Volker Möller

 

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Möller, V., Williams-Jones, A.E. and Heiligmann, M., 2011.

The nature and evolution of the Nechalacho REE deposit, NWT, Canada (GAC MAC 2011 Ottawa)

 

The Nechalacho rare metal deposit at Thor Lake, NWT, Canada comprises one of the world’s largest high-grade Rare Earth Element (REE) and High Field Strength Element (Y, Zr, Nb, Ta) deposits, and is of particular economic interest due to its enrichment in heavy REE (HREE). The principal REE-bearing minerals, zircon, allanite, fergusonite, bastnäsite, synchisite and monazite are present in a variably disseminated or condensed upper zone enriched in light REE and a lower layered cumulate zone enriched in HREE. The host rock is an aegirine-nepheline-sodalite syenite of miaskitic to agpaitic composition, the Nechalacho Syenite. This unit is part of a large, layered peralkaline complex characterized by highly variable grain size (from pegmatitic to fine-grained), low-viscosity flow textures, compositionally distinct cumulus layers, cyclic micro-layering, and a variety of secondary alteration features. In the upper zone, significant metasomatic remobilization of REE is indicated by veins of bastnäsite and vein-style aggregates of zircon.

Our study focuses on the contact relationships within the complex, the primary mineral assemblages and late-magmatic alteration processes in the host rocks and mineralized zones. Recent borehole data suggests that the Nechalacho Syenite intruded a gradational igneous sequence represented by the Thor Lake Syenite and the overlying Grace Lake Granite. The crystallization sequence and geochemical evolution of the complex were established on the basis of petrographic relationships and mineral and whole-rock chemical analyses. The primary mineral assemblage is persodic, iron-rich and oxidized, as indicated by abundant near end member aegirine, containing up to 0.6 wt. % zirconium.  Primary carbonate minerals and fluorite are present in the deep succession. Late magmatic, auto-metasomatic alteration was more reducing, leading to the formation of magnetite at the expense of aegirine. Locally, aegirine was altered to riebeckite or replaced by zircon. Late-stage hydrothermal replacement of magnetite by hematite indicates a return to oxidizing conditions. In the micro-layered cyclic sequence of foyaites and microsyenites of the cumulate zone, aegirine was completely replaced by magnetite and hematite, and primary zircono-silicates were pseudomorphed by zircon, allanite, quartz and fluorite. At depth, the magmatic assemblage grades into layered biotite-aegirine-amphibole-sodalite-syenite hosting zircon, pyrochlore, Na-zirconosilicates and sodic amphiboles, which occur as primary phases, but also form late-magmatic overgrowths on biotite.

The transition from silica over- to under-saturated lithologies, the formation of cumulus layers, the spatial variation of REE mineralogy, and the presence of volatiles in the magmatic and alteration stages were all key factors in the formation of the Nechalacho deposit.

 

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