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Article

REY Spatial Distribution and Mineral Association in Coal, Carbonaceous Shale and Siltstone: Implications for REE Enrichment Mechanisms

by
Laura Wilcock
1,*,
Lauren P. Birgenheier
1,
Emma A. Morris
2,
Peyton D. Fausett
1,
Haley H. Coe
1,
Diego P. Fernandez
1,
Ryan D. Gall
3 and
Michael D. Vanden Berg
3
1
Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA
2
Department of Earth and Ocean Sciences, University of North Carollina Wilmington, Wilmington, NC 28403, USA
3
Utah Department of Natural Resources, Utah Geological Survey, Salt Lake City, UT 84116, USA
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(8), 869; https://doi.org/10.3390/min15080869
Submission received: 28 May 2025 / Revised: 26 July 2025 / Accepted: 11 August 2025 / Published: 18 August 2025
(This article belongs to the Special Issue Green and Efficient Recovery/Extraction of Rare Earth Resources)
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Abstract

Rare earth elements (REYs) are crucial components of billions of products worldwide. Transitioning from foreign to domestic REY 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, promising secondary REY resources, are the focus of this study. Understanding REY mineral associations in unconventional resources is essential to quantifying resource volume and identifying viable mineral separation and processing techniques. Highly REY-enriched (>750 ppm) coal or mudstone samples from the Uinta Region, Utah, USA, were selected for scanning electron microscopy (SEM) analysis. Energy dispersive X-ray spectroscopy (EDS)-determined REY enrichment occurs in: (1) a silt-size fraction (5–30 μm) of monazite and xenotime REY-enriched grains, (2) a clay-size fraction (2–5 μm) of monazite REY-enriched grains dispersed in the clay-rich matrix, and (3) organically confined REY domains < 2 μm. Findings suggest possible REY enrichment from multiple sources, including: (1) detrital silt-size grains, (2) volcanic ash fall, largely in clay-size grains, and (3) organic REY uptake in the peat swamp depositional environment.

1. Introduction

Due to their similar properties, critical minerals (CMs), rare earth elements (REYs), the lanthanide series (La through Lu) of elements, and often yttrium (Y) and scandium (Sc) are crucial in military defense applications and green energy technologies. Despite their relative abundance in crustal rock, sufficient quantity and quality in surficial deposits are much more difficult to source [1,2,3,4,5,6,7,8,9,10].
Moving away from foreign to domestic sources of CMs and REYs requires utilization of both primary ore resources (e.g., carbonatites, alkaline igneous rock), secondary source materials (e.g., ion-adsorption clays, placer deposits), and alternative or unconventional resources (coal, waste rock from coal mines, phosphorites) known to contain viable amounts for industrial processing [11,12]. The most abundant resources for economically viable REYs are typically found in primary igneous deposits and are predominantly concentrated in bastnasite [(Ce, La)(CO3)F], monazite [(Ce, La)PO4)], xenotime (YPO4), loparite [(Ce, Na, Ca)(Ti, Nb)O3], apatite [(Ca, REY, Sr, Na, K)3Ca2(PO4)3(F, OH)], and ion-adsorption clays [2,5,6,13,14]. Bastnasite, monazite, and xenotime make up 95% of the world’s REY reserves [14,15].
China currently dominates the REY industry with abundant production and distribution of REY resources from secondary sources, supplying over 90% of the global demand [6,8,16]. Due to limited surficial resource abundance and geopolitical concerns, increased attention has focused on coal and coal by-products as unconventional resources for REYs [5,17,18,19,20,21,22]. Current research points toward the possible utilization of REYs in coal and coal-adjacent strata as viable secondary sources for REYs and CMs [10,14,21,22,23,24]. Coal and other coal-related materials are particularly promising as sources of valuable HREEs, particularly compared with primary igneous sources [5,19,25]. This study leverages a recent comprehensive study by Coe et al. (2024) [10] that supports REY enrichment (>200 ppm) in the coal-related materials of the Uinta Region of Utah and western Colorado specifically.
Understanding the mineral association of REYs in these unconventional resources is imperative for future work aimed at quantifying and characterizing potential resource volume. Extraction of REYs from coals and carbonaceous shales/mudstones relies on several factors such as the REY-bearing mineral type, crystallinity, and spatial distribution, as well as the nature of associations with other minerals and/or the sample matrix [19,21,22,24,26,27,28].
Previous work highlights the complicated nature of identifying definitive REY-enrichment mechanisms in coal and coal-adjacent strata due to REYs and/or CMs displaying both organic and inorganic associations [21,22,23,29,30]. Depositional conditions can control these associations as well as sediment provenance geochemistry, volcanic input via ash, redox conditions, or other diagenetic processes that may occur after deposition [14,30,31,32,33]. Multiple REY sources and mineral associations may be present, making correlations of REY-enrichment mechanisms difficult to interpret.
Both LREEs and HREEs in coal have been associated with organic and inorganic sources [17,34,35]. Modes of occurrence are dependent on depositional as well as geological factors. Differentiation of LREEs vs. HREEs has been shown, in previous work, as a metric for analyzing possible mechanisms for REY enrichment, particularly when attempting to determine grain provenance or diagenetic conditions leading to present anomalous concentrations of select REYs [17,32,36,37].
This research builds on prior studies by analyzing REY-enrichment patterns at the micro- to nano-scale in coal and coal-adjacent samples in the Uinta Region. The Uinta Basin Region has been the subject of previous work and presents an ideal location for the purpose of this study. In active and historical coal mines, along with robust data-dense sample sets, heterogeneity provides variability in samples, which could add to the understanding of enrichment patterns of REYs in coal and coal-adjacent strata. We develop a novel, systematic, and thorough approach to scanning samples for REYs using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) to (1) identify mineral associations of REYs in samples from the Uinta Region and (2) document the spatial distribution, grain size fractions, and morphology of REY-hosted minerals. Observational data regarding REY-bearing grain size, morphology, and spatial distribution, as well as EDS-based REY mineral association interpretation, are used to uniquely illuminate depositional and diagenetic mechanisms for REY enrichment in coals and coal-adjacent strata.

