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At the surface it measures roughly 8 x 17 kilometers, and has been exposed for 1700 meters of thickness. The nearest village is Narsaq, a small coastal town with a population of about 1700.
The name derives from a prominent neighboring mountain, Ilímaussaq, located along the northeast border of the intrusion. The local scenery is austere; rocks are easily friable and outcrop boldly in shades of gray. Little vegetation is seen. Adjacent fjords are deep blue and contain great blocks of ice much of the year. The Ilímaussaq intrusion is mid-Proterozoic in age (~1160 Ma), one of ten in South Greenland that make up the ‘Gardar’ igneous province (~1350–1120 Ma). The chemistry and mineralogy of Ilímaussaq keep it unique, however—unique among these ten, and among the world. It is classed as peralkaline—that is, Na + K oxides dominate over Al oxides on a mole basis, and it includes highly sodic rocks, as well as rocks variously enriched in nearly half the periodic table. In fact, some 220 mineral species have been identified from Ilímaussaq thus far, making it one of the most richly speciated locales known [see species listing for detail]. Nine are known only from Ilímaussaq, and the world’s finest examples of many—such as the enchantingly fluorescent tugtupite—hail from Ilímaussaq. The unique-to-the-deposit fluorescent mineral sorensenite is perhaps a ‘signature’ species for Ilimaussaq’s bizarre chemistry: an odd sodium silicate of tin and beryllium, it’s named for a pioneering researcher in Ilímaussaq mineral science, Prof. Henning Sørensen.
Ilímaussaq is presumed to be the product of extended fractionation of an ancient alkaline basalt melt, partially contaminated with continental rock, first formed in an unknown deep magma chamber. Among the three phases or pulses of magma forming the intrusion, the last—a series which includes agpaitic* nepheline syenites—makes up the vast bulk of the deposit. Further fractionation within this last magma phase was extensive. Cumulate textures are common. Some rocks (kakortokite) became strongly enriched in Zr, Nb, Ti. This proceeded at least in part rhythmically; kakortokite outcrops with their red bands of eudialyte form one of the most spectacular examples of layered intrusion known. Other, younger rocks—specifically some medium- to coarse-grained lujavrites—became highly enriched in Be, Li, REE, U, and Th, as well as late-stage volatiles. Hydrothermal veins and pegmatites occur here as well.
Most of the fluorescent minerals of interest to us here are found in these rocks. For a superb summary of recent research on Ilimaussaq’s geology,
petrology and geochemistry, as well as an updated mineral listing, see
Sørensen et al. (2001). **Agpaitic rocks are peralkaline nepheline syenites which characteristically have Zr and Ti contained in complex silicates such as eudialyte and rinkite, rather than more commonplace zircon and titanite. Ilímaussaq is the ‘type’ locale for such rocks, which are also known from the Lovozero and Khibina massifs on Russia’s Kola Peninsula, and Mont Saint-Hilaire in Quebec, Canada.
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