Geology and architecture of the Norra Kärr Zr-REE deposit, Southern Sweden


Geology and architecture of the Norra Kärr Zr-REE deposit, Southern Sweden

Atanasova, P.; Leijd, M.; Gutzmer, J.

The Norra Kärr rare metal deposit in Southern Sweden represents one of the largest resources of rare earth elements (REE) in Europe. The deposit is hosted by a metamorphosed and deformed peralkaline nepheline syenite intrusion, which covers an area of 350m by 1100m. This intrusion is situated within a suite of Proterozoic gneisses and granites referred to as the Växjö Granite. The Växjö Granite belongs to the Trans-Scandinavian Igneous belt (1.85-1.65 Ga); the age of the peralkaline intrusion is rather poorly constrained at about 1545 ± 61 Ma (Blaxland 1977, recalculated by Welin 1980). Gross magmatic layering and orientation of early deformation fabrics suggest the intrusion was emplaced as a sill within the granitic basement (Rankin 2011). The granitoid rocks close to the contact to the peralkaline intrusion exhibit signs of fenitization (Adamson 1944, Eckermann 1968).

The Norra Kärr nepheline syenite and the surrounding basement have been deformed by weak to moderate N-S directed compression (Rankin 2011). This late-stage fold event has produced rhombic bulging and necking of the intrusion. The body is preserved within an overturned synformal hinge suggesting potential for other nepheline syenite bodies within the region. The intrusion has undergone not only modification of its shape by 3 phases of folding, but also possible transport of the body from its original emplacement locus by both N-S movement and E-vergent movement. In addition, the overprinting of the regional NNE-trending structural corridor by later episodes of extensional faulting also obscures the original geometry.

The Norra Kärr REE mineralization is notably enriched in the coveted heavy REE (HREE) and has been granted by the Geological Survey of Sweden a “national interest” status. The intrusion was first discovered by the Swedish Geological Survey in the earlier year of the 20th century and first described by A. E. Törnebohm in 1906. Since its discovery it has been explored – and on a small scale exploited - for nepheline, Zr and Hf. Since August 2009 Tasman Metals pursues an aggressive exploration program that focuses on rare metals mineralization, in particular HREE, in the intrusion. Currently, resources in the deposit are 41.6 Mt @ 0.57 % TREO with 51 % HREO/TREO and 1.7 % ZrO2 (indicated) and 16.5 Mt @ 0.64 % TREO with 49% HREO/TREO and 1.7 % Zr2O (inferred).

The lithotypes that comprise the intrusion are mostly identified by local names only; they share an agpaitic composition but their spatial and genetic relationship of the lithotypes remains unclear at present. The most common lithotype present is referred to as grennaite, best described as a fine to medium grained meta-syenite consisting of alkali feldspar, nepheline, aegirine, natrolite, eudialyte and catapleiite (Adamson 1944, Blaxland 1977). Less common lithotypes include lakarpite (arfvedsonite-albite nepheline meta-syenite), pulaskite (microcline-arfvedsonite-albite nepheline meta-syenite) and kaxtorpite (eckermannite-microcline-aegirine-pectolite nepheline meta-syenite). Exploration has revealed that much of the rare metal mineralization is associated with pegmatitic and migmatitic intervals within the complexly zoned intrusion, suggesting an important influence of (fluid-induced?) recrystallization and remobilization on the character and distribution of rare metal mineralization. Mineralogical studies have shown REE bearing minerals to include eudialyte group minerals and very minor mosandrite and cerite. The Zr-silicates catapleiite and eudialyte host the majority of the zirconium.

Keywords: REE; rare earth elements; zirconium; deposit; alkaline; rare metals

  • Poster
    Geochemistry of mineral deposits, 15.-20.07.2012, Andover, NH, United States

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Publ.-Id: 17180