2. Materials and Methods

2.1. Geological Setting

Coal-bearing strata and adjacent units in the Uinta Region of Utah include the Ferron Sandstone Member of the Mancos Shale as well as the Blackhawk Formation (Figure 1). Triassic igneous provinces along the western margin of the North American continent shed vast quantities of sediment eastward into the Western Interior Seaway with peak plutonism occurring during the Late Cretaceous [38,39]. Late Cretaceous crustal thickening and subsidence due to loading from the Sevier Fold and Thrust Belt led to the development of the Western Interior foreland basin [40,41,42,43]. Cretaceous coal-bearing units of the Uinta Region record coastal plain and deltaic deposition along the shoreline of the Western Interior Seaway, as clastic sediment was shed off the fold and thrust belt into the foreland basin [38,39,40].
The Turonian-age Ferron Sandstone is a relatively thin member of the Mancos Shale (Figure 1). It is underlain by the Naturita Sandstone and Tununk Shale and overlain by the Blue Gate Shale and Emery Sandstone Members of the Mancos Shale [50,51,52].
The Ferron Sandstone contains coal, sandstone, and mudstone, including carbonaceous shale. Interpreted as a primarily fluvial coastal plain with river-to-storm-dominated deltaic deposits, the Ferron Sandstone has abundant bituminous coal resources currently in production from the Emery underground mine.
The Campanian-age Blackhawk Formation is overlain by the fluvial Castlegate Sandstone and underlain by the shallow marine Star Point Sandstone, which in turn overlies the Mancos Shale (Figure 1). The Blackhawk Formation contains bituminous coal resources as well as shale, siltstone, carbonaceous shale, and sandstone that records fluvial, coastal plain, and shallow marine depositions [53]. The Blackhawk Formation progrades eastward into the Western Interior Seaway and interfingers with the offshore marine mudstone of the Mancos Shale [54,55].
Prior work completed in the Mancos Shale showed abundant illite, with minor kaolinite and smectite presence also reported [56]. The abundance of illite may be explained by the diagenetic conversion of smectite during thermal maturation, which can be evidenced by the presence of bituminous ranked coal beds throughout the adjacent strata [57].

2.2. Sample Selection

A large, previously developed portable X-ray fluorescence (pXRF) and inductively coupled plasma mass spectrometry (ICP-MS) geochemical dataset consisting of samples collected from historical mines, inactive mines, outcrops, and stratigraphically complete cores throughout the Uinta Region was utilized for scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS) sample selection [10,11]. This robust dataset includes coal and coal-adjacent claystone, siltstone, carbonaceous shales, and sandstones from the Blackhawk Formation, n = 3113 [10], and the Ferron Sandstone, n = 6641 [11]. The dataset from the Blackhawk Formation and Mesaverde Group, n = 3113, reveals economically viable levels of REY enrichment in shales, with over 40% of the shale samples showing REY concentrations that are considered to be enriched (>200 ppm), when analyzed via pXRF or ICP-MS [10]. Similarly, the Ferron Sandstone Member displays comparable, if not higher, levels of REY enrichment in ongoing research [11].
Highly REY-enriched samples from the Ferron Sandstone Formation and Blackhawk–Mesaverde datasets, as determined by ICP-MS or pXRF analysis, were prioritized to maximize REY detection using SEM–EDS imaging and quantification techniques [58,59]. Preliminary analysis suggested that the quality of the EDS spectra of the REYs was particularly robust for samples with REY concentrations greater than 750 ppm, as determined through ICP-MS or pXRF geochemical characterization. ICP-MS geochemical analysis included the entire suite of REYs (Sc, Y, and the lanthanides), while the pXRF geochemical dataset only included Sc, Y, Nd, Ce, and La.
For this study, baseline levels of REY enrichment greater than 750 ppm were determined to be most suitable for SEM–EDS analysis. Six samples from the Blackhawk Formation and the Ferron Sandstone were selected based on known combined concentrations of La, Ce, Pr, Nd, Y, and Sc being present and being greater than 750 ppm when using either pXRF or ICP-MS analysis. REY-enrichment quantification and geochemical analysis methods of the larger dataset (n = 9754) and the subset of samples in this study (n = 6) are further described in Coe et al. (2024) [10] (Table 1).
Lithologies of the six samples include: two coals, two carbonaceous shales, and two siltstones. Of the two coals, one sample was sourced from the Ferron Sandstone Member and the other from the Blackhawk Formation. The Ferron Sandstone uses letter nomenclature to identify coal seams. The Ferron Sandstone coal used in this study comes from the ‘C’ coal seam (MC-C) and was collected from an outcrop near Miller Canyon in the Emery Coal Field as part of a measured section, with sampling and geochemical data reported in Birgenheier et al. (2024) [11] (Figure 2). The second coal sample, BVCA-4, from the Castlegate A coal seam, is a Utah Geological Survey archived hand sample from the Beaver Creek closed historic mine [10].
Both carbonaceous shale samples are from the Ferron Sandstone, though sourced from different locations and coal seams within the Emery Coal Field. MC-G-02-03 (MC-G) is a carbonaceous shale located stratigraphically below the ‘G’ coal seam in Miller Canyon from an outcrop [11]. WF-M-10-03 (WF-M) is a carbonaceous shale from the Walker Flats shallow mine and is stratigraphically between two subsections of the ‘M’ coal seam [11]. Carbonaceous shales have varying mineral compositions, are typically rich in clay, and contain a slightly higher than 10% fraction of organic constituents.
One siltstone sample, IPA-1-1673.6 (IPA-1), was sourced from the IPA-1 core of the Blackhawk Formation in the Book Cliffs Coal Field, sampled at a 1673.6 ft (510.10 m) core depth. IPA-1 is a fine-grained siltstone that is stratigraphically below the Sunnyside coal seam of the Blackhawk Formation [10]. The Rockland-JM-02-07-08 (Rockland) sample is a coarse- to medium-grained siltstone that is stratigraphically above a subsection of the Ferron Sandstone ‘J’ coal. It was taken from the measured section of the Rockland shallow mine in the Emery Coal Field [11].

