Geochemistry of tertiary tholeiites and picrites from Qeqertarssuaq (Disko Island) and Nuussuaq, West Greenland with implications for the mineral potential of comagmatic intrusions

P C Lightfoot, Chris Hawkesworth, K Olshefsky, T Green, W Doherty, R R Keays

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    71 Citations (Scopus)

    Abstract

    Tertiary continental flood basalts on Qeqertarssuaq and Nuussuaq in West Greenland contain similar to 3 km of picrites and variably contaminated tholeiites. The picrites are in the Naujanguit member of the Vaigat Formation and they have 7-29 wt% MgO, La/Sm = 0.9-2.1, and Nd-143/Nd-144 = 0.51263-0.51307. They appear to have crystallised from high-Mg parental magmas (14.4-16.4 wt% MgO) with isotope and trace element ratios similar to recent Icelandic picrites. Discrete horizons of tholeiites, including the Asuk and Kuganguaq have elevated SiO2 (50-58 wt%), La/Sm = 3-7 Sr-87/Sr-86 = 0.70550-0.71224, and low Nd-143/Nd-144 = 0.51234-0.51174. These lavas have low Cu and Ni abundances (typically 10-50 ppm Ni or Cu), and in the case of the Asuk on Qeqertarssuaq, they contain droplets of native iron. The low Cu and Ni contents are attributed to scavenging by magmatic sulphides formed in response to crustal contamination of picritic magmas. Two contamination trends are recognised, one to a sediment end-member with high Th/Nb and Archaean model Nd ages, and the other to a metaigneous component with high La/Sm, low Th/Nb and Rb/Nb, and Proterozoic source ages. Overall, Pb-206/Pb-204 varies from 16.47-21.68. Both contamination trends are associated with low Cu and Ni, and high SiO2, and it is argued that the magmatic sulphides were triggered by the increases in silica, rather than simply by the introduction of additional crustal-derived sulphur. Geochemically, the Asuk and Kuganguaq rocks resemble the most contaminated Nadezhdinsky lavas of the Siberian Trap, which are widely regarded as the source of the Ni and Cu mineralisation in the giant Noril'sk deposits. Mass balance considerations indicate that the parental liquids to the contaminated magmas contained sufficient Ni, Cu, S and platinum group elements to form substantial magmatic sulphide deposits. However, unlike the lavas at Noril'sk, the contaminated (low Cu and Ni) West Greenland basalts are in isolated units with no evidence for a gradual recovery in Ni and Cu abundances with height in the lava column. Comparison with Noril'sk suggests that although significant quantities of metals were scavenged by sulphides in West Greenland, the metal contents of the sulphides may not have been upgraded by continued interaction with subsequent magma batches.

    Original languageEnglish
    Pages (from-to)139-163
    Number of pages25
    JournalContributions to Mineralogy and Petrology
    Volume128
    Issue number2-3
    Publication statusPublished - Jul 1997

    Keywords

    • CONTINENTAL FLOOD BASALTS
    • TRACE-ELEMENT
    • NORILSK REGION
    • SIBERIAN TRAP
    • ISOTOPIC EVIDENCE
    • INSIZWA COMPLEX
    • ICELAND PLUME
    • METALLIC IRON
    • MANTLE PLUME
    • SR-ISOTOPE

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