Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings: Storehouses of Rare Earth Elements

Abstract

The rare earth elements (REE) are critical raw materials for much of modern technology, particularly renewable energy infrastructure and electric vehicles that are vital for the energy transition. Many of the world's largest REE deposits occur in alkaline rocks and carbonatites, which are found in intracontinental, rift-related settings, and also in syn- to post-collisional settings. Post-collisional settings host significant REE deposits, such as those of the Mianning-Dechang belt in China. This paper reviews REE mineralisation in syn- to post-collisional alkaline-carbonatite complexes worldwide, in order to demonstrate some of the key physical and chemical features of these deposits. We use three examples, in Scotland, Namibia, and Turkey, to illustrate the structure of these systems. We review published geochemical data and use these to build up a broad model for the REE mineral system in post-collisional alkaline-carbonatite complexes. It is evident that immiscibility of carbonate-rich magmas and fluids plays an important part in generating mineralisation in these settings, with REE, Ba and F partitioning into the carbonate-rich phase. The most significant REE mineralisation in post-collisional alkaline-carbonatite complexes occurs in shallow-level, carbothermal or carbonatite intrusions, but deeper carbonatite bodies and associated alteration zones may also have REE enrichment.