2.3. Petrography and SEM–EDS

Twelve ultra-thin sections—six individual samples with a duplicate—were prepared at a 15–20-micron thickness by Wagner Petrographic, Lindon, Utah, USA. Bedding direction could not be inferred from the collected hand samples; thus, they were cut randomly relative to bedding.
Prior to SEM–EDS analysis, thin sections were petrographically imaged at the University of Utah, Utah, USA, for sample characterization and potential grain analysis. Petrographic images were used to develop a mount map to guide strategic SEM analysis in order to reduce the redundancy of scans and/or to avoid missing possible targets (Figure 3).
Thin sections were carbon coated to reduce charging inside the SEM chamber. Processing of samples was completed on an FEI Teneo SEM with a Trinity Detection System and an EDAX Octane Elite system at the Utah Nanofab, located at the University of Utah, Salt Lake City, UT, USA. An average accelerating voltage between 15–20 keV, depending on the sample, was used—with an average working distance between 8.5–11 mm.
Typically, the accelerating voltage is set at 2–3 times the critical ionization energy of the elements of interest [60]. The use of overvoltage limits resolution to about 2 μm, meaning particles smaller than 2 μm were detectable but difficult to discern or accurately image in detail.
SEM–EDS analysis was completed using a modified methodology outlined by Fu et al. (2024) [23] and Ji et al. (2022) [61]. Fu recognized that the intensity of backscattered electron (BSE) signals is directly proportional to the average atomic number within the minerals present in the sample. To strategically analyze samples, Ji et al. (2022) [61] further refined the methodology for probing areas of interest with EDS mapping. Areas of interest that contained suspected heavy elemental concentrations (bright grains or zones) were scanned using EDS briefly (≤10 scans) to determine if REYs were present in the scan. If this brief EDS analysis of the area of interest showed detectable levels of REY enrichment, then a more detailed EDS analysis of the zone was performed. Specifically, the more detailed EDS analysis was longer (≥30 scans) and at a higher magnification (Figure 4 and Figure 5).
Scans of the samples and images collected were performed with backscattered electrons before EDS analysis. Grid pattern exploration was utilized to better analyze areas of interest. Spots where minerals or zones were targeted for EDS analysis were marked on the sample map to prevent re-scans of the same mineral or zone.
Since Iridium (Ir), Platinum (Pt), Lithium (Li), Cobalt (Co), Nickel (Ni), Vanadium (V), Manganese (Mn), Germanium (Ge), and Gallium (Ga) were deemed to be particularly valuable non-REY CMs, they were also included in EDS analysis and search criteria.
LREEs were the primary fraction of REYs detected during EDS and consisted predominantly of Nd, Pr, La, and Ce. Though some HREEs were present in minor quantities, LREEs were the most abundant REY fraction identified in substantial, detectable concentrations during detailed EDS mapping.

3. Results

3.1. ICP-MS Geochemical Sample Characterization

ICP-MS data were normalized to upper continental crust (UCC) values in order to more strategically analyze the sample dataset’s REY relative enrichments or depletions as well as to identify any distinct patterns in element mobility (Table 2) [62]. To better constrain and differentiate between geochemically distinct groups of REYs, this study utilizes REYs to denote relative totals of Sc, Y, and the lanthanides, whereas LREE is used in reference to La through Eu and HREE is used in reference to Gd through Lu.

3.2. Petrography and SEM–EDS Grain Characteristics

General petrographic observations of grain assemblages of carbonaceous shales and siltstones show that the samples contain both clay-size and silt-size grains with size fraction characteristics typical of the host lithology. Quartz is abundant in mudstones and occurs in both clay-size and silt-size fractions. Coal samples also contain smaller fractions of fine to very fine silt and clay-size grains of various mineralogy, largely quartz, though these individual grains are interspersed throughout the clay within the sample’s matrix. The growth of framboid pyrite and microquartz in pore spaces is also observed in several samples. SEM observations of clay morphologies show consistent dominance of illite in prepared thin sections.
A total of 135 grains (n = 135) were analyzed through EDS. Grain selection for EDS processing was based on the intensity and brightness as described in the methodology. Of the 135 grains analyzed in EDS, a total of 15 (n = 15) individual grains and/or zones within the sample were confirmed to bear REYs (Table 3; Figure 6).

