Mineralogy and geochemistry of REE-Zr-Nb mineralised nepheline syenites in the peralkaline Ilímaussaq complex, South Greenland

• Main Author: Borst, Anouk Margaretha
• Format: Book
• Language: English
• Published: Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen 2016
• Subjects: Greenland
• Online Access: https://curis.ku.dk/portal/da/publications/mineralogy-and-geochemistry-of-reezrnb-mineralised-nepheline-syenites-in-the-peralkaline-ilimaussaq-complex-south-greenland(00069bde-6a65-4024-aa84-bacd19488afd).html; https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122734937405763

Summary: The rare earth elements (REE) share unique physical, chemical and light-emitting properties that are of great importance to the high-tech industry. Among the many rocks containing appreciable amounts of REE, alkaline igneous rocks and carbonatites provide important resources for these elements. This thesis focusses on the Mesoproterozoic Ilímaussaq complex (c. 1160 ± 5 Ma) in Southern Greenland, a well-known alkaline intrusion that hosts significant resources of REE, as well as zirconium, niobium, tantalum, uranium, lithium and zinc. The Ilímaussaq complex is part of the rift-related Gardar province and comprises a series of highly unusual magmatic rocks described as agpaitic nepheline syenites. Agpaitic rocks are characterised by the presence of complex Na-Zr-Ti-silicates such as eudialyte and rinkite instead of common accessory Zr and Ti minerals like zircon and titanite. The present study focusses on the lower part of the magmatic sequence in the southern part of the complex (Kringlerne), comprising a c. 250 m thick sequence of spectacularly layered syenites called kakortokites. The kakortokites are of particular interest for their enrichment in Zr, Nb, Ta and REE, largely hosted in the mineral eudialyte and its alteration products. Mineralogical, geochemical and isotopic data were obtained and discussed with the overall aim to better understand the respective roles of magmatic and hydrothermal processes in the evolution of the kakortokites and their impact on the economic potential of these rocks. It is shown that widespread hydrothermal alteration of primary magmatic eudialyte has led to the formation of fine-grained intergrowths of secondary Zr, Nb and REE phases. The alteration was not associated with significant loss or gain of the valuable metals, although the increased mineralogical complexity may have important consequences for extraction of the metal-bearing minerals from the ore. Geochronological studies (U-Pb, Sm-Nd and Rb-Sr) of a single kakortokite unit provide additional insights in the isotopic effects of hydrothermal alteration and element transport at the subsolidus stage in different minerals, confirming that the Rb-Sr system is particularly sensitive to fluid interaction. Detailed compositional analyses of eudialyte across the sequence provide new constraints on bulk and mush melt evolution, the emplacement history and layering dynamics. Overall, the study contributes to an improved understanding of mineralisation and magma chamber processes in alkaline systems.