit (the Mountain Pass mine in CA), but this is not enough to fulfill the domestic demand for these materials. However, samplings from 2015 to recent times by the ND Geological Survey have identified a tremendous concentration of critical minerals throughout the Williston Basin.
The sampling project has produced one of, if not the most detailed data sets of enriched coal deposits in North America through the analysis of over 300 sites and 1,700 samples from western and south-central North Dakota. While these formations only represent a small amount of the total estimated lignite reserves in the state, testing has already suggested that ND has many deposits of critical earth elements that have not yet been tapped into. The largest of these, a brightly-colored area containing many enriched minerals, is known as the Bear Den Member of the Golden Valley Formation — which can most often be seen in upland areas covering 340 square miles over west-central North Dakota.
Samples of lignite coal and mudstones from the lower aspects of Bear Den Member were noted to contain up to 2,570 parts per million rare earth elements — which is believed to be the highest spot concentration ever reported from North American coal deposits (far exceeding the threshold of 300 per million in most areas considered ‘economic’). Enriched concentrations of critical minerals including cobalt, gallium, germanium, and lithium were also present in these samples.
The Geological Survey has already identified a handful of other weathering zones in the Williston Basin, which will be discussed in future reports sent out by the DMR. Another of these deposits, located 1,000 feet stratigraphically below the Bear Den Member, has also been confirmed to contain high concentrations of rare minerals and has been used to supply enriched lignite for research at the University of North Dakota.
The full details of the discovery are available to download on this page at the DMR’s Geological Survey website.
Lynn Helms, Director of the North Dakota Department of Mineral Resources, and Ed Murphy, North Dakota State Geologist, announced that the North Dakota Geological Survey has developed a comprehensive exploration model for lignite coals and organic-rich mudstones enriched in critical minerals. This model is based on the recent discovery of a thirtyfoot-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 which is typically present in upland areas covering 340 square miles 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.
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Critical minerals are commodities defined by the U.S. Government as essential to the economic or national security of the United States. They are increasingly vital components of modern technologies, especially the electronic components needed for an electrified energy infrastructure and advanced defense applications. There is little or no production of critical minerals in the United States, and domestic manufacturers depend on steady supplies from
foreign countries, many of them adversarial to U.S. national interests. 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 U.S. 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. Contrary to what their name implies, rare earths are present in many types of rocks and sediments, but these metallic elements rarely concentrate into ores that can be economically mined.
The U.S. has historically only developed one such deposit, the Mountain Pass mine in California, but it won’t be able to meet future domestic demand for rare earths. Igneous ores at Mountain Pass and other major mines are mostly enriched in the “light” rare earths, like cerium and lanthanum, resulting in overproduction and low prices for some elements and ongoing global supply shortages for others. These hard rock sources can also harbor large amounts of
the radioactive contaminant thorium, making the ores expensive to refine. Coal and coal ash generally contain much lower overall concentrations of rare earths, but with comparatively cheap mining costs, low amounts of thorium, and a higher proportion of the more valuable heavy 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. Most of the global supply of the more valuable, heavy rare earths currently comes from South China, where elements are naturally leached from igneous rocks and concentrated in thick soil horizons. The wet, humid climate is prone to acidic soils that break down granite into the clay mineral kaolinite, which absorbs rare earth elements from groundwater as it carries them through the soil profile. Water moves elements through rocks and sediments in North Dakota in similar ways, but because the climate is comparatively cool and dry, the weathering intensity is
much lower. Conditions were much different, however, in North Dakota’s past. The coal-bearing strata of the state were deposited in subtropical rivers, lakes, and swamps, and even preserve the record of one of the warmest and wettest climate events in Earth's history, the Paleocene-Eocene thermal maximum.
Intense weathering during this event, dated at 56 million years ago, left behind a thick sequence of kaolinite in the Bear Den Member of the Golden Valley
Formation, which is traceable across portions of seven counties in west-central North Dakota. Recent sampling by the ND Geological Survey shows rare earth elements and other critical minerals can become significantly enriched, especially when coal is present in the lower portion of this ancient soil profile.
Samples of thin lignite coals and organic-rich mudstones from the lower Bear Den Member contain up to 2,570 parts per million rare earth elements, believed to be the highest spot concentration yet reported from a North American coal and far exceeding the threshold of 300 parts per million considered potentially economic. Concentrations of many additional critical minerals were enriched in the same samples, including cobalt, gallium, germanium, and lithium.
