REE Spatial Distribution and Mineral Association in Coal and Mudstone: Implications for REE Enrichment Mechanisms

Rare earth elements (REE) are crucial components of billions of products worldwide. Transitioning from foreign to domestic REE sources requires utilizing both primary (i.e., carbonatites, alkaline igneous rocks, pegmatites, skarn deposits) and secondary (unconventional) sources (i.e., ion adsorption clays, placer deposits, weathered rock, black and/or oil shales). Coal and coal-bearing strata, a promising secondary REE resource, are the focus of this study. Understanding REE mineral associations in unconventional resources is essential to quantify resource volume and identify viable mineral separation and processing techniques. Highly REE-enriched (>750 ppm) coal or mudstone samples from the Uinta Region were selected for scanning electron microscopic (SEM) analysis. Energy Dispersive X-ray Spectroscopy (EDS) determined REE enrichment occurs in: 1) a silt-size fraction (5–30 μm) of monazite and xenotime REE-enriched grains, 2) a clay-size fraction (2–5 μm) of monazite REE-enriched grains dispersed in the clay-rich matrix, and 3) organic-hosted REE domains >2 μm. Findings suggest possible REE enrichment from multiple sources, including: 1) detrital silt-size grains, 2) volcanic ash fall, largely in clay-size grains, and 3) organic REE uptake in the peat swamp depositional environment.