Downing, Todd; (2025) Investigating the Deportment of REEs and HFSEs in the Ilímaussaq Complex, South Greenland. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Ilímaussaq Complex of South Greenland (1.16 Ga ± 5 Ma) is the type locality for agpaitic rocks and hosts rhythmically layered nepheline syenites known as kakortokite. These rocks comprise 29 macrorhythmic units of black, red, and white layers and contain variable eudialyte, a Na-Ca-zirconosilicate that is the principal host of REEs and HFSEs. Although the mineralisation is of magmatic origin, the primary assemblage has been variably modified by late- to post-magmatic processes, leading to uncertainty over the scale, mechanisms, and timing of REE-HFSE redistribution. This thesis evaluates whether REE-HFSE deportment in kakortokite is controlled by primary magmatic crystallisation or modified during late-magmatic or secondary alteration involving external fluids. Integrated petrography, in-situ trace-element mapping, and stable-isotope analyses demonstrate that eudialyte is a primary magmatic phase and that Unit 0 evolved largely as a closed system. Preservation of magmatic zoning, trace-element ratios, and δ¹⁸O values indicates crystallisation through crystal-mat-driven differentiation governed by evolving halogen budgets. These results provide new mineralogical, geochemical, and isotopic evidence that resolves the respective roles of magmatic and late-magmatic processes in REE-HFSE distribution. Alteration was driven by a single, internally derived late-magmatic fluid that was F-rich within pseudomorphs and locally Cl-enriched along fractures. This caused selective but conservative element redistribution: REEs and HFSEs remained at the grain scale and were sequestered into secondary zirconosilicates (catapleiite) and Nb-REE minerals (nacareniobsite), while Na-Si-Cl were lost. Any externally sourced input was minor, fracture-bound, and insufficient to modify the bulk REE-HFSE budget. Overall, this study demonstrates that the kakortokite REE-HFSE inventory is predominantly magmatic and internally buffered during alteration, refining models of fluid evolution in agpaitic systems and improving predictions of critical-metal preservation in peralkaline rare-metal deposits.

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