Critical Minerals in Sedimentary Rocks

Critical minerals are used in modern technology products and are necessary for energy and defense. Rare Earth Elements (REE), such as Nd, Dy, Pr, are particularly essential for magnets and metal alloys for energy related products. Critical mineral exploration is still nascent and growing exponentially to meet the energy and defense demand. Delineating geologic controls on critical mineral enrichment trends is key to developing predictive exploration tools. Exploratory research across Utah and western Colorado the past five years reveals key geologic trends in the occurrence of REE in coal-bearing strata and phosphatic limestones. Portable X-ray fluorescence (pXRF) was combined with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to quantify REE enrichment within a stratigraphic context. In coal-bearing strata, REE-enrichment (> 200 ppm) is most commonly found in mudstone units overlying, underlying or within coal zones, with a strong preference of REEs in claystone sublithologies. Coal lithologies are generally not REE-enriched, with notable exceptions. REE-enriched coals exhibit lower coal quality (i.e., dull and brown colored bituminous coals). Additional petrography combined with scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) reveal silt-sized detrital monazite grains, as well as clay-sized REE-enriched domains in the matrix and organic matter. REE-enrichment results from a combination of paleo-depositional and diagenetic mechanisms. We provide a recommended coupled pXRF and ICP-MS approach for quantifying REE in sedimentary resources, broadly. These geologic trends can be used as an REE resource prediction tool, particularly in bituminous coal-bearing successions worldwide.

In Permian phosphatic limestones from actively mined phosphatic ore show critical mineral enrichment. Specifically, grainstones are REE and Cr enriched, likely hosted in francolite peloids. Specific geologic and climatic conditions during the Permian Phosphoria Basin deposition allowed for extensive, REE-enriched, phosphorite deposition.

This work identifies several key geologic controls on critical mineral enrichment which can be used as a predictive energy and mineral exploration tool in sedimentary rocks globally.