Field observations of peralkaline microsyenite units in the Motzfeldt Complex, South Greenland

This report documents field observations made on peralkaline units of the Motzfeldt Centre during summer 2016 as part of a joint initiative between the Geological Survey of Denmark and Greenland (GEUS) and Korea Institute of Geosciences and Mineral Resources (KIGAM).

The 1.275 Ga Motzfeldt Centre is a large intrusive centre within the Mid-Proterozic Gardar Province of igneous activity in South Greenland. It comprises multiple intrusions of dominantly miaskitic syenites that exhibit a wide range of textural and mineralogical characteristics. Magmatism was concentrated into two main phases that started with intrusion of the Motzfeldt Sø Formation. Extensive in-situ fractionation produced volatile- and incompatible element-rich peralkaline residua; crystallization of this residua, together with hydrothermal activity led to the generation of a Ta-Nb-Th-U-Zr-REE mineralization. The mineralization increases in intensity towards the roof and margins of the formation.

The geography of the complex is characterised by topographic variations that exceed 1700 m, and a network of valleys that deeply dissect the intrusive complex. As a consequence, large vertical sections expose the geology of Motzfeldt in exceptional detail. This makes the Motzfeldt Complex and excellent natural laboratory for studying crystallization and mineralization processes in alkaline intrusions.

The fieldwork focused on two sections of the Motzfeldt Sø Formation with the aim of collecting observations that further the current understanding of U and REE enrichment in the Motzfeldt Centre and broader Gardar Province.