Summary:
The REE and Zr-bearing Norra Kärr deposit is hosted by an alkaline igneous intrusion dominated by various sub-categories of nepheline syenite. The intrusion is termed “agpaitic grennaite” (named after the nearby town of Gränna), which is described as a peralkaline nepheline syenite containing complex and exotic silicate minerals such as eudialyte and rinkite (containing zirconium, titanium, rare earth elements). It is part of the Precambrian age (~1 Ga) Trans Scandinavian Igneous Belt (“TIB”), which comprises a giant elongated array of batholiths extending 1,400 km across Scandinavia from southeast Sweden to northwest Norway. The REE and Zr mineralisation is hosted by the complex silicate mineral eudialyte. There are minor amounts of secondary REE-bearing britholite and trace mosandrite. The eudialyte at Norra Kärr is relatively rich in REE compared to most other similar deposits and also contains a very high proportion of HREO.
The Norra Kärr complex has been emplaced in a rift setting (Lake Vattern) within the Trans Scandinavian Igneous Belt (“TIB”).
Geologically, the Norra Kärr complex is best defined as a zoned agpaitic, peralkaline, nepheline syenite - similar in many respects to other well-known peralkaline complexes, e.g. Ilimaussaq in Greenland, Dubbo in Australia, Kipawa in Canada and Lovozero on the Kola Peninsula of Russia.
The Norra Kärr intrusive complex is located close (1.52 km) to the eastern shore of Lake Vattern, a large rift-controlled lake. Following the LEM exploration efforts, the Norra Kärr complex is known to be an ovoid mass with an aerial extent of some 1.3 km by 450 m.
The intrusive underwent 3 significant phases of deformation:
a) D1 – N-S to NE-SW compression with development of subhorizontal to shallow-dipping foliation (variable intensity). S1 is sub-parallel to So magmatic layering. Folding appears restricted to small-scale intrafolial folds.
b) D2 – E-W compression with development of a regional N-S F2 synform. The synform dips moderately to the west and plunges shallow to moderate to the SW. The Norra Kärr intrusive is preserved in the synformal keel. A variable intensity F2 foliation overprints S1. This is generally a flattening fabric but is locally associated with reverse – thrust mylonite development on the overturned western limb (along the intrusive contact).
c) D3 – N-S compression with development of E-W and minor conjugate NE- / NW- trending kink folds across the intrusive. The kink folding produces localised complexities in the orientation of the dominant S0 / S1 fabric.
The mineralisation at Norra Kärr is at once both simple and complex. Simple in mineralogical terms in that nearly all the REE mineralisation is hosted in the complex zircono silicate mineral eudialyte. There are minor amounts of the (probably) secondary Ca-LREE-F-silicate (britholite) and trace mosandrite. The eudialyte at Norra Kärr is, relatively rich in REE compared to most other similar deposits and also contains a very high proportion of HREO.
Various studies (e.g. Sjöqvist et al, 2013), have shown that within the resource, the TREO content of the eudialyte varies between 6-10%. The percentage of HREO also varies from about 30% in the more central GTM zone to above 70% in more distal GTC zone. The total content and also distribution of the REEs within the eudialyte also varies throughout the deposit. Sjöqvist et al defined three compositional varieties of eudialyte group minerals at Norra Kärr:
• Fe-rich, REE-poor, classical pink eudialyte;
• Fe-Mn-bisected, HREE-rich eudialyte from “pegmatitic” grennaite; and
• Mn-rich, LREE-rich eudialyte from “migmatitic” grennaite.
Equally, it may be shown that ~60% of the zirconium is hosted by eudialyte with the remaining 40% hosted by the catapleiite (Ca/Na2ZrSi3O9•2H2O). Consequently, whilst the GTC domain contains LREO and HREO and in particular zirconium, the greater concentrations of LREO and HREO found in the GTM and PGT domains, coupled with the improved liberation characteristics of these domains make the PGT and GTM the main economic targets at Norra Kärr.