Norra Kärr alkaline complex

Introduction
Alkaline rocks form less than 1 percent of the total volume of igneous rocks on Earth, but their names make up half of all igneous rock names (Sørensen, 1974). Despite this, their remarkable mineralogical diversity has brought them repeatedly to the attention of petrologists and mineralogists, with the result that alkaline rocks account for about half of all igneous rock names. Sorensen (1974) lists no fewer than 400 alkaline rock types. This diversity springs largely from an abundance of alkalis and deficiency in silica which together generate a large number of mineral species not stable in more silica-rich, alkali-poor magmas.

However, a large part of the attention given to alkaline rocks is due to their characteristic high concentrations of incompatible or large-ion lithophile elements (LILE). These are often of more than academic interest as most of the world's resources of niobium, tantalum, diamonds and the rare-earth elements are found in or around alkaline igneous rock bodies. Also, alkaline rocks provide us with physical samples of the Earth’s deep interior in the form of mantle xenoliths.

One of the pioneers of the modern study of alkaline rocks was Danish petrologist Henning Sørensen whose life’s work mainly revolves around the enigmatic Ilímaussaq alkaline complex in South Greenland. In 1974 he was the editor of a textbook named "The Alkaline Rocks", which was a ground-breaking book that sought to advance and unify the field of alkaline rock geology. The definition of alkaline rock, according to Sørensen is: a rock of igneous origin where the molar ratio of alkalis (Na2O+K2O) to alumina (Al2O3) and to silica (SiO2) exceeds 1:1:6, where either alumina or silica or both are deficient.

Agpaitic rocks
Agpaitic rocks are peralkaline nepheline syenites, characterised by the presence of complex zirconosilicate minerals such as eudialyte, catapleiite, and members of the wöhlerite and rosenbuschite groups as rock-forming minerals. The original definition of agpaitic rocks by Ussing required whole-rock molar proportions of (Na + K)/Al ≥ 1.2. This ratio is known as the agpaitic index (A.I.) or peralkaline index (P.I.). Sørensen introduced an alternative definition, based on the presence of complex Na-Ca-Ti-Zr-silicates instead of, e.g., zircon and ilmenite; which is accepted in current petrographic nomenclature.

Agpaites generally have low concentrations of elements such as Mg, (Ca), Sc, V, Co, Cu, Ni, (Sr), and Ba, but they are commonly exceptionally rich in elements such as Li, Be, Na, Y, Zr, Nb, lanthanides, Hf, Ta, and halogen elements, mainly F and Cl. The common petrological model to explain these rocks is that they evolve by fractional crystallisation of mantle- derived melts, such as alkali basalt and nephelinite or nepheline benmorite. Low contents of elements such as Mg, Sc, Co, and Ni imply fractional crystallisation of mafic minerals. Low contents of Ca, Sr, and Eu in agpaites such as Ilímaussaq and Norra Kärr additionally witness to plagioclase crystallisation and removal.

The type locality for agpaitic rocks is the Ilímaussaq alkaline complex in southern Greenland, which contains different agpaitic nepheline syenites such as foyaite, naujaite, kakortokite, and lujavrite. Other important localities of agpaites are the Khibina and Lovozero complexes, Kola Peninsula, Russia, the Mont Saint-Hilaire complex, Quebec, Canada, parts of the Tamazeght complex, Morocco, the Pilanesberg complex, South Africa, and the pegmatites at Langesundsfjord, Norway.

Norra Kärr alkaline complex

The Norra Kärr peralkaline complex is about 300 km southwest of Stockholm in southern Sweden (Fig. 1). Norra Kärr is a small, Proterozoic complex of agpaitic nepheline syenite rocks. The complex was emplaced along a north-south trending corridor of ductile shear zones in the westernmost part of the Paleoproterozoic Trans Scandinavian Igneous belt (TIB). The TIB comprises an elongate array of mainly felsic plutonic and volcanic rocks extending for about 1400 km along the Scandinavian Peninsula from southeastern Sweden to northwestern Norway.
The intrusion is roughly elliptical, with a northsouth long axis of about 1300 m and an east-west short axis of about 500 m. Structural observations from drill core suggest that the intrusive defi nes a west-dipping (45-60°) doubly plunging synform. Based on the magmatic layering and orientation of early deformation fabrics, the body may have been emplaced as a sill (possibly lopolithic) into the Palaeoproterozoic granite-gneissic basement of the TIB.

