The association between coal and REE deposits has been well documented elsewhere, but little data had been previously analyzed relevant to Utah and Colorado’s once busy coal fields, which have fallen on hard times as domestic demand for coal has shriveled. Among a longer-term decline, however, remaining active coal mines in Utah and Colorado report that they can’t mine fast enough in recent years to meet demand and high coal prices.

“The goal of this phase-one project was to collect additional data to try and understand whether this was something worth pursuing in the West,” said study co-author Michael Vanden Berg, Energy and Minerals Program Manager at the Utah Geological Survey. “Is there rare earth element enrichment in these rocks that could provide some kind of byproduct or value added to the coal mining industry?”

The study targeted the coal-producing region stretching from Utah’s Wasatch Plateau east across the Book Cliffs deep into Colorado. Researchers analyzed 3,500 samples from 10 mines, four mine waste piles, seven stratigraphically complete cores, and even some coal ash piles near power plants.

The study included Utah’s active Skyline, Gentry, Emery and Sufco mines, recently-idled Dugout and Lila Canyon mines in the Book Cliffs, and the historic Star Point and Beaver Creek No. 8 mines. The Colorado mines studied were the Deserado and West Elk.

Analyzing rock samples by the thousands

“The coal itself is not enriched in rare earth elements,” Vanden Berg said. “There's not going to be a byproduct from mining the coal, but for a company mining the coal seam, could they take a couple feet of the floor at the same time? Could they take a couple feet of the ceiling? Could there be potential there? That's the direction that the data led us.”

To gather samples, the team worked directly with mine operators and examined coal seam outcrops and processing waste piles. In some cases, they analyzed drilling cores, both archived cores and recently drilled ones at the mines. The team entered Utah mines to collect rock samples from the underground ramps that connect coal seams.

Researchers deployed two different methods to record levels of REE’s present, expressed in parts per million, or ppm, in the samples. One was a hand-held device for quick readings in the field, the other used Inductively Coupled Plasma-Mass Spectrometry, or ICP-MS, in the on-campus lab overseen by Fernandez.

“We’re mostly using this portable X-ray fluorescence device, which is an analysis gun that we hold to the rock for two minutes, and it only gives us five or six of the 17 rare earth elements,” Birgenheier said. If samples showed concentrations higher than 200 parts per million, or ppm, they ran a more complete analysis using the mass spectrometry equipment on campus.

The Department of Energy has set 300 ppm as the minimum concentration for rare earth mining to be potentially economically viable. However for the study, researchers deemed concentrations greater than 200 ppm to be considered “REE enriched.”

The study found the highest prevalence of such concentrations in coal-adjacent formations of siltstone and shale, while sandstone and the coal itself were mostly devoid of rare earths.

The team has analyzed 11,000 samples to date, far more than were used in the published study. The next steps include determining how much rare earth ore is present, likely to be done with colleagues at the University of Wyoming and New Mexico Institute of Mining and Technology.

“We still have results that are ongoing and papers that'll be coming out soon,” Birgenheier said. “We’re writing a proposal now for phase two. We can't make resource volume estimates yet because we don't have that data. This next phase will push us towards answering, ‘how do we actually calculate a volume of rare earths in these deposits?’”

How did the elements get there?

The study did not identify the geological process that enriched the coal-adjacent formations, but Birgenheier has a few theories. Many of the Utah coal-bearing formations were deposited during the Cretaceous period that ended 66 million years ago, a time when the western U.S. was volcanically active.

“There are two models. One is maybe volcanic ash brought rare earths into ancient peat bogs,” she said. “The other is there's evidence that terrestrial organic material in the peat bog actually takes in heavy rare earths.”

Then, through time, heat and burial, the peat bogs enriched in rare earths became Utah and Colorado coal deposits.

“We think rare earths were in the coals and have migrated into the adjacent mudstones or siltstones above and below the coals,” Birgenheier explained, “probably through a process called diagenesis, basically any fluid movement that happens in the rock after it was deposited.”

Titled "Rare earth element enrichment in coal and coal-adjacent strata of the Uinta Region, Utah and Colorado," the study appeared on April 26 in Frontiers of Earth Science.