OFFICIAL most likely are confined to the uppermost 0.5-1 km of the complex, and that ultra-/mafic rocks constitute the rock volume deeper down (Ramberg, 1973). The complex is approximately circular (~3 km diameter) and lies ~13 km west of the Oslo Rift syenite plutons; surface exposure is locally poor due to Holocene post-glacial marine clay cover reaching up to ~50 m. The Fen Complex evolved through five partly overlapping stages. (1) Early emplacement of alkaline silicate rocks and compositionally variable carbonatites; this stage was accompanied by intense alteration (“fenitization”) of the country rock. (2) Subsequent intrusion of calcite carbonatite (søvite) dikes, phlogopite–apatite–pyrochlore dikes, and dolomite carbonatite dikes. (3) Emplacement of Fe-rich dolomite carbonatites (FDC), representing the principal REE-enriched lithology. (4) Intrusion of ultramafic damtjernite as dikes and diatremes, interpreted as near-direct mantle-derived injections. (5) Late hydrothermal alteration and hematitization generated rødberg (“red-rock”) hematite ores and further modified earlier units including secondary REE upgrading. Stages 3–5 likely overlapped, as indicated by red-rock clasts within FDC and damtjernite breccias. REE enrichment within the FDC is typically pervasive and laterally consistent across exploratory drill holes, with structural permeability (faulting/brecciation) acting as the key control on hematitization intensity and local REE concentration. Figure 1 shows a simplified geological map of the Fen Complex in the project area. Mining of different commodities has previously been carried out within the Fen Complex. Iron mines were operated from the 1650s to 1927 on hematite ores within the “rødbergite” (Dahlgren, 2006). Søvite was mined for niobium in the period 1953 to 1965. Figure 1 – Simplified geological map of the Fen Complex (Dahlgren, 2006). REE enrichment of the FDC unit (Fe-Dolomite Carbonatite, “Rauhaugite”), generally is pervasive and consistent with an elevated level of REEs evident throughout the majority of exploration drill holes within this unit.
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