Rare Earth Elements in Alkaline-Silicate Roof Zones: Late-Stage Magmato-Hydrothermal Processes in the Motzfeldt Igneous Centre, South Greenland

The Motzfeldt Centre forms part of the Igaliko Complex: one of the major complexes of the Mesoproterozoic Gardar Igneous Province of Southern Greenland. This syenite hosts a Ta-Nb-Rare Earth Element (REE)-Zr alkaline-silicate roof zone, containing several metals classed as “critical” to the economy by the EU.

This study carries out detailed mineralogical, geochemical, and microtextural analysis of REE-enriched syenite variants from Motzfeldt using BSE-SEM imaging, EPMA, WDS mapping, and RAMAN spectroscopy. These data provide evidence that an aggressive, magmatically derived, F-rich fluid (F₁) altered primary magmatic pyrochlore (P₀), replacing it with mineralogically heterogeneous pseudomorphs formed of secondary pyrochlore (P₁) and several alteration phases. Also found also encrusted on the surface of P₁, these intergrown secondary phases are formed of elements lost from P₀ during alteration including Nb, REEs, F, Ca, Zr. In a second hydrothermal event (F₂), these crusts are “scrubbed” from the surface of P₁, dissolving into and enriching that fluid (F₂) in the elements scavenged from those minerals. The escape structures formed by the fluid F₂ as it exits the system are breccia pipes, cemented by a rock mineralogically reminiscent of carbonatite. Previously interpretations were that these structures were entirely magmatic, but our findings indicate these structures are late-stage hydrothermal in origin. This may be a novel mechanism by which carbonatite-type rocks are formed.

These processes display how early intensive hydrothermal events can prime an ore body for further alteration, through conversion of refractory primary phases into secondary phases more vulnerable to hydrothermal attack.