Incompatible element

T.Tukiainen Nb-Tn Mineralisation in the Motzfeldt Centre

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• published: 14 May 2016
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Abstract The Motzfeldt Centre (1310 ± 10 Ma) is one of the major, central complexes in the Gardar Province of alkaline igneous activity. The Motzfeldt Centre is made up of multiple intrusions of syenite with a wide range of textural and compositional characteristics. The syenites were emplaced as two main igneous phases into the Proterozoic Julianehåb Granite and the unconformably overlying Gardar supracrustal rocks. The syenites of the early igneous phase occur as isolated bodies which are truncated by the syenite units of the main igneous phase, the Motzfeldt Ring Series. The bulk of the Motzfeldt Ring Series constitutes three major, steep-sided, outward dipping intrusions of predominantly peralkaline syenite and nepheline syenite which young inwards. The apparent intrusion mechanism was a combination of ring fracture and block subsidence. Large quantities of the roofing sandstone and volcanics have been incorporated to the outermost unit of the Motzfeldt Ring Series, the Motzfeldt Sø Formation, where they are preserved as large rafts. The sandstone has, however, been largely assimilated and has given the outer zone of the Motzfeldt Sø Formation its unique (for the centre) quartz normative character. The Motzfeldt Sø Formation underwent an extreme in situ differentiation, probably due to effective crystal fractionation, which resulted in the formation of a peralkaline residuum rich in volatile and incompatible elements. The peralkaline residua gave rise to a complex of late peralkaline sheets of microsyenite and pegmatite and hydrothermal alteration with associated Th-U-Nb-Ta-Zr-REE mineralisation which increases in intensity towards the margins, and especially towards the roof of the intrusion. The mineralisation is probably the result of the combination of an incompatible element/volatile-enriched magmatic residuum and an influx of silica and meteoric water, which resulted in a dramatic increase in oxygen fugacity, acidity and hydrothermal activity. A volatile-saturated outer shell developed which facilitated the migration, accumulation and precipitation of the incompatible elements. The Motzfeldt Sø Formation hosts zones of economically interesting pyrochlore enrichment. The pyrochlore, characterized by the enrichment of LREE, Ta and U, shows a marked compositional variation depending on its relative depth in the Motzfeldt Sø Formation. The Ta content and the Nb/Ta ratio of the pyrochlore varies within wide limits, from 1.5 to 10.0% Ta2O5 and from 8 to more than 50 respectively. The pyrochlore at the deeper levels is enriched in Ta and Ca, whereas the pyrochlore at the higher levels of the igneous column is more enriched in Nb, U and LREE.