Raising the roof for rare metals : controls on rare earth mineralisation in magmatic roof-zones, Motzfeldt, Southern Greenland

Rare Earth Elements (REE) and other High Field Strength Elements (HFSE) are known as “critical metals”: raw materials with unstable supply lines essential for modern technology such as electric vehicles, renewable energy and defence systems. Deposits of these metals are often hosted by alkaline-silicate igneous rocks, however, models and processing pathways for these deposit types are underdeveloped compared to those hosting conventional resources. This thesis concerns the deposit type “Alkaline Silicate Roof-Zones”, focussing primarily upon controls on REE mineralisation in these systems. This is investigated here through study of the REE-Nb-Ta-Zr mineralised roof-zone of the Motzfeldt Sø Centre, Southern Greenland. Field and lab work is presented investigating mineralogy and mineral chemistry through field relations, petrography and mineral chemical analysis (Electron Microscopy, EPMA, LA-ICP-MS). Hydrothermal activity plays a key role in remobilising the REE and HFSE in the pyrochlore microsyenite as recorded by pyrochlore chemistry. Through use of an alteration proxy and textural analysis, it is shown REE and other A-site elements decrease during hydrothermal alteration, and Nb and B-site elements are released by dissolution of pyrochlore. At least two hydrothermal events are identified, recorded in the formation and later mobilisation of secondary crusts around pyrochlore. New field evidence identifies REE-HFSE mineralised hydrothermal chimney structures and metamorphosed xenoliths with REE-HFSE rich anatectic melts. SEM and EDS analysis reveals these features to be formed by the identified hydrothermal events through enrichment in HFSE, alkalis, F and Fe. These features are incorporated into the deposit model and have potential as exploration vectors and secondary mineral deposits. A method is developed to calculate REE deportment utilising WDS mapping and phase mapping software, identifying pyrochlore to be the major REE host in pyrochlore microsyenite (~60% of LREE), leading to discussion of appropriate processing methods and estimated yields of the most valuable REE species. These findings significantly improve both the deposit model for and understanding of the complex magmatic, hydrothermal and metamorphic processes involved in the critical metal enrichment of alkaline silicate roof zones and other alkaline rocks worldwide.