GEOCHEMICAL AND PETROGRAPHIC INSIGHTS INTO THE DISTRIBUTION OF RARE EARTH ELEMENT ENRICHMENT IN THE HALLECK CREEK DEPOSIT, SOUTHEASTERN WYOMING

The unique chemical properties of rare earth elements (REEs) make them attractive commodities for the manufacturing of advanced technologies. In recent decades, vulnerability in the supply chain and increased demand have necessitated active exploration of domestic REE sources. The Mesoproterozoic Halleck Creek Deposit in the Red Mountain Pluton (RMP) of southeastern Wyoming is a potentially significant new REE resource. RMP has a surficial area of approximately 20km² and ranges from monzonite to syenite in composition, with bulk rock total REE concentrations reaching 0.5 wt.% (Anderson, 1995). Individual measurements of bulk total REE concentration place the RMP at between 3 to 34 times more enriched than the average continental crust, with the majority of samples in excess of the 0.15 wt.% total rare earth oxide (TREO) cut-off grade established by American Rare Earths Ltd.

Anderson (1995) previously identified allanite as the principal REE-bearing mineral contributing to predominantly light REE enrichment across the RMP. Allanite is typically found as an accessory mineral in felsic and intermediate igneous rocks and pegmatites, though rarely in sufficient concentration to be mined as REE ore. Petrographic analysis of allanites from the RMP have revealed textural variations in grains across the pluton facies, however no prior investigation has determined whether these variations correspond with variable REE concentration. Furthermore, the spatial distribution of REE enrichment is poorly constrained across the pluton and the extent of REE contribution from other minerals such as zircon and apatite are unknown. We will present petrographic observations to understand the crystallization sequence of the RMP and use microtextural associations of allanite to determine its role in pluton crystallization and REE enrichment. Similar study will be applied to other potential REE-bearing minerals (e.g., apatite, zircon) to determine the relative contribution of REEs from different phases. Additionally, bulk rock geochemistry data will be used to assess the relationship between REE enrichment and previously mapped plutonic facies in the RMP.