A RECENTLY DISCOVERED TRACHYTE-HOSTED RARE EARTH ELEMENT-NIOBIUM-ZIRCONIUM OCCURRENCE IN NORTHERN MAINE, USA

Critical minerals are increasingly important for diverse types of industries and a low-carbon energy future. In the United States, the U.S. Geological Survey in 2022 designated 50 mineral commodities as being critical, i.e., essential to the economy and security of the nation, but for which there is risk of supply chain disruption (Nassar and Fortier, 2021; U.S. Geological Survey, 2022a). Included in this list are rare earth elements (REE), niobium, and zirconium. In 2021, the United States was 100% reliant on imports of niobium and 90% reliant on imports of REE; in previous years the United States had been 100% reliant on imports for both (U.S. Geological Survey, 2022b). Except for the Mountain Pass REE mine in southern California, no current production from U.S. deposits exists for REE or for niobium. In this report, we present preliminary field and laboratory data on a recently discovered trachyte-hosted REE-Nb-Zr occurrence in northern Maine and compare it with similar deposits in Australia and China.

The U.S. Geological Survey Earth Mapping Resources Initiative (Earth MRI), a national mapping and data collection effort by the U.S. Geological Survey and state geological surveys, was developed to improve knowledge of the nation’s geology and document areas that may have potential for critical mineral resources (Day, 2019; U.S. Geological Survey, 2022c). This initiative includes the collection of airborne geophysical data. In the summer of 2021, an airborne magnetic and radiometric survey was contracted and flown in northern Maine over the Munsungun-Winterville belt and vicinity, covering an area of ca. 9,600 km² (Shah, 2022; Fig. 1). This region was selected because of the presence of the large, 30-Mt Ordovician Bald Mountain Cu-Zn-Ag-Au(-Co-Sb-Se-Te-Bi) volcanogenic massive sulfide (VMS) deposit (Slack et al., 2003) and large stratiform Mn deposits hosted in Silurian sedimentary rocks northeast of the volcanic terrane (total 327 Mt @ 9.0 wt % Mn; Pavlides, 1962; Slack et al., 2022), and the potential for additional VMS and other deposit types that might contain critical metals. The new airborne radiometric data (gamma-ray spectrometry for K, eU, and eTh) generally show low values of eTh except for an anomaly on the eastern side of Pennington Mountain (Fig. 2A) that resides within a slightly larger region having elevated K. The area associated with these anomalies subsequently has been characterized geologically via mapping and sampling, and by a ground-based radiometric survey.

The airborne geophysical data were collected with the goal of imaging the geologic framework of the VMS and sedimenthosted Mn deposits known to be present in the region. The data were collected along NW-flight lines, oblique to the northeast structural trends of the region, with a line spacing of 250 m and nominal flying height of 80 m above ground; areas with rugged topography were flown at greater heights for safety reasons. The airborne radiometric data, which represent sources within the upper 0.5 to 1 m (excluding vegetation), have a horizontal resolution of several hundred meters. Downloadable data and details of the processing are provided in Shah (2022).

Over most of the survey area, the airborne data show that radiometric thorium (commonly referred to as equivalent thorium or eTh because of multiple decay chains involved in the estimate) is typically subdued with eTh <10 ppm. These generally low values are due to glacial cover throughout the area and to low amounts of Th-bearing minerals in both the igneous and sedimentary rocks; lowest values are observed over lakes as gamma rays cannot escape the fluid medium. However, in the northeastern corner of the survey area and on the eastern side of Pennington Mountain is a single anomaly with eTh values up to 26 ppm. This area with elevated eTh is approximately 900 m long by 400 m wide.

The airborne Th anomaly is crossed by only three flight lines, so a detailed ground radiometric survey was subsequently conducted to better delineate the horizontal extent of rock with elevated Th (Shah and Wang, 2022). Continuous gamma-ray spectrometer measurements were recorded by walking over the anomaly area in a survey pattern with several cross traverses; average values were logged every 30 s. The results (Fig. 2B) show that Th-bearing minerals are widespread over the eastern part of the mountain and decrease in concentration over lower elevations associated with glacial sediments, both at the edge of the mountain and locally within stream valleys.

