Dr. Ian Hillenbrand
USGS

Topic: Beyond Plumes and Rifts: A New Origin Story for Rare Earth Element Deposits

Academic host: Tyler Wickand

Abstract: Rare earth elements (REEs) are among the most "critical" of the critical minerals due to their essential role in modern technology and the geopolitical vulnerabilities of their supply chains. Despite their significance, the petrologic and tectonic processes that generate world-class REE deposits remain poorly understood. Carbonatite-hosted deposits, the predominant global source of light REEs, have traditionally been attributed to continental rifting and mantle plumes. In this talk, I test this classic paradigm through the lens of the world-class Mountain Pass REE deposit in California and global geochemical databases. Multi-mineral dating (titanite, apatite, and biotite) of mylonites at Mountain Pass suggests that REE mineralization was contemporaneous with transpressional, post-collisional deformation and the 1.4 Ga Picuris orogeny, inconsistent with a simple rift models. Zircon trace element signatures at Mountain Pass are diagnostic of a mantle source region modified by ancient subduction rather than a mantle plume. Extending this approach globally, machine learning models trained on zircon trace element compositions reliably distinguish between barren and REE-fertile intrusions. Mountain Pass-like REE-enriched deposits are temporally linked to supercontinent assembly while carbonatites also containing Nb mineralization are associated with supercontinent breakup, suggesting different metallogenic processes across the supercontinent cycle. These results establish a new framework for REE metallogenesis with post-collisional, transpressional terranes with subduction-enriched mantle sources as fertile, underexplored targets for critical mineral exploration.