Critical Minerals in Late Cretaceous Coal Beds of The San Juan Basin, New Mexico
— Virginia T. McLemore and Evan J. Owen

Critical minerals, including rare earth elements (REE), are becoming increasingly more important in our technological society
and are used in many of electronic devices, batteries, and magnets. Rare earth elements include the 15 lanthanide elements (atomic numbers
57 to 71), yttrium (Y, atomic number 39), and scandium (Sc, atomic number 21). They are lithophile elements (elements enriched in the
crust) that have similar physical and chemical properties and occur together in nature. In New Mexico, low to moderate concentrations of
REE are found in 26 coalfields in Late Cretaceous coal-bearing strata of the San Juan Basin. These rocks are being characterized as part of
the U.S. Department of Energy’s (DOE) CORE-CM (Carbon Ore, Rare Earth, and Critical Minerals) program. Rare earth elements can be
measured in either the entire coal sample before burning (whole-rock basis) or in the ash after burning (ash basis). Because REE become
concentrated in ash after burning, REE concentrations in ash are greater than in whole rock. Measuring REE concentrations in ash approximates
REE concentrations in the fly or bottom ash remaining after coal is burned at a power plant. Rare earth elements can be leached from
power plant ash. Generally, REE concentrations in ash samples are higher on average in lower ash content of coal samples (560 ppm total
REE for samples containing <10% ash compared to 234 ppm for samples with >90% ash). Some of the highest total REE concentrations
(ash basis) are found in coal ash from the closed La Plata mine in the Fruitland Formation (2,103 ppm total REE), the closed Mentmore
mine in the Gallup Sandstone (807 ppm), as well as the Crownpoint (1,684 ppm), Standing Rock (523 ppm), Barker Creek (528 ppm), Mt.
Taylor (696 ppm), Star Lake (795 ppm), and Monero (1,026 ppm) coalfields in the San Juan Basin. Additional chemical and mineralogical
analyses are necessary to better understand the distribution and origin of critical minerals in coal deposits. As the demand for critical minerals,
especially REE, increases because of increased use and restricted supplies, the dollar value per ton of coal, coal mine waste, and ash
will rise, enhancing the feasibility of extracting and recovering these economically important minerals.