The Pennington Mountain radiometric eTh anomaly is confined to a trachyte body, here informally named the trachyte at Pennington Mountain (Fig. 3). The trachyte occurs in the northeast-trending Winterville inlier, which forms part of the Ordovician Munsungun-Winterville belt, both of which are in a region of northern Maine characterized by sub-greenschist (prehnite-pumpellyite) grade metamorphism. The inlier is underlain predominantly by volcanic rocks of the Middle Ordovician Winterville Formation (Boone, 1958) with lesser Upper Ordovician graywacke and black slate of the Ferguson Brook Formation of Wang (2022a), all unconformably overlain by pre-Acadian sedimentary rocks of the Lower Devonian Seboomook Group and clastic sedimentary rocks of the post-Acadian St. Froid Lake Formation of Wang (2022a) (Fig. 1). The rocks have been significantly displaced by Acadian and post-Acadian faulting dominated by southeast-directed thrust faults of at least two generations. The Winterville Formation is chiefly basalt with lesser pyroclastic rocks (tuff, lapillistone, and breccia), diabase, rhyolite, dacite, and trachyte. The extrusive flows and pyroclastic rocks are tilted due to Acadian and post-Acadian folding and faulting, and strike mainly northeast-southwest. The basalt is mostly aphyric, typically showing pillows and vesicles/amygdules and, less commonly, peperite and hyaloclasite textures. Whole-rock geochemical analyses of eight basalt samples collected from the Fish River Lake-Pennington Mountain area (Wang, 2022a, b) demonstrate that the basalt is largely transitional between calc-alkaline and tholeiitic, depleted in Ta and Nb, moderately enriched in light REE (LREE), and has normalized trace element patterns and tectonic discrimination signatures that suggest formation in an ensialic volcanic arc. These results are similar to the conclusions of previous geochemical studies of the Winterville Formation by Hynes (1976) and Winchester and van Staal (1994). The trachyte at Pennington Mountain, approximately ~1.2 km² in area, forms an irregular, shallow intrusive body within northeast-striking basalt and minor tuff, rhyolite, and diabase sills or dikes of the Winterville Formation. The western lobe of the intrusion cuts Winterville volcanic rocks. The eastern lobe is bounded by a low-angle thrust fault against Upper Ordovician graywacke and black slate of the Ferguson Brook Formation (Fig. 3). The trachyte is massive, fine grained with a microporphyritic texture, and featureless in the field except for three outcrops in the western lobe that have vesicular and amygdaloidal textures. The elongated vesicles and quartz amygdales plunge to the west at about 15º, probably due to thrust faulting during the Early Devonian Acadian orogeny.

Gamma-Ray Spectrometry (GRS) using a handheld GF Instruments Gamma Surveyor instrument was used to help in mapping the area and for selecting samples for further analysis. Ten samples of trachyte were collected for thin sections and polished thin sections for petrographic, micro-X-ray fluorescence (µXRF), microprobe energy-dispersive spectrometry (EDS), and wavelength-dispersive spectrometry (WDS) analyses and imaging. Twenty-two samples evenly collected from the eastern lobe were analyzed for major and selected trace elements by portable X-ray fluorescence (pXRF) spectrometry. Twelve trachyte samples were selected for whole-rock major, trace, and REE analyses. Figure 3 shows outcrop observation stations and sampling sites within the trachyte.

Whole-rock analyses for major, trace, and REE were performed at ALS Laboratories by inductively coupled plasmamass spectrometry after a Li borate fusion and acid digestion. Selected trace elements (As, Co, Ni, Cu, Zn, Cd, Pb, Mo, Ag, Tl) were analyzed separately by inductively coupled plasmaatomic emission spectrometry following a four-acid digestion. Details of the analytical procedures including standards are available at www.alsglobal.com. pXRF analyses (Lemière, 2018) were done at the University of New Brunswick (UNB) using an Olympus Vanta pXRF to screen samples for further analysis. A silica blank and four certified reference materials, OREAS620, OREAS146, NIST2710a, and NIST2711a, were used for calibration checks. Both the rare element geochemical mode [2 energy levels, 15 kV (120 s), 50 kV (120 s)] and soil mode [3 energy levels 10 kV (60 s), 25 kV (60 s), and 50 kV (60 s)] were used. We selected the best data analysis mode with the best R² value (extrapolated through the origin) for reporting the data. µXRF-EDS elemental scans of polished thin sections of selected samples were used to document textures and guide more detailed microbeam mineralogical studies. Data were obtained at UNB using a Bruker Nano M4 Tornado μXRF instrument, following the methods outlined in Flude et al. (2017). Operating conditions were 40 kV and 40 nA with a step size of 25 μm for 3 ms, which generated 20-μm square spots used to produce the scans.

