The Significance of Metasomatism in the Formation of the Tanbreez REE Deposit in South Greenland

The layering of the lower layered kakortokite in the per-alkaline Ilímaussaq complex has been interpreted as an orthocumulus rock. Petrographic observation and mineral chemical data from the topmost and the lowest part of the layered kakortokite show signs that indicate massive metasomatic overprint. The occurrence of globular structures in the top part of kakortokite and fine-grained inclusions in the lower layered kakortokite are interpreted as the precursor of kakortokite and the result of a subsolidus reaction between a fluid phase and the underlying rock, respectively. Two different processes led to the formation of kakortokite, a precursor where a clear repetitive layering occurs and a chemical reaction between a fluid phase and the underlying rock where different kakortokite types are randomly interstratified. Both metasomatic events led to a higher rare earth element (REE) grade of the original REE mineralization.