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.

Alkaline-Silicate REE-HFSE Systems

“…Alkaline-silicate magmas have a mantle source, although the exact nature of that source varies between and within provinces (Bell and Simonetti, 2010;Hutchison et al, 2019Hutchison et al, , 2021. They are most commonly associated with lithospheric-scale extension, although some postcollisional alkaline magmatic provinces are associated with transcurrent tectonics (Hou et al, 2009;Banks et al, 2019;Goodenough et al, 2021). Alkaline-silicate complexes commonly occur in provinces, which are regional-scale features (10s to 100s km extent) comprising several individual igneous complexes.…”

“…1, 2). In the latter, magmatism is generally associated with a change in stress regime from compressional to transcurrent that postdates continental collision by millions to tens of millions of years (Goodenough et al, 2021). Alkaline magmatism also represents a minor but ubiquitous component of oceanic island systems (Fitton and Upton, 1987;Willbold and Stracke, 2006).…”

“…In such settings, as seen in the modern East African rift (Mahood, 1984;Hutchison et al, 2016a;Rooney, 2020b), the East Greenland Tertiary province (Brooks and Nielsen, 1982), and the Mesoproterozoic Gardar rift of southwest Greenland (Upton et al, 2003;Upton, 2013) magmas tend to be sodic, rather than potassic, and commonly attain peralkaline compositions (Marks et al, 2011). Though several major mineralized alkaline-silicate and carbonatite systems are known from postcollisional settings, the academic community have thus far focused on the intraplate and rift-related systems, and the literature on postcollisional settings is therefore less comprehensive (Goodenough et al, 2021). Melt compositions in postcollisional settings tend to be potassic, and mantle melting here has been attributed to slab break-off, lithospheric delamination, and adiabatic decompression associated with orogenic collapse (Kay and Mahlburg Kay, 1993;Huw Davies and von Blanckenburg, 1995).…”

“…Many rift-related and intraplate alkaline-silicate rocks have Nd-Hf isotope compositions that overlap with those of high-µ (HIMU) oceanic island basalts (Bell and Simonetti, 2010), indicating that their melt source is mantle dominated and contains a recycled oceanic crust component (Chauvel et al, 1992). Some alkaline-silicate systems, especially in postcollisional settings, have compositions that extend toward enriched mantle 2 and enriched mantle 1 (EM-2 and EM-1) oceanic island basalt compositions (Goodenough et al, 2021), reflecting contributions from continental crustal sources (Willbold and Stracke, 2010). Here, continental material is either introduced to magmas through assimilation during ascent or more likely via incorporation into the mantle source, for example via subduction of sedimentary rocks.…”

Lanthanides as impurities in the Bayer production cycle of the aluminum hydroxide from Sierra Leone bauxite

“…The search for new natural resources of critical metals has encouraged research works about identifying some new available raw materials supply and new strategies for intensive valorization. The recent published reviews are available on such subjects as occurrence, exploration and analysis of the critical lanthanides minerals [4][5][6][22][23][24][25][26][27][28][29][30][31], as well as the new strategies to bring all these resources in the circular economy [32][33][34][35]. Also, valuable reviews cover different subjects concerning the mobilization of huge deposits residual materials from other large-scale industries, like red mud and other poly-metallic resources [36][37][38][39][40][41][42][43][44][45][46][47], phosphate materials and phospho-gypsum [10-12, 18, 27, 48-51] and the coal and coal ashes [52][53][54][55][56].…”