Mineral identifications were done on polished thin sections at the University of Maine in Orono via EDS and WDS using a Cameca SX-100 electron microprobe with PeakSight software and a Roentec XFlash EDS detector (4036-channel resolution; EdWin software). Operating conditions for backscattered electron (BSE) imaging done via EDS were 15 kV, 20 nA, a focused beam, and 15.3-μs dwell time, yielding images of 1,024 × 768 pixels; WDS imaging conditions, used for identification of C and F, and for thin-section scans of K, Y, and Yb, were 15 kV, 200 nA, a focused beam, 5-ms dwell time, and a 15-μm step size in the production of six tiled 821× 838-pixel images.

The trachyte is homogeneously massive and fine grained with a microporphyritic texture (Fig. 4). Euhedral perthite phenocrysts are set in a microcrystalline groundmass composed predominantly of potassium feldspar and subordinate albite, together with minor ferromagnesian silicates (likely hornblende and biotite altered to chlorite). The eastern lobe is more strongly mineralized and altered than the western lobe, and is characterized by pervasive fractures and breccia textures (Fig. 5). The REE-Nb-Zr minerals occur mostly in a texturally diverse matrix to the breccia fragments but are difficult to recognize in outcrops and hand specimens, except locally as aggregates or veins containing very small (<0.5 mm) white or pale brown grains. The µXRF-EDS scans of entire thin sections obtained from strongly mineralized trachyte samples show distinctive breccia textures (Fig. 6) caused by intensive fracturing and element enrichments for K, Zr, Nb, La + Ce, and Zn. Results for K indicate that the highest concentrations occur in angular 0.1- to 1.0-cm fragments, with lower levels present in the surrounding matrix (Fig. 6A). High contents of Zr, Nb, La + Ce, and Zn are largely confined to the matrix, with the fragments having only minor enrichments of these metals in veinlets and disseminations (e.g., Fig. 6B-F).

The REE-Nb-Zr minerals occur dominantly in the matrix of the breccia as seams, lenses, and veins, and as disseminations. Within our samples imaged by µXRF, the brecciated fragments each range in size from 0.1 to at least 1 cm. Based on textures shown in the µXRF-EDS maps (Fig. 6A) and BSE images (Fig. 7A, B), the following generalized paragenesis is proposed: (1) significant brecciation of igneous trachyte, particularly within the eastern lobe; (2) pervasive alkali metasomatism of fragments by potassium feldspar ± albite coupled with hydrothermal mineralization of the matrix by REE-Nb-Zr phases + potassium feldspar + albite ± magnetite ± sphalerite ± pyrite; and (3) filling of younger crosscutting fractures by albite, bastnäsite [(LREE)CO₃F], or barite. In some samples, relict potassium feldspar phenocrysts are partly replaced by albite (Fig. 7C). Principal secondary minerals in the breccias are jarosite, hematite, and one or more unidentified K-Al-Si clays.