Although the enriched lignite beds were too thin to be economically mined at the first few sites investigated in this report, lignite beds can thicken laterally over short distances and the potential exists for several feet of coal to be enriched at other sites. The identification of widespread enrichment below a prominent weathering zone, linked to a well-studied global warming event, also has important implications for coal-hosted critical mineral exploration in
other rocks in North Dakota, and in other U.S. basins. Enrichment may now be better explained in the context of sometimes subtle and localized evidence of ancient soil development rather than looking for influence from volcanic ash or groundwater flowing from distant sources.
The ND Geological Survey has already identified a handful of other weathering zones in the Williston Basin which 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.
The ND Geological Survey geologists concluded that lignites in the lower Bear Den Member of the Golden Valley Formation contain the highest and most widespread rare earth element concentrations yet described from the Williston Basin. Three samples from this interval contained over 1,820 ppm REE on a dry coal basis, which represent the first reported occurrences of significant rare earth element enrichment in North Dakota, defined as concentrations ten-fold that of average rocks and sediment near the Earth’s surface. One of these samples is, to the authors’ knowledge, the highest spot concentration yet
reported from a North American coal.
The Bear Den Member has been well-studied, as it records an ancient weathering profile created during an unusually warm and humid climate event called the Paleocene-Eocene Thermal Maximum. The acidic waters that percolated through the soil profile during this period weathered the feldspar and smectite-rich sediments to form a thick sequence of kaolinite, which primarily appears today as a bright white bed with orange iron staining that is easily traced across Dunn, Stark, and adjacent portions of the surrounding counties.
Rare earth elements and other critical minerals, which were originally present in low to moderate abundances in these sediments, were leached from the upper part of the weathering profile and redistributed into its lower portions. Mudstones and claystones in these lower positions can become slightly enriched (364 ppm REE or higher), but where lignites occur, organic complexes incorporate REE and other cations to a much higher degree. Enrichment over 910 ppm is common and significant enrichment occasionally occurs, depending on the position of the lignite within the profile. Significant enrichment of other critical minerals like antimony, arsenic, cobalt, germanium, lithium, molybdenum, and uranium also occurs within lignites in this zone.
Concentrations of beryllium, chromium, gallium, indium, tellurium, vanadium, and zirconium show at least slight enrichment and are noteworthy for their
similar overall distributions to REE through the profile. The identification of REE-enriched lignites within a well-characterized pedogenic context has important implications for explaining how REEs were transported to other lignites identified in this and other basins. The Bear Den Member is the most studied paleosol complex in North Dakota (because it is associated with a globally significant paleoclimate event), but although the earlier Paleocene climate was slightly less warm and humid, it was still capable of producing significant weathering profiles and thus leaching REEs. Thin, localized, weakly developed paleosols likely explain many of the otherwise “anomalous” 300 ppm REE samples encountered throughout the Fort Union Group over the course of this project. A more strongly developed, kaolinite-rich weathering profile approaching the thickness of the Bear Den occurs in the upper-middle portions of the Sentinel Butte Formation, and a second well-known Williston Basin paleosol, the Rhame Bed at the top of the Slope Formation, may have experienced an equivalent degree of weathering.
No comparable thermal events are known from the Paleocene, so these kaolinized zones may represent longer periods without deposition. The geochemical signals from these ancient periods of weathering are all overprinted by more recent fluids carrying uranium and more mobile elements through the rock column since the Miocene. To a smaller degree, REE mobilization has also likely occurred below some long-lived surfaces exposed to the low-intensity weathering of the Quaternary climate.
The enriched lignites identified in this report are likely too thin to be an economic source of REEs, and where thick coals did occur below the Bear Den Member, they were too deep to receive significant REE contributions from the overlying weathered zone. These lignites exhibit considerable lateral variability in thicknesses over short to intermediate distances, and this report investigated only a small handful of outcrops. The potential exists for a thick coal (several feet or more) to occur within the lower Bear Den Member.
Questions remain as to the total theoretical REE content a coal in this optimal position (low enough that it wasn’t leached by acidic waters but not so low that the mobilized REE were distributed through a considerable thickness of overlying sediment) would accommodate, but it would likely be the most economic lignite in the basin.
Work is currently underway by the ND Geological Survey to identify locations with thicker lignites in the lower Bear Den Member and to characterize the other known weathering profiles within the Fort Union Group.
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