113-13 Mineralogical and Geochemical Controls of the Allanite-Hosted Halleck Creek Rare Earth Element Deposit, Southeastern Wyoming

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones (Posters)

Presenting Author:

Authors:

Abstract:

Rare earth elements (REEs) are a critical constituent in several advanced and clean energy technologies. Hosted within monzonites and syenites of the Mesoproterozoic Red Mountain Pluton (RMP), the Halleck Creek deposit represents an unconventional new REE resource. Several ~1.4 Ga granitoid plutons are already known to host REE deposits across North America, demonstrating the prospectivity of the coeval RMP.

Allanite is identified as the primary REE host phase across the RMP, with a modal abundance reaching 3.6% (Anderson, 1995). Trace element analysis indicates light REE enrichment, attributed to the high modal abundance of allanite and relative scarcity of other REE mineral phases such as zircon and apatite. Anderson et al. (2003) suggest that a major chemical control on the crystallization of allanite in the RMP is high Fe₂O₃ concentration and limited P₂O₅ and TiO₂ in the melt, which is consistent with the observed geochemical and mineralogical trends. Individual samples have yielded bulk rock total REE compositions up to 1.1 wt % and exhibit a positive correlation with Th concentration. Petrographic analysis reveals that allanite occurs as glomerocrysts of hornblende ± clinopyroxene ± biotite and as isolated crystals, with metamictization pervasive across both textures. Allanite is typically found dispersed throughout the pluton but is accumulative in localized areas. Despite the identification of bulk rock REE enrichment across the RMP, the variation of REE concentration within allanites remains poorly constrained. It is also unclear whether textural variation and degree of metamictization between allanites correlate with variable REE concentration. Here, we will present field observations, whole rock and mineral chemistry, and petrography from the RMP. Using semi-quantitative microscopy and microanalytical techniques, we will determine if there is variable REE enrichment between individual allanite crystals. Together, these data will reveal additional chemical or textural vectors to inform more targeted exploration of the Halleck Creek deposit and provide further insight into the petrogenesis of the RMP.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9084

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