Hydrothermal minerals in the matrix are dominated by potassium feldspar that contains minor to major amounts of Ba (several wt %; cores are enriched relative to rims). Other abundant hydrothermal minerals are zircon and monazite [(LREE,Th)PO₄]. The zircon typically forms bands of granular to spongey concentrations of very small (<1 μm) grains (Fig. 7D-G); we use the term zircon here in a preliminary sense, since complete quantitative analysis for this mineral is not yet available. Uncommon, somewhat coarser zircon that forms aggregates of anhedral grains 5 to 20 μm in diameter also occurs in the matrix (Fig. 7E). Monazite typically forms 1- to 3-μm grains disseminated in zircon bands (Fig. 7H), or less commonly within potassium feldspar (Fig. 7D), and varies in composition from relatively low Th + U to high Th + U (Th > U). Minor hydrothermal minerals, present in some but not all samples, are (1) bastnäsite occurring as 1- to 20-μm grains in zircon and albite, as 1- to 3-μm grains in pyrite, and in crosscutting veinlets (Fig. 7C); (2) euxenite [(Y,Ca,Ce,U,Th)(Nb,Ta,Ti)₂O₆] as disseminated 5- to 10-μm grains (Fig. 7B); (3) Fe-Mg chlorite as disseminations or small irregular masses; (4) barite in euhedral disseminated grains (Fig. 7E) and veinlets; (5) columbite [FeNbO₆] as irregular grains up to 100 μm in diameter commonly peripheral to zircon bands (Fig. 7F); (6) pyrite as disseminated 50- to 150-μm grains associated with zircon; (7) sphalerite as subhedral to euhedral grains 1 to 50 μm in diameter intergrown with or contained in zircon (Fig. 7G), and in aggregates of 10- to 100-μm grains within albite and potassium feldspar; and (8) euhedral magnetite as disseminations and aggregates in zircon (Fig. 7I). Sparse to rare hydrothermal minerals, which are typically fine-grained (<10 μm), include betafite [(Ca,U)₂(Ti,Nb,Ta)₂O₆(OH)], ilmenorutile [(Ti,Nb,Fe)O₂], siderite, galena, and cassiterite; quartz occurs in late veinlets. Heavy REE-rich minerals such as xenotime were not found. Importantly, numerous EDS spectra obtained on the Zr-silicate phase indicate that this mineral contains small amounts of P and HREE, consistent with the results of detailed microbeam studies of zircon from other settings that show these elements can occur as crystallographic substitutions (e.g., Hoskin and Schaltegger, 2003). Noteworthy in this context is the HREE-rich hydrothermal zircon occurring in some peralkaline granites of Scotland that contains up to 0.99 wt % Dy₂O₃ and 1.27 wt % Yb₂O₃ (Belkin and Macdonald, 2021).

The 22 pXRF analyses (App. Table A1) represent averages of three spots per sample. These results confirm the notably anomalous U and Th contents of all samples identified by GRS and show highly anomalous Zr (up to 2.0 wt %), Y (up to 0.14 wt %), Nb (up to 0.69 wt %), La (up to 0.03 wt %), Zn (up to 0.5 wt %), Cu (up to 0.03 wt %), and Sn (up to 0.01 wt %). Although the data are considered semiquantitative, sample selection done for follow-up lithogechemical analysis used these pXRF results and the GRS data for screening purposes. Sample locations are shown in Figure 3.

Table 1 lists basic statistics for seven strongly mineralized samples collected from the eastern lobe and for five weakly mineralized samples from the western lobe of the trachyte at Pennington Mountain. Appendix Table A2 lists complete whole-rock data for all analyzed samples. Sample locations are shown in Figure 3.

Compared to the western lobe, average values for major elements in the strongly mineralized samples (eastern lobe) are characterized by relatively low SiO₂ (59.4 ± 1.33 wt %), TiO₂ (0.16 ± 0.05 wt %), Fe₂O₃^T (3.77 ± 1.45 wt %), MnO (0.04 ± 0.02 wt %), MgO (0.33 ± 0.33 wt %), and P₂O₅ (0.14 ± 0.08 wt %). In contrast are relatively high averages determined for Na₂O (5.75 ± 1.18 wt %) and K₂O (5.91 ± 1.55 wt %). Low major-element totals mostly reflect high zirconium contents and unanalyzed sulfur and fluorine. The trace elements with the highest concentrations are Zr (11,706 ± 2,144 ppm), Nb (1,656 ± 405 ppm), Ba (3,132 ± 4,506 ppm; one sample has 1.32 wt %), and Ce (1,479 ± 410 ppm). Relatively high averages are also evident for Y (1,140 ± 305 ppm), Hf (324 ± 59.4 ppm), Zn (689 ± 616 ppm), Ta (122 ± 22.2 ppm), Th (124 ± 16.5 ppm), U (36.5 ± 12.5 ppm), and Sn (106 ± 22.0 ppm). Among LREE, in addition to Ce, the highest average concentrations observed are for La (763 ± 245 ppm) and Nd (489 ± 146 ppm); for HREE, Dy and Er are the most abundant on average (167 ± 42.7 and 114 ± 31.1 ppm, respectively). Total Y+REE oxides average 5,613 ± 1,512 ppm. Chondrite-normalized Ce anomalies are negligible (1.11 ± 0.07); Eu displays uniformly moderate negative anomalies (Eu/Eu* = 0.16 ± 0.01).

