The Ilímaussaq Alkaline Complex, South Greenland - 2015

Rare Earth Deposits & Mining

Marks and Markl reported in 2015 that the Ilímaussaq complex in South Greenland is a well-studied multiphase alkaline to peralkaline intrusion of Mesoproterozoic age. Most of the Ilímaussaq rocks are extremely enriched in alkalis, iron, halogens, high-field-strength elements (HFSE) and other rare elements, forming one of the most differentiated peralkaline rock suites known.

The major factors causing the extreme differentiation trends are low oxygen fugacity and silica activity as well as very low water activity in the melts. These inhibit the early exsolution of aqueous NaCl-bearing fluids and facilitate the enrichment of alkalis and halogens in the melts, thereby increasing the solubility of HFSE. The unusually long crystallization interval of these rocks and the suspected continuous transition from melt to fluid results in extensive (auto)metasomatism and hydrothermal overprint. Primary mineral assemblages are therefore partially resorbed in most rock units and replaced by secondary minerals to various extents.

The Ilímaussaq complex is a well-known example of magmatic layering in peralkaline plutonic rocks. Recent investigations of mineral chemical trends in the layered rocks permit better understanding of their formation. The mechanism of crystal mats formation in the cooling magma causing crowding effects during settling of the layering-forming minerals is believed to govern the formation of the Ilímaussaq layered sequence.

Despite more than 100 years of research and hundreds of publications on Ilímaussaq, important aspects on the origin of some Ilímaussaq rocks, the architecture and deep structure of the complex and the significance of hydrocarbons and bitumens present in the peralkaline rocks remain unclear. Thus, plenty of room for further studies on these unusual rocks exists and interdisciplinary research is needed to better understand the genesis of this unique magmatic complex.

The authors concluded that in all, there is no direct information on the actual melt compositions coexisting with the observed mineral assemblages—neither for the Ilímaussaq complex nor for the Gardar province in general, since to date, no melt inclusion study on the Ilímaussaq rocks or other Gardar rocks has been performed. Such a study would reveal many important details on the geochemical evolution and the liquid line of descent of the fascinating Ilímaussaq magmatic system.

Original URL: https://www.i2mconsultants.com/downloads/corp/MarksandMarkl2015.pdf