3.3. Silt-Size REY-Enriched Grains

Based on SEM petrographic observation, the identified REY-enriched grains were binned into three size fractions for differentiated characterization (Table 3; Figure 6). The three size categories include: (1) 5 μm to 30 μm size grains, (2) 2–5 μm, and (3) <2 μm. The identified REY-enriched grains that are between 5 μm to 30 μm are defined as silt-size grains as per the Udden–Wentworth grain scale. They include fine and medium silt-size grains. The identified REY-enriched grains that are between 2-5 μm in size are defined as largely clay-size grains (<4 μm) and include very fine silt-size grains (4–5 μm). For simplicity, this study refers to grains of 2–5 μm as “clay-size” and grains of 5 μm to 30 μm as “silt-size”. Finally, zones of REY enrichment less than 2 μm were also identified. These zones cannot be properly sized or characterized due to limitations in SEM imaging resolution at high accelerating voltages. However, observed zones of REY enrichment less than 2 μm are only documented in two cases in one sample, MC-G (Table 3). There, they occur within organic matter in the sample.
Silt and clay-size grain size fractions are consistent within and between samples. For example, all REY-enriched grains found within the Rockland sample are silt-size grains (Figure 6). REY-bearing grain size fractions that are present vary by sample lithology. Of the fifteen grains or organic zones that contain REY-enriched grains, five are silt-size grains (Table 3; Figure 6). These five silt-size grains are found within three samples—one carbonaceous shale (WF-M) and the two siltstones (IPA-1 and Rockland). Eight dispersed clusters of clay-size grains were found only in the two coal samples, which include coal from the Blackhawk Formation and the Ferron Sandstone (Table 3; Figure 6).
The identified REY-enriched silt-size grains are generally subrounded to subangular with varying degrees of sphericity (Figure 6). The REY-enriched silt-size grains share similar shape and size distributions as well as random orientation as the surrounding non-REY-bearing grains (Figure 7A). The spatial distribution of REY-enriched silt-size grains is unevenly spread throughout the sample, indicating that there is no strong spatial trend or zones in which the REY-enriched grains are concentrated.
EDS analysis shows that REY-enriched silt-size grains are primarily phosphates with some trace signatures of other elements such as Ca, Mg, Na, S, Cl, and Fe. Trace element presence not typical in phosphates may indicate spectral interference from the adjacent grains and/or the surrounding clay matrix [61]. REY-enriched phosphates also carry a typical associated Th signature. This, combined with the presence of Ce, Pr, Nd, and/or La concentrations, indicates that silt-size phosphate grains are predominately monazite, specifically monazite-Ce, due to the higher spectral signature of Ce in relation to the other REYs (Figure 7B,C). Although the primary enrichment of REYs comes from monazite, xenotime is also present as a single silt-size grain in one sample of siltstone (Table 4).

3.4. Clay-Size REY-Enriched Grains

Two samples contain large fractions of clustered REY-enriched clay-size grains or zones 5 μm to 2 μm in size (Figure 6). Both samples that contain this size fraction of REY-bearing grains or zones are coal: MC-C-01-06 (MC-C) and BVCA-4. Resolution limitations with the high accelerating voltage required to detect REYs, coupled with the clustered and ambiguous nature of REY-enriched clay-size grains and/or zones, make exact measurement of grain size and shape difficult to precisely discern via SEM petrography. Where resolution allows for differentiation between individual grains, REY-enriched clay-size grains in coal sample MC-C appear globular and show highly angular grain boundaries in some instances (Figure 6 and Figure 7D). Clay-size REY-enriched grains in MC-C are typically 5 μm or less in size and are considerably smaller than other non-REY-enriched clay-size grains found within the sample. Clay-size zones of REY enrichment were identified in coal sample BVCA-4.
The abundance and overall spatial distribution of REY-enriched clay-size grains or zones within the two coal samples are notable. REY-bearing clay-size grains can be found in small to large clusters across the denser aluminosilicate-rich domains of the two coal samples analyzed (Figure 7D–F and Figure 8).
Non-REY-enriched clay-size grains in the coals do not display the same spatial distribution pattern and are far more randomly dispersed throughout the rest of the sample. It is notable that REY-bearing clay-size grains and zones appear to be spatially distributed only across or within clay zones in coal samples BVCA-4 and MC-C (Figure 9).
EDS analysis shows that the elemental assemblages of clay-size grains within the coals are phosphates. REY-enriched clay-size grains are determined to be monazite given their elemental properties and high concentrations of Ce, which further support that they are monazite-Ce (Figure 7D–F).

3.5. Organically Associated REYs

Sample MC-G, a carbonaceous shale, did not contain any identifiable REY-enriched individual grains, though two zones of weak REY enrichment within the organic material were isolated and analyzed (Figure 6: sample MC-G-02-03). Weak REY enrichment found within the organic zones is difficult to quantify due to spectral overlap and background interference; however, EDS analysis shows two distinct spots of minor and dispersed concentrations of Ce, La, Pr, and Nd present within the organic constituents of sample MC-G (Figure 10). Further, EDS spectral analysis of the adjacent clay and organic matter showed no detectable REY enrichment (i.e., the spectral peaks from the surrounding clay and organic domains are distinctly different than Figure 10C), making these two fractions of organically bound REY enrichment unique within the analyzed areas of the sample.

3.6. LREE-to-HREE Ratios and Non-REY CMs

Lithological differences between LREEs/HREEs are present, as siltstone displays slightly lower LREE/HREE concentration ratios compared with coals and organic-rich shales (Table 5, Figure 11). Overall, carbonaceous shales and coals show a higher enrichment of LREEs to HREEs when normalized to average UCC values (Figure 11). Significant concentrations of non-REY CMs were not observed or detected in EDS analysis of the analyzed samples.