Weakly mineralized samples, from the western lobe, on average contain appreciably higher SiO₂, TiO₂, and Fe₂O₃^T, and lower Al₂O₃ (Table 1). Among trace elements of interest for this study, the weakly mineralized samples have much lower average contents of REE, Y, Zr, Nb, Hf, Ta, Ba, Th, U, Sn, and Zn than the strongly mineralized samples from the eastern lobe. Also noteworthy in two samples are slightly higher positive Ce anomalies and much smaller negative Eu anomalies relative to those from the eastern lobe of the trachyte body. Among all 12 analyzed samples, very strong positive correlations (R²) exist between Th and Zr (0.98), Nb (0.94), Y (0.95), Ce (0.92), Yb (0.95), and Sn (0.94); moderately strong correlations occur for Th vs La (0.87), Nd (0.87), and U (0.84).

Figure 8 shows a chondrite-normalized plot of REE data for strongly and weakly mineralized samples of trachyte from Pennington Mountain together with data for several major REE deposits, including the carbonatite-hosted Bayan Obo (China) and Mountain Pass (California) deposits, and the alkaline igneous-hosted deposits at Bokan Mountain (Alaska) and Toongi (Dubbo) (Australia). Toongi is a REE-Nb-Zr deposit also hosted by trachyte. Shown for comparison are the range and average for seven Cenozoic unmineralized trachytes, which have REE abundances ca. 10× lower than those of the strongly mineralized Pennington Mountain samples. Chondrite-normalized data for the strongly mineralized Pennington Mountain samples display relatively flat patterns in which La abundances are ~1,500× to ~4,000× and Yb abundances are ~500× to ~1,000× chondritic values. No significant Ce anomalies are evident. All samples from this eastern lobe of the trachyte body show moderately negative Eu anomalies.

The strongly and pervasively mineralized nature of the eastern lobe of the trachyte body is delineated on the basis of a detailed ground GRS survey (Fig. 2B), pXRF analyses of 22 evenly distributed samples (App. Table A1), and whole-rock geochemical data for seven samples (App. Table A2). Together with the eTh values obtained from the airborne radiometric survey, these data indicate that the REE-Nb-Zr mineralization is widespread in this part of the trachyte and that the analyzed samples are representative. Importantly, very strong positive correlations (R²) of 0.92 to 0.98 that exist in the whole-rock geochemical data (n = 12) between Th and Zr, Nb, Y, Ce, Yb, and Sn indicate that the GRS data for eTh represent valid proxies for concentrations of these metals in the trachyte.

A preliminary interpretation of the genesis of REE-Nb-Zr mineralization at Pennington Mountain is presented here on the basis of field studies, petrography, and µXRF and BSE imaging. The first stage was an episode of major brecciation that formed angular fragments mostly 1 mm to 1 cm in diameter (locally up to ~20 cm in outcrops), possibly due to a near-surface explosion during emplacement of the trachyte magma. Next was pervasive alkali metasomatism (Ba-rich potassium feldspar and albite) of the fragments chiefly in the eastern lobe of the trachyte that likely contributed to elevated radiometric K in the area and elevated radiometric eTh (Fig. 2B). Contemporaneous was formation of the breccia matrix composed of (1) Ba-rich potassium feldspar and albite, (2) coeval REE, Nb, Zr, and Ba minerals, and (3) minor pyrite and sphalerite (Figs. 6 and 7). This main stage of mineralization has a metal signature (in decreasing order of average concentration; Table 1) of Zr-REE-Ba-Nb-Y-Zn-Hf-Th-Ta-Sn-U. Most of the zircon observed to date is very fine grained (<1 μm) and forms textures consistent with zircon of hydrothermal origin (e.g., Rubin et al., 1989; Corfu et al., 2003). A key unanswered question is whether the hydrothermal mineralization at Pennington Mountain is related genetically to the host trachyte or instead to a younger, unexposed intrusion of similar or different composition. Geochronological work is needed in order to establish the age of the intrusion and of the hydrothermal mineralization.

