Two-stage rare earth element mineralization in the Bowery Peak carbonatite dyke, Bearpaw Mountains, Montana, USA: An integrated mineralogical, geochemical, and geophysical study

Rare earth elements (REEs) are essential to the advancement of modern economies and technology-driven industries. Understanding the geological processes that control their formation, particularly in carbonatite-hosted systems, is critical for guiding exploration and identifying new deposits. This study presents the first detailed mineralogical and geochemical characterization of the Bowery Peak carbonatite complex in the Bearpaw Mountains. Using an integrated suite of analytical techniques, X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), hyperspectral spectroscopy, and magnetic susceptibility, we document widespread light REE enrichment and localized heavy REE concentrations. The primary REE-hosting minerals—bastnäsite-(Ce), parisite-(Ce), synchysite-(Ce), and xenotime-(Y)—occur within a calcite-rich matrix, confirmed by carbonate absorption bands at 1933.5 ± 5 nm and 2335.5 ± 7 nm, along with consistent reflectance maxima at 1837.5 ± 20  nm, 2118 ± 18 nm, and 2390.5 ± 3 nm, with low magnetic susceptibility values (1.9×10⁻³ to 4.3×10⁻⁶ SI). Mean oxide concentrations across all samples include 14.32 wt.% La₂O₃, 25.48 wt.% CeO₂, 8.32 wt.% Nd₂O₃, 3.59 wt.% Pr₂O₃, and 2.19 wt.% Sm₂O₃, confirming highly-concentrated light REE mineralization. Elevated heavy REE concentrations, up to 40.26 wt.% Y₂O₃ and notable levels of Dy, Er, Gd, Ho, and Tb oxides, indicate a late-stage hydrothermal enrichment zone. SEM images show textures such as irregular grain boundaries, veins that cut across earlier minerals, and barite and hematite infill, features that indicate hydrothermal fluids altered and replaced the original magmatic REE minerals. These findings support a two-stage mineralization model involving early magmatic crystallization of light REE minerals followed by hydrothermal overprinting and redistribution of heavy REEs. This study identifies Bowery Peak as a promising mineralization zone for both light and heavy REE and provides a practical framework for assessing similar carbonatite complexes globally. The integrated approach demonstrated in this study strengthens early-stage critical mineral assessments by combining mineralogical, spectral, and geochemical tools.