THE NEWLY DISCOVERED TRACHYTE-HOSTED PENNINGTON MOUNTAIN REE-NB-ZR DEPOSIT IN NORTHERN MAINE: PRELIMINARY GEOLOGY, MINERALOGY, AND GEOCHEMISTRY

Given potential for critical mineral resources, in 2021 the U.S. Geological Survey Earth Mapping Resources Initiative selected the Munsungun-Winterville belt in northern Maine for a regional airborne magnetic and radiometric survey to complement its multi-year STATEMAP bedrock geologic mapping project. The survey revealed anomalously high equivalent Th residing in a slightly larger area with elevated K near Pennington Mountain. This anomaly has been confirmed as a REE-Nb-Zr deposit by geological, mineralogical, geochemical, and ground-based gamma-ray spectrometric studies. The deposit occurs within a shallow trachyte intrusion hosted by volcanic rocks (predominantly basalt) of the Ordovician Winterville Formation. The trachyte intrusion (~1.2 km²) is fine grained with a micro-porphyritic texture defined by K-feldspar phenocrysts. The entire eastern lobe (900 m × 400 m) of the intrusion is pervasively and strongly brecciated and hydrothermally altered with a matrix containing seams, lenses, and veinlets composed mainly of potassium feldspar, albite, and very fine-grained (<3 μm) zircon and monazite. Minor minerals within the matrix include columbite and bastnäsite, as well as local chlorite, euxenite, pyrite, sphalerite, magnetite, and barite. Whole-rock geochemical analyses for seven strongly mineralized samples from the eastern lobe document high average contents of Zr (1.17 ± 0.21 wt %), Nb (1,656 ± 405 ppm), Ba (3,132 ± 4,506 ppm), Y (1,140 ± 305 ppm), Hf (324 ± 59.4 ppm), Ta (122 ± 22.2 ppm), Th (124 ± 15.5 ppm), U (36.5 ± 12.5 ppm), and Sn (106 ± 22.0 ppm). Among light REE, the highest average concentrations are shown by Ce (1,479 ± 410 ppm) and Nd (489 ± 146 ppm). For heavy REE (HREE), Er and Yb are the most abundant on average (114 ± 31.1 ppm and 105 ± 28.0 ppm, respectively). The HREE apparently occur in monazite, bastnäsite, and the fine-grained zircon. In our model, pervasive brecciation of the trachyte focused coeval or later hydrothermal fluids that deposited the mineralized matrix of REE-Nb-Zr-Ba minerals, Ba-rich potassium feldspar and albite, and minor pyrite and sphalerite. Potential may exist for similar trachyte-hosted REE-Nb-Zr mineralization elsewhere in the Winterville Formation of northern Maine.