Trachyte-hosted REE-Nb-Zr deposits similar to the occurrence at Pennington Mountain have not been reported previously in the United States but are known in Australia and China. The unmined Dubbo (Toongi) deposit in New South Wales, occurring within a small (~0.3 km²) Cretaceous trachyte laccolith, consists of complex Na-Ca-Fe-Zr-Y-REE silicate minerals (mainly eudialyte) together with lueshite/natroniobite and minor bastnäsite dispersed uniformly throughout the intrusion (Spandler and Morris, 2016). This deposit has a [Australasian] Joint Ore Reserves Committee (JORC)-defined proven reserve of 18.9 Mt averaging 1.85% ZrO₂, 0.871% ΣY+REE oxides, 0.440% Nb₂O₅, 0.040% HfO₂, and 0.029% Ta₂O₅, and a measured + inferred resource of 75.2 Mt averaging 1.89% ZrO₂, 0.740% ΣY+REE oxides, 0.440% Nb₂O₅, 0.040% HfO₂, and 0.030% Ta₂O₅ (ASM Ltd., 2018). By comparison, the seven strongly mineralized and altered surface samples analyzed in this study from the Pennington Mountain occurrence have lower average concentrations of these components (1.58% ZrO₂, 0.561% ΣY+REE oxides, 0.237% Nb₂O₅, 0.038% HfO₂, and 0.015% Ta₂O₅). Importantly, abundances of HREE and HREE/LREE ratios for samples from Pennington Mountain are higher than those of carbonatite-hosted deposits and the Dubbo (Toongi) deposit, and are similar to those of the Bokan Mountain deposit (Fig. 8).

The unmined Brockman REE-Nb-Zr deposit in Western Australia is within a Paleoproterozoic rhyolitic tuff in a predominantly trachytic volcanic sequence (Taylor et al., 1995a, b). This aerially extensive deposit has a JORC indicated + inferred resource of 41.4 Mt averaging 0.90% ZrO₂, 0.210% ΣY+REE oxides, and 0.359% Nb₂O₅ (Hastings Technology Metals Ltd., 2022). Major economically important minerals are hydrous, gel-like (<1 μm) zircon (similar to Pennington Mountain) that has very high average concentrations of Y (4.49 wt %), Nb (0.77 wt %), Th (0.70 wt %), and HREE (e.g., 0.60 wt % Yb), together with variable amounts of columbite, bastnäsite, thorite, parisite, synchysite, and sphalerite in gangue dominated by fluorite and Fe-rich carbonates (Ramsden et al., 1993; Taylor et al., 1995a).

Numerous unmined REE-Nb-Zr deposits in the south Qinling orogenic belt of Central China are hosted in trachyte intrusions of Permian and Triassic age (Jiang et al., 2020; Yin and Song, 2022). The Tudiling deposit, apparently the largest, comprises 122.5 Mt at low grades averaging 0.17% ZrO₂, 0.08 % Nb₂O₅, 0.03 % ΣREE oxides, and 0.005 % Ta₂O₅ (Xiong et al., 2018; Yan et al., 2021). Other major deposits in this belt include Zhujiayuan, Pingli, Zhuxi, and Tianbao (Nie et al., 2020a, b, 2021; Wang et al., 2021; Yan et al., 2022). These deposits form tabular lenses and vein concentrations, in both intrusive and extrusive trachyte, consisting of fine-grained monazite, bastnäsite, aeschynite, allanite, parisite, loparite, columbite, fersmite, ilmenorutile, rutile, and zircon.

Maine has a diverse metallogeny that includes a high potential for undiscovered major critical mineral deposits of stratiform manganese in sedimentary rocks, nickel-copper(-cobalt-PGEs) in mafic and ultramafic intrusions, and lithium in granitic pegmatites (Slack et al., 2022). Prior to this discovery at Pennington Mountain, no potentially economic concentrations of REE, Nb, or Zr were known in Maine (Mariano and Mariano, 2012; Van Gosen et al., 2017, 2019).