4. Discussion

The enriched silt-size grains of monazite and xenotime are interpreted as largely detrital in origin and are typically found in the siltstone samples. The REY-bearing silt-size grains share similar size and shape characteristics as the other non-REY-bearing grains throughout the samples, despite being geochemically and mineralogically unique from the surrounding non-REY-bearing grains. The REY-bearing and non-REY-bearing silt-size grains within the sample have low sphericity and are subangular to subrounded, as is typical of detrital silt-size grains [64,65,66]. The REY-hosted silt-size grains are uncommon in all the samples, and the spatial distribution does not bear any notable pattern compared with the other sample grains, suggesting that they were transported and deposited concurrently with all the other detrital silt grains in the sample.
Minor to significant negative EuN anomalies present in the REY-hosted silt-size grains provide further evidence of the detrital nature of the grain’s depositional mechanism (Figure 12). Negative EuN anomalies can be attributed to felsic detrital sediment input [37]. The nature of the grain distribution, size, and shape along with the negative EuN anomalies present support a detrital origin of REY enrichment of some of the silt-size grains found in the siltstone samples (Figure 13A).
Enriched clay-size grains are interpreted as volcanic in origin and are found exclusively in coal samples (MC-C and BVCA-4). REY-hosted clay-size grains, identified as monazite-Ce, are atypical of the sample’s other, non-REY-bearing grains in both size and shape. Monazite-Ce clay-size grains—present as clustered groups and distributed within clays in the sample—appear to be ≤5 μm in average size, are generally high in sphericity, and are highly angular in shape, which would be expected for a volcanic air fall origin [70,71].
High sphericity likely developed in lapilli and rapid cooling during air fall, whereas highly angular grain shapes from igneous crystals would also be expected. Evidence of notable episodic eruptions from the volcanic arc is recorded in the numerous bentonite deposits documented throughout the Mancos Shale [38,39]. Reworking of bentonite beds may also contribute to volcanic ash and epiclastic material in the study area strata. Coal sample BVCA-4 displays small clusters of phosphate and REY-enriched clay-size particles that appear randomly dispersed in clay zones within the organic matter (Figure 6). The nature of these observed REY enrichment patterns in clay-size grains can be interpreted as probable adsorption of REYs into clay particles through fluid mobility during digenesis [72]. This uptake of REYs lends support to volcanic ash fall in that the mechanism of adsorption occurred post deposition when REYs leeched from volcanic ash that was deposited adjacent to the coal [72].
Alternatively, because clay-size grains are found in clay mineral domains and not in organic matter domains, it is possible REYs were adsorbed onto clays through diagenetic fluid mobility [72]. It is also possible that depositional and diagenetic mechanisms contributed in combination to explain why REY-enriched monazite grains are restricted to clay zones and not dispersed throughout the sample’s organic domains. Previous studies have noted that leaching of volcanic ash minerals from underlying and/or overlying strata may have contributed to REY incorporation of authigenic monazite during diagenetic fluid interaction [72,73].
The coal samples used in this study are highly enriched with LREEs compared with average UCC values and even among the other samples in this study (Figure 13). The relatively high LREE concentrations of coal samples further demonstrate plausible enrichment mechanisms through the deposition or adsorption of inorganic LREE-enriched volcanic ash and detrital input from sediment source regions rather than organic uptake that would typically favor HREEs. However, it is important to note that there are limitations on the detection of HREEs in EDS using the methodology outlined in this study (Figure 13A) [14,30,31,32,33,67].
Coupled with high LREE concentrations, positive Ce and Eu anomalies are present with clay-size grain monazite-Ce found in coal samples (Figure 12). Positive Ce and Eu anomalies are typically attributed to volcanogenic influence on peat mire environments, with the latter anomaly also indicating possible injection of hydrothermal fluids during late deposition/diagenesis (Figure 13A,B) [17,32,36,37]. However, the lack of fracture sulphate, carbonate, or phosphate infillings in the observed samples suggests that diagenetic hydrothermal fluid interaction was unlikely.
The organic zones of enrichment found in one carbonaceous shale sample are interpreted as a result of REY uptake into plants, deposited in peat mires, and hosted in organic matter that was then subject to mineral influxes from diverse sources as well as diagenetic processes [14,31,32,67,74,75]. This interpretation is supported by zones of organic matter showing minor detectable levels of Ce, Pr, Nd, and La.
Identified REY-bearing silt-size grains, clay-size grains or zones, and zones of inferred organic REY associations highlight multistep diagenetic and paleodepositional processes that led to the enrichment of the Uinta Region samples (Figure 13A,B). The REY enrichment in silt- to clay-size grains appears to be predominately influenced by direct detrital and volcanic ash input, respectively, into peat swamps, coastal plains, and paralic environments that developed along the shores of the Cretaceous Western Interior Seaway. High positive and persistent Gd anomalies across the sample dataset support the marine influence on developing peat mires [37,76] (Figure 12).
Detrital silt-size grains were likely sourced from the Sevier Orogenic Belt, as the previous literature supports [41,50,51,77,78,79,80,81]. While detrital phosphates influenced the REY enrichment in silt-size grain sediments, volcanic input directly into peat or deposited adjacent to the developing coal led to substantial REY enrichment of the clay-size fraction, particularly in LREE. Volcanic ash falls as clay-size grains were sourced from periodic activation of the Sierra Nevada Arc during the mid to late Cretaceous [79,80,81].
ICP-MS-measured REY anomalies are noted in the study dataset and can be utilized to provide some support to inferred enrichment mechanisms. High positive CeN and EuN anomalies have been shown to indicate volcanogenic influence on REY enrichment in coals [17,32,36,37,82,83]. Negative EuN has been used to indicate REY enrichment via detrital felsic input while positive EuN and YN anomalies can be used to suggest probable influence of hydrothermal and/or fluid interaction leading to REY migration during diagenesis [17,32,36,37,82,83].

