Rare earth and niobium mobility at the magmatic-hydrothermal transition: Motzfeldt Sø Centre, Greenland

Alkaline igneous complexes host the world’s largest deposits of rare earth elements (REE) and high field strength elements (HFSE) (e.g. Nb, Ta). The genetic models for these metals are underdeveloped, and the role hydrothermal fluids play in modifying the deposit is unclear. The Motzfeldt Sø Centre, Greenland offers an exceptional 3D slice through a collapsed roof-zone in which REE and HFSE mineralisation is found within pyrochlore-group minerals. We use textural and chemical analyses of pyrochlore to interrogate hydrothermal activity in the roof-zone, revealing a two-step model to form an REE-HFSE enriched deposit. First, magma stalls in the roof of the intrusion, accompanied by an aggressive, F-rich fluid from the cooling pluton below. Intensive hydrothermal alteration converts fluorcalciopyrochlore (P₁) by adding to A-site vacancy (□) to create an Fe/□-dominated pyrochlore (P₂). This liberates REEs, alkalis, and F from the pyrochlore A and Y sites. The elements form a secondary coating on the pyrochlore surfaces. Further alteration then dissolves the coating, enriching the fluid in REE, Nb, and Fe. This process forms mineralised veins containing columbite and REE-fluorcarbonates, which escape as “chimneys” in the upper-roof zone. Late-stage fluids play a key role in determining the nature of critical metal mineralisation, through reworking primary REE and Nb/Ta mineral assemblages into secondary hydrothermal phases, both proximal and distal to the primary deposit. This expands the genetic model for this deposit type, providing a robust framework for future resource exploration in alkaline mineralised roof-zone systems.