Our preliminary evaluation of the Pennington Mountain occurrence presented here assumes that trachyte is the key host rock for containing hydrothermal REE-Nb-Zr mineralization in the region. Based on the discordant geometry of the western lobe relative to strata of the host Winterville Formation and the geometry of the eastern lobe, formation as a laccolith is suggested. The trachyte intrudes volcanic rocks of the Middle Ordovician Winterville Formation. We consider it probable that this trachyte is also Middle Ordovician, but a younger yet also pre-Acadian age cannot be ruled out; the trachyte body is cut on the east side by a thrust fault (Fig. 3), which is an Acadian structure of Early Devonian age. Importantly, alkalic basalts of possible trachytic affinity have been identified elsewhere in the Winterville Formation (Winchester and van Staal, 1994) including east of Winterville, south of Portage Lake, and in the Castle Hill inlier northeast of Ashland (Fig. 1). The Middle Ordovician Pointe Verte Formation in northern New Brunswick also contains trachytic volcanic rocks (Winchester et al., 1992). The Winterville Formation has not been mapped in detail, and as a result this unit may have additional unrecognized intrusive trachyte bodies, some of which could be highly mineralized as at Pennington Mountain. Although this area was identified by a prominent eTh anomaly found during an airborne radiometric survey (Fig. 2), such surveys only image the upper 0.5 to 1 m of the land surface and hence even small amounts of glacial or soil cover will mask possible Th anomalies. Consequently, the absence of strong eTh anomalies in other areas of the Winterville Formation (Fig. 2A) does not preclude the possibility that REE-Nb-Zr occurrences like that at Pennington Mountain exist elsewhere in northern Maine. Moreover, metallogenically similar mineralization rich in REE (and HREE) could occur within alkaline rhyolite associated with trachyte bodies, based on the geologic setting of deposits of this type such as in the Peak Range in Queensland, Australia (Chandler and Spandler, 2020).

This study of the Pennington Mountain occurrence is constrained by a lack of information on the presence and character of potential metal enrichments at depth within the trachyte body and on the age of the REE-Nb-Zr mineralization. The radiometric eTh anomaly is likely related to Th-rich monazite, which is spatially and paragenetically linked to the REE-Nb-Zr mineralization based on data from µXRF and BSE images (Figs. 6B-D; 7A, B, D, H). No separate HREE-rich minerals have been identified among our surface samples, but based on a strong correlation in whole-rock data between HREE and Th (R² = 0.96), monazite is a potential major host for some of the HREE given the lack of other identified Th-rich minerals. However, the uniformly low LREE/HREE ratios in whole-rock analyses (Fig. 8) suggest that in addition to the HREE-bearing zircon, unrecognized xenotime or other HREE-rich minerals could also be present, with or without associated monazite and independent of radiometric anomalies. Also relevant is the heterogeneous distribution of LREE and Nb minerals, on a hand-specimen scale, in contrast to the microcrystalline zircon that appears to be the most widespread hydrothermal mineral in the occurrence other than potassium feldspar.

The metal signature of hydrothermal REE-Nb-Zr mineralization at Pennington Mountain is similar to that of many deposits hosted in alkaline granites and syenites (e.g., Dostal, 2016; Beard et al., 2023). We speculate that a mineralized intrusion with this geochemical signature may exist at depth beneath the trachyte and could have been the source for the hydrothermal fluids that formed the occurrence. Importantly, the possibility of unmapped or buried alkaline granites or syenites/trachytes occurring elsewhere in the Winterville Formation cannot be ruled out and should be evaluated to better delineate potential REE-Nb-Zr resources throughout northern Maine. Similar mineralization also could exist in other areas of the Appalachian-Caledonian orogen where volcanic and subvolcanic trachytes have been recognized.

The airborne geophysical data presented here were collected and processed by Sander Geophysics and funded by the U.S. Geological Survey Earth Mapping Resources Initiative (https://usgs.gov/special-topics/earthmri). Bedrock geologic mapping in northern Maine was funded through the U.S. Geological Survey STATEMAP component of the National Cooperative Geologic Mapping Program and by the Maine Geological Survey. We thank Maine State Geologist Steve Dickson for support, Steve Pollock for discussions of regional geology, David Putnam for fieldwork support, and student interns Preston Bass and Liam Daniels for field assistance. Helpful reviews and suggestions on an early version of the manuscript were provided by Brad Van Gosen, Warren Day, and Editor Larry Meinert. We also thank Iain Samson for a thorough and constructive review that significantly improved the final paper.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Chunzeng Wang is a geology professor at the University of Maine at Presque Isle and on the graduate faculty of University of Maine and of Guilin University of Technology in China. He graduated with a B.A. in exploration geology from Guilin College of Geology in 1984, received an M.S. degree in structural geology from China University of Geoscience in 1989, and a Ph.D. in earth and environmental sciences from the City University of New York in 2001. Wang’s research interests include tectonic and deformational history and mineral potential and metallogenesis of the northern Maine Appalachians, as well as in South China.