5. Conclusions

This study identifies and characterizes REY-bearing grains and associated mineral associations within six highly REY-enriched coal and stratigraphically adjacent carbonaceous shale and siltstone samples from the Uinta Region. The identified REY-bearing grains range in size from ≥2 µm to 30 µm. Small zones of weak REY organic enrichment within the matrix in zones < 2 µm are also present but are beyond the limits of the SEM imaging at the resolution utilized for EDS data collection. Highly REY-enriched samples are predominantly host to monazite-Ce and, to a lesser degree, xenotime.
REY-bearing grains between 5–30 µm are classified as silt-size grains and are found exclusively in siltstone and carbonaceous shale samples. Silt-size REY-bearing grains demonstrate homogenous size and shape relative to surrounding non-REY-bearing grains. The spatial distribution of monazite and xenotime silt-size grains in siltstones and carbonaceous shales are irregular. These spatial and morphological observations suggest that these REY-bearing grains are detrital in origin and are transported from up-dip sediment source areas and deposited together with non-REY-hosted grains.
Clay-size (2–5 µm) monazite-Ce grains are also identified exclusively in both coal samples used in this study. The spatial distribution patterns and morphologies of REY-bearing clay-size grains are distinct from silt-size grains. Clay-size grains appear in coal samples as small to extensive clusters within the clay matrix particles. Clay-size monazite grains ~2 µm in size are amorphous, lacking definitive shape, though larger grains (~5 µm) are generally highly spherical or very angular.
The shape and distribution of REY-hosted clay-size grains of monazite and REY-enriched phosphates suggest a volcanogenic origin and are likely to represent volcanic ash fall into peat swamps during heightened periods of arc activity or ash fall. Because REY-enriched clay-size grains are only found within clay domains rather than organic matter domains within coal samples, it is possible that diagenetic fluid mobility and absorption of REYs on clay minerals was alternatively or additionally an important enrichment mechanism.
A higher LREE-to-HREE ratio further supports grain characteristic and provenance interpretations. The dominance of LREEs compared with HREEs is indicative of higher detrital content and volcanoclastic ash that introduced REYs into developing peat mires. LREEs are attributed to mineral-content-sourced REY enrichment, whereas HREEs typically have a greater adsorption affinity for organic matter, particularly in lower rank coal [14,28,29,30,31,66]. In addition, a negative EuN anomaly further supports detrital and volcanic influences responsible for REY enrichment found in the study samples.
This study adds to prior work by confirming the presence of complex and varied enrichment mechanisms of coal and coal-bearing strata as well as identifying mineral associations and mechanisms for emplacement. Analysis from this work also shows that visual grain observations and mineral association interpretations, coupled with LREEN/HREEN ratios and REY anomalies, can provide insight into paleodepositional conditions and diagenetic mechanisms leading to REY enrichment.

Author Contributions

Conceptualization, L.W. and L.P.B.; methodology, L.W.; investigation, L.W., E.A.M., P.D.F. and H.H.C.; resources, D.P.F., M.D.V.B. and R.D.G.; writing—original draft preparation, L.W. and L.P.B.; writing—review and editing, L.W., L.P.B., E.A.M., P.D.F., H.H.C., R.D.G. and M.D.V.B.; supervision, project administration. and funding acquisition, L.P.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Department of Energy’s CORE-CM: Transforming Uinta Basin Earth Materials into Advanced Products project, DE-FE0032046.

