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NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Bailey, J. C., Gwozdz, R., Rose-Hansen, J., & Sørensen, H. (2001). Geochemical overview of the Ilímaussaq alkaline complex, South Greenland. Geology of Greenland Survey Bulletin, 190, 35-53. https://doi.org/10.34194/ggub.v190.5172 _______________ The Ilímaussaq alkaline complex is one of the latest members of the Mesoproterozoic Gardar igneous province. It was emplaced in three phases, (1) augite syenite, (2) alkali acid rocks and (3) the dominant agpaitic nepheline syenites. The immediate parent for the nepheline syenites is the augite syenite whose geochemical features, such as the low Zr/Nb ratio (3.7), match benmoreites from a nearby alkali basalt series. The agpaitic nepheline syenites are characterised by exceptionally high contents of Zr, Hf, Nb, Ta, REE, Th, U, Sn, Li, Be, Rb, Zn, Pb, Sb, W, Mo, As and Ga, and the volatile elements F, Cl, Br, I and S but exceptionally low levels of Ba, Sr, Co, Cu, Ni, Sc, V and Cr. Fractionation of cumulus phases with a distinctive geochemistry, such as sodalite (rich in Cl, Br, I, B), eudialyte (Zr, Hf, Nb, Ta, W, As) and arfvedsonite (Co, Sc, V, Cr), caused exhaustion of these elements in the residual magmas. The agpaitic magmas underwent extreme fractionation with the final lujavrite forming after 99% crystallisation of the augite syenite. Zirconium was apparently soluble in Ilímaussaq melts up to a concentration of c. 9000 ppm. Whole-rock analyses define a number of discrete Zr–U arrays which are restricted to certain intervals of the cumulate stratigraphy and are taken as evidence for liquid layering in the Ilímaussaq magma chamber. The distribution of the less common trace elements (Bi, Cd, Co, Ge, In, I, Hg, Sc, Se, V, W) and of the more abundant trace elements which failed to form minerals (Br, Cs, Ga, Hf, Rb, Sr) is summarised.

Many nickel–copper sulfide orebodies contain Cu- and Fe-rich portions. The Fe-rich ore is generally richer in Os, Ir, Ru, and Rh and poorer in Pt, Pd, and Au than the Cu-rich ore. In komatiite-hosted ores Ni tends to be concentrated in the Cu-rich ore, whereas in tholeiitic ores it tends to be concentrated in the Fe-rich ore. The origin of this zonation could be due to crystal fractionation of Fe-rich monosulfide solid solution from a sulfide liquid. The crystal fractionation would produce an Fe-rich cumulate enriched in Os, Ir, Ru, and Rh and a fractionated liquid enriched in Cu, Pt, Pd, and Au. This model can be tested for zoned orebodies by applying experimentally determined partition coefficients for the metals into monosulfide solid solution. We have compared our experimental results with those of other workers to show that the partition coefficients are strongly influenced by the sulfur content of the system. There is a positive correlation between the partition coefficients and sulfur content of the monosulfide solid solution and between the partition coefficients and the sulfur content of the liquid. In sulfur-saturated and sulfur-over-saturated experimental systems, the metals behave in a manner consistent with the model, that is, Os, Ir, Ru, and Rh are compatible with monosulfide solid solution, Cu, Pd, and Pt are incompatible, and Ni has a partition coefficient close to 1. The use of the experimental partition coefficients is demonstrated in the numerical modelling of a zoned komatiite-related ore (Alexo, Abitibi Greenstone Belt) and a zoned tholeiite-related ore (Oktyabr'sky, Noril'sk region, Siberia). In both cases, the experimental partition coefficients numerically model the composition zones of the actual ores. This supports the model of fractional crystallization of a monosulfide solid solution from a sulfide liquid to form zoned orebodies. Furthermore, it indicates that the experimentally determined partition coefficients are geologically reasonable.

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