On the occurrence of Steenstrupine in the Ilímaussaq massif, Southwest Greenland

Abstract

The Ilímaussaq alkaline massif, South Greenland. It is now known to be of widespread occurrence in this massif, being present in agpaitic rocks (naujaite, sodalite foyaite and lujavrite) and in late veins of hydrothermal affinity.

In the present paper three small areas within the Ilímaussaq massif have been selected for a rather detailed study of the mode of occurrence of the steenstrupine, namely the small island of Qeqertaussaq made up of naujaite which is cut by very thin veins of a black, lujavrite-like rock, felted ægirine and albite-analcime-natrolite rocks ; the head of Kangerdluarssuk with thicker veins of black lujavrite and green ægirine felt cutting the naujaite ; and the north coast of Tunugdliarfik which represents a deeper level in the intrusion, being composed of lujavrite with numerous inclusions of naujaite. The naujaite of the last-named area is cut by thin veins of aegirine felt, acmite, albite and analcime-natrolite. The lujavrite here has apparently assimilated naujaitic material.

Steenstrupine occurs in these three areas in lujavrite, late veins and in the naujaite adjacent to these rocks. It is also present in albite-analcime-natrolite-bearing replacement bodies in some zoned naujaite pegmatites.

Some features favour a metasomatic origin of the lujavrite, but it is concluded that the combined observations are best explained by a magmatic interpretation. The lujavrite is clearly later than the naujaite, its intrusion being guided by the joints of the naujaite. Considerable tilting and rotation of the naujaite inclusions in lujavrite is seen in places. In some lujavrite veins it is seen that the mise en place of the lujavrite was preceded by the formation of ægirine felt along the naujaite joints. The intrusion of the lujavrite was accompanied and succeeded by the formation of thin veins containing one or more of the minerals acmite, arfvedsonite, albite, analcime and natrolite.

In the lujavrite a very pure maximum microcline and a pure low albite have been formed in equilibrium indicating a low temperature of formation, probably of the order of 400° C. The albite and in cases also the microcline, nepheline and sodalite of the lujavrite, may be replaced, to varying extent, by analcime, and, more rarely, by natrolite. This also favours a low temperature of formation of the

lujavrite. At these low temperatures and in this very sodium-rich environment liquid immiscibility may have played a role as is mentioned in the discussion of peculiar spheroidal structures in the lujavrite.

Some of the analcime of the lujavrite may be a primary precipitate of the lujavrite magma, but most analcime is probably secondary after the primary alkali-aluminium silicates. This analcitization is considered to be late magmatic. The sodium-water-rich rest liquid of the lujavrite may be squeezed out and has effected analcitization of the naujaite inclusions. These late fluids are also responsible for the formation of the late veins made up of albite, analcime, natrolite and a number of rare minerals containing rare earths, Nb, Th, Mn, Li, P, F, Zn and S.