Data Availability Statement

Data are contained within the article and are available online at the Open Science Framework doi:10.17605/OSF.IO/FU8PH.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Stratigraphic column showing Cretaceous-age coal-bearing units mined in the Uinta Region of Utah, including the Ferron Sandstone and the Blackhawk Formation [42,43,44,45,46,47,48,49]. * disconformity.
Figure 1. Stratigraphic column showing Cretaceous-age coal-bearing units mined in the Uinta Region of Utah, including the Ferron Sandstone and the Blackhawk Formation [42,43,44,45,46,47,48,49]. * disconformity.
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Figure 2. Uinta Basin regional map of sample locations and source coal fields. Historic mines, outcrops, and core samples were utilized for this study.
Figure 2. Uinta Basin regional map of sample locations and source coal fields. Historic mines, outcrops, and core samples were utilized for this study.
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Figure 3. Thin section map of Rockland siltstone showing six zones of petrographic images (marked 1–6). Red boxes indicate areas of interest where detailed SEM–EDS analysis was completed.
Figure 3. Thin section map of Rockland siltstone showing six zones of petrographic images (marked 1–6). Red boxes indicate areas of interest where detailed SEM–EDS analysis was completed.
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Figure 4. Methodology modified from Ji et al. (2022) [61] showing the process for identifying areas of interest for EDS analysis.
Figure 4. Methodology modified from Ji et al. (2022) [61] showing the process for identifying areas of interest for EDS analysis.
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Figure 5. Example of EDS methods employed for grain analysis on sample BVCA-4. (A) initial BSE scan showing clay zone ≥ 40 μm that displays several bright spots indicative of heavier elements. (B) EDS phase mapping of entire area of interest. (C) BSE imaging at a higher magnification of bright spots for further EDS analysis. (D) EDS analysis of bright spots outlined by green polygon showing high phosphate content. (E) EDS spectral analysis of green polygonal region (D) showing elevated enrichment of REYs.
Figure 5. Example of EDS methods employed for grain analysis on sample BVCA-4. (A) initial BSE scan showing clay zone ≥ 40 μm that displays several bright spots indicative of heavier elements. (B) EDS phase mapping of entire area of interest. (C) BSE imaging at a higher magnification of bright spots for further EDS analysis. (D) EDS analysis of bright spots outlined by green polygon showing high phosphate content. (E) EDS spectral analysis of green polygonal region (D) showing elevated enrichment of REYs.
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Figure 6. BSE images of identified REY-enriched grains and REY-hosted organic matter categorized by size.
Figure 6. BSE images of identified REY-enriched grains and REY-hosted organic matter categorized by size.
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Figure 7. Enriched silt- and clay-size monazite-Ce grains. (A) BSE image of silt-size monazite grain identified in the Rockland siltstone sample. (B) Qualitative concentrations of REYs present in the Rockland monazite grain. (C) EDS spectrum of REY-enriched monazite grain in the Rockland siltstone sample. (D) BSE image of clay-size monazite grains found in coal sample MC-C. (E) Qualitative EDS analysis of REY concentrations present in clay-size monazite grains. (F) EDS spectrum of REY-enriched monazite grain cluster found in the MC-C coal sample.
Figure 7. Enriched silt- and clay-size monazite-Ce grains. (A) BSE image of silt-size monazite grain identified in the Rockland siltstone sample. (B) Qualitative concentrations of REYs present in the Rockland monazite grain. (C) EDS spectrum of REY-enriched monazite grain in the Rockland siltstone sample. (D) BSE image of clay-size monazite grains found in coal sample MC-C. (E) Qualitative EDS analysis of REY concentrations present in clay-size monazite grains. (F) EDS spectrum of REY-enriched monazite grain cluster found in the MC-C coal sample.
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Minerals 15 00869 g008
Figure 8. (A) Bright spots are interpreted as a phosphate grain field in clay from coal sample MC-C shown in secondary electrons (SE) (labeled ETD) and (B) BSE. The white grains pictured are all similar in geochemical makeup and are interpreted as containing varying levels of REY enrichment. The red box indicates an area of interest where detailed EDS analysis was completed and REY enrichment was identified in clustered clay-size phosphate (monazite) grains (see also Figure 7D).
Figure 8. (A) Bright spots are interpreted as a phosphate grain field in clay from coal sample MC-C shown in secondary electrons (SE) (labeled ETD) and (B) BSE. The white grains pictured are all similar in geochemical makeup and are interpreted as containing varying levels of REY enrichment. The red box indicates an area of interest where detailed EDS analysis was completed and REY enrichment was identified in clustered clay-size phosphate (monazite) grains (see also Figure 7D).
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Minerals 15 00869 g009
Figure 9. BSE image of a REY-bearing monazite (Mz) clay-size grain field (white dispersed grains) in coal sample MC-C. Monazite clay-size grains are concentrated primarily in the clay matrix. Notably, REY-bearing monazites are not present in the organic matter (Org) and are likely coal domains.
Figure 9. BSE image of a REY-bearing monazite (Mz) clay-size grain field (white dispersed grains) in coal sample MC-C. Monazite clay-size grains are concentrated primarily in the clay matrix. Notably, REY-bearing monazites are not present in the organic matter (Org) and are likely coal domains.
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Minerals 15 00869 g010
Figure 10. (A) BSE image of carbonaceous shale sample MC-G-02-03. Blue box represents the identified area of interest for EDS spectra analysis. (B) Organic matter EDS spectral analysis of the area of interest identified in (A) displaying elemental assemblages contain varying amounts of Si, Al, and Mg, with fractions containing predominately C along with trace S and Na. Blue box overlaying spectra represents where point and polygon detailed EDS analysis was completed. (C) Red spectral EDS signature is representative of the entire imaged zone (A). Inset spectra show slightly higher than background (blue line) concentrations of REYs Ce, La, Nd, and Pr, indicating possible weak enrichment contained within the organic matter.
Figure 10. (A) BSE image of carbonaceous shale sample MC-G-02-03. Blue box represents the identified area of interest for EDS spectra analysis. (B) Organic matter EDS spectral analysis of the area of interest identified in (A) displaying elemental assemblages contain varying amounts of Si, Al, and Mg, with fractions containing predominately C along with trace S and Na. Blue box overlaying spectra represents where point and polygon detailed EDS analysis was completed. (C) Red spectral EDS signature is representative of the entire imaged zone (A). Inset spectra show slightly higher than background (blue line) concentrations of REYs Ce, La, Nd, and Pr, indicating possible weak enrichment contained within the organic matter.
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Minerals 15 00869 g011
Figure 11. ICP-MS total REY concentrations normalized to average UCC values [63]. Samples show higher concentrations of LREEs to HREEs, particularly in the coal samples.
Figure 11. ICP-MS total REY concentrations normalized to average UCC values [63]. Samples show higher concentrations of LREEs to HREEs, particularly in the coal samples.
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Minerals 15 00869 g012
Figure 12. REY anomalies EuN, CeN, YN, and GdN in the study samples. Slightly anomalous values are at 1.0 ± < 0.1, while highly anomalous values are designated as 1.0 ± > 0.1 in the study samples.
Figure 12. REY anomalies EuN, CeN, YN, and GdN in the study samples. Slightly anomalous values are at 1.0 ± < 0.1, while highly anomalous values are designated as 1.0 ± > 0.1 in the study samples.
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Minerals 15 00869 g013
Figure 13. Proposed REY-enrichment mechanisms in coal and coal-adjacent strata. (A) Depositional environmental scenarios leading to REY enrichment from volcanic ash fall (1), detrital input (2), and organic uptake in developing peat mires (3). (B) Post depositional REY-enrichment mechanisms with coal formation during early (1) to late diagenesis (2,3). This figure is a visual representation based on compiling interpretations from previous work [11,14,30,31,32,33,67,68,69].
Figure 13. Proposed REY-enrichment mechanisms in coal and coal-adjacent strata. (A) Depositional environmental scenarios leading to REY enrichment from volcanic ash fall (1), detrital input (2), and organic uptake in developing peat mires (3). (B) Post depositional REY-enrichment mechanisms with coal formation during early (1) to late diagenesis (2,3). This figure is a visual representation based on compiling interpretations from previous work [11,14,30,31,32,33,67,68,69].
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Table 1. Samples selected for SEM–EDS analysis.
Table 1. Samples selected for SEM–EDS analysis.
SampleFormationGeologic AgeSimplified
Lithology		Coal FieldLocation TypeMeasured Height
(m)		Measured
Depth
(m)		Named Coal SeamAdjacency to CoalAsh % 1ICP-MS tREY
(ppm)		pXRF
REY
(ppm)
BVCA-4BlackhawkCampanianCoalWasatchMine--Castlegate A-23934.07473.00
IPA-1-1673.6BlackhawkCampanianSiltstoneBook CliffsCore-510.1SunnysideBelow-244.323038.33
MC-C-01-06FerronTuronianCoalEmeryOutcrop0.1-CBelow165165.651711.40
MC-G-02-03FerronTuronianCarbonaceous ShaleEmeryOutcrop0.9-GWithin84250.011852.00
WF-M-10-03FerronTuronianCarbonaceous ShaleEmeryMine0.6-MAbove87421.70873.90
Rockland-JM-02-07-08FerronTuronianSiltstoneEmeryMine2.10-JAbove92227.13867.78
1 Based on sample preparation for ICP-MS analysis—specifically, sample incineration. tREY includes Sc, Y and the lanthanides as determined by ICP-MS analysis.
Table 2. REY concentrations in ppm as measured by ICP-MS.
Table 2. REY concentrations in ppm as measured by ICP-MS.
SampleScYLaCePrNdSmEuGdTbDyHoErYbLuΣREY
BVCA-4N0.972.066.076.586.095.815.144.554.383.142.922.372.252.332.0456.70
BVCA-4A13.5443.20188.30414.6343.21156.7724.144.5517.532.2011.401.975.194.660.63931.91
IPA-1-1673.6N0.781.401.351.391.361.381.501.451.511.251.341.231.241.331.2619.77
IPA-1-1673.6A10.8929.4441.7987.629.6637.367.061.456.020.875.221.022.862.670.39244.32
MC-C-01-06N2.837.9542.6438.4630.8125.9722.3321.7618.3512.9311.338.537.517.316.70265.41
MC-C-01-06A39.57167.011321.882422.88218.74701.14104.9621.7673.429.0544.187.0817.2814.632.085165.65
MC-G-02-03N0.901.211.551.451.391.371.421.331.351.131.221.151.201.351.2619.28
MC-G-02-03A12.5925.3348.0491.559.9036.876.671.335.380.794.760.962.752.700.39250.01
WF-M-10-03N0.320.443.043.262.692.422.131.781.631.040.830.560.440.300.2321.08
WF-M-10-03A4.519.3094.11205.0819.1065.2410.001.786.510.723.230.461.000.590.07421.70
Rockland-JM-02-07-08N0.581.211.301.341.271.261.381.151.291.121.241.191.291.551.4918.66
Rockland-JM-02-07-08A8.0925.3840.2684.568.9833.906.481.155.160.794.850.992.963.110.46227.13
Note: REYN—REY has been normalized to average UCC values based on Rudnick and Gao (2003) [63] and REYA—absolute values.
Table 3. Identified REY-enriched grains from SEM–EDS analysis.
Table 3. Identified REY-enriched grains from SEM–EDS analysis.
SampleFormationLithology# Grains Processed (EDS)# REY-Enriched GrainsSize
BVCA-4BlackhawkCoal3352–5 μm
IPA-1-1673.6BlackhawkSiltstone2015–30 μm
MC-C-01-06FerronCoal2632–5 μm
MC-G-02-03FerronCarbonaceous Shale212<2 μm
WF-M-10-03FerronCarbonaceous Shale1715–30 μm
Rockland-JM-02-07-08FerronSiltstone1835–30 μm
Table 4. Identified REY-enriched grains.
Table 4. Identified REY-enriched grains.
SampleFormationLithologyREY-Bearing Minerals Identified
BVCA-4BlackhawkCoalLa, Ce, and Nd clusters of phosphate clay-size grains (≤5 µm) found in zones of the clay matrix (<1 µm)
IPA-1-1673.6BlackhawkSiltstoneCe-bearing monazite grain > 10 µm
MC-C-01-06FerronCoalLa, Ce, Pr, and Nd clusters of monazite clay-size grains (≤5 µm) throughout zones of the clay matrix (<1 µm)
MC-G-02-03FerronCarbonaceous ShaleLa, Ce, Pr, and Nd unevenly dispersed throughout two small (<1 µm) zones of the organic matrix
WF-M-10-03FerronCarbonaceous ShaleLa, Ce, and Nd-bearing monazite grain > 10 µm; primary signal is Ce
Rockland-JM-02-07-08FerronSiltstoneLa, Ce, Pr, and Nd in two monazite grains > 10 µm; primary signal is Ce
Y-enriched xenotime was also identified in a >10 µm grain
Table 5. LREE and HREE concentrations (ppm) and ratios based on ICP-MS analysis.
Table 5. LREE and HREE concentrations (ppm) and ratios based on ICP-MS analysis.
SampleLithologyLREEHREERatio
BVCA-4Coal831.6043.5719.08
IPA-1-1673.6Siltstone184.9419.059.70
MC-C-01-06Coal4791.36167.7128.56
MC-G-02-03Carbonaceous Shale194.3617.7310.95
WF-M-10-03Carbonaceous Shale395.3012.5931.38
Rockland-JM-02-07-08Siltstone175.3418.329.57
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Wilcock, L.; Birgenheier, L.P.; Morris, E.A.; Fausett, P.D.; Coe, H.H.; Fernandez, D.P.; Gall, R.D.; Vanden Berg, M.D. REY Spatial Distribution and Mineral Association in Coal, Carbonaceous Shale and Siltstone: Implications for REE Enrichment Mechanisms. Minerals 2025, 15, 869. https://doi.org/10.3390/min15080869

