ALKALINE PEGMATITES

Compared to granitic pegmatites our knowledge of alkaline pegmatites is very limited. One of the obvious reasons being that granitic bodies are more abundant than alkaline complexes and that granitic pegmatites have yield precious stones, hence more incentive for studying these. However, with the increased interest in REE and other elements alkaline complexes and their pegmatites have attracted new interest. A major difference between granitic pegmatites and those of this presentation is that the former are usually formed in the country rock away from the intrusive body whereas in the alkaline systems most pegmatites are formed within the complex.

Alkaline complexes are enriched in elements like REE, Nb and Be similar to the geochemistry of granitic pegmatites. However, the mineralogy and processes are different and for example Nb is found to be extremely mobile in alkaline environments. Actually, secondary Nb-minerals are often among the last minerals to form after zeolites. Many alkaline pegmatites will show a core rich in zeolites with clear replacement textures, but whether these are formed during the last bit of solidification or a much later event is not always obvious. However, it is clear that this zeolitisation is a contributing factor to the wealth of minerals in alkaline systems.

This presentation will focus on observations mainly from pegmatites in the Ilímaussaq alkaline complex in South Greenland and from the Larvik Plutonic Complex in Norway with an emphasis on rare element minerals and element mobility. However, rather than provide conclusions I wish to encourage input from the audience about how to generate a more systematic study of alkaline pegmatites and what we can learn from the work carried out on granitic pegmatites.