AMA Style

Wilcock L, Birgenheier LP, Morris EA, Fausett PD, Coe HH, Fernandez DP, Gall RD, Vanden Berg MD. REY Spatial Distribution and Mineral Association in Coal, Carbonaceous Shale and Siltstone: Implications for REE Enrichment Mechanisms. Minerals. 2025; 15(8):869. https://doi.org/10.3390/min15080869

Chicago/Turabian Style

Wilcock, Laura, Lauren P. Birgenheier, Emma A. Morris, Peyton D. Fausett, Haley H. Coe, Diego P. Fernandez, Ryan D. Gall, and Michael D. Vanden Berg. 2025. "REY Spatial Distribution and Mineral Association in Coal, Carbonaceous Shale and Siltstone: Implications for REE Enrichment Mechanisms" Minerals 15, no. 8: 869. https://doi.org/10.3390/min15080869

APA Style

Wilcock, L., Birgenheier, L. P., Morris, E. A., Fausett, P. D., Coe, H. H., Fernandez, D. P., Gall, R. D., & Vanden Berg, M. D. (2025). REY Spatial Distribution and Mineral Association in Coal, Carbonaceous Shale and Siltstone: Implications for REE Enrichment Mechanisms. Minerals, 15(8), 869. https://doi.org/10.3390/min15080869

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