The bulk of Norra Kärr consists of an agpaitic, fine-grained, foliated, greyish-green, peralkaline nepheline syenite, which is dominantly aegirine-bearing. The alkali feldspar consists of essentially pure endmember albite and microcline. The dominant feldspathoid is nepheline, which is sometimes replaced by zeolite minerals. Eudialyte is a characteristic major phase and can occur both in the fine-grained matrix as well as centimetre-sized grains. Catapleiite is a common mineral, which dominantly occurs as centimetre-sized, bluish-white streaks with abundant inclusions of the other matrix minerals.

Fenite
Surrounding the Norra Kärr Alkaline Complex is a heterogeneous aureole in which the host granitoids were affected by alkali metasomatism (fenitisation). The thickness of the metasomatised zone is 25–100 m wide.

Grennaite
Approximately 80% of the intrusion consists of varieties of grennaite, a mesocratic fine-grained aegirine-rich nepheline syenite carrying the zirconosilicate minerals, eudialyte and catapleiite. The term grennaite is derived from the nearby town of Gränna.

Towards the center of the complex the three main grennaite varieties are:

• Grennaite with catapleiite: The outermost layerof grennaite is the typical catapleiite-rich rock. Locally, instead of orin addition to catapleiite are centimetre-sized eudialyte grains. In some cases, catapleiite occurs asrims on eudialyte.

• Grennaite (Pegmatitic): This unit is further characterised by the variable abundance of medium- to coarse - grained leucocratic nepheline syenite veins and dykes (pegmatites). The pegmatites dyke largely consist of nepheline, albite, microcline, aegirine, and eudialyte. A spectacular variety also contains blue catapleiite crystals.

• Grennaite (migmatitic): Is the innermost grennaite unit, wrapping around the central kaxtorpite. It is composed of the same minerals as the other grennaite types, but zeolites are more common and pectolite occurs as a minor phase. These rocks differs from the other grennaites both in texture and chemical composition. The rock is commonly tightly folded, has a paler color, and generally exhibits a recrystallized or slightly migmatitic texture.

Kaxtorpite
Kaxtorpite occurs at the center of the complex and is a melanocratic nepheline syenite, which is a foliated and commonly folded. The rock type was named after the farm Kaxtorp. It is made up mainly of nepheline, microcline, albite, eckermannite, aegirine, and catapleiite.
Pectolite and lorenzenite occur frequently, as well as secondary zeolite minerals. Willemite, normally a secondary mineral after sphalerite, is an accessory mineral in the kaxtorpite, but sphalerite has at present not been identified in this unit, although it has been observed elsewhere.

Lakarpite
Törnebohm described a rock type from the northern parts of the complex, in which he identified rosenbuschite, and named it lakarpite after the nearby farm Lakarp. This rock is a mesocratic nepheline syenite that is mainly composed of albite, arfvedsonite, and nepheline with microcline, rosenbuschite, mosandrite, apatite, titanite, and abundant fluorite.
Another type of lakarpite is found near the eastern boundary of the complex, usually in or near the transition from the pegmatitic grennaite and grennaite with catepleiite domain. This rock is generally mafic and is made up of arfvedsonite and aegirine, microcline, albite, nepheline, pectolite, pink eudialyte, mosandrite, and fluorite. This unit, with pink eudialyte, is named lakarpite with Eudialyte. Lakarpite mainly has lower concentrations of MgO and CaO than kaxtorpite and is richer in ZrO2. Lakarpite generally has a more massive texture than kaxtorpite.

Pulaskite
Pulaskite occurs mainly along the western and northern contacts of the intrusion. It is a medium-coarse grained rock low in REEs and typically carries large rounded microcline augen in a groundmass of albite, aegirine, amphibole, microcline and minor biotite and nepheline. Rosenbuschite, apatite, titanite and fluorite occur as accessories. Pulaskite may occur together with fine-grained grennaite as alternating zones.

norrakarr2019(15).jpg

Outcrop at Norra Kärr. From: Hildegard Wilske

Pegmatitic grennaiite with large eudialyte crystals (red): Hildegard Wilske

Pegmatitic grennaiite with large eudialyte crystals (red): Hildegard Wilske

Grennaiite with eudialyte crystals (red): Hildegard Wilske

Grennaiite with catepleiite crystals (red-brown). From: Hildegard Wilske

Grennaiite with catepleiite crystals (red-brown). From: Hildegard Wilske

Grennaiite with catepleiite crystals (red-brown). From: Hildegard Wilske

Kaxtorpite with large microcline crystals set in a aegirine-rich groundmass. From: Hildegard Wilske

Kaxtorpite with large microcline crystals set in a aegirine-rich groundmass. From: Hildegard Wilske

Lakarpite with nepheline (grey) and arfvedsonite (dark). From: Hildegard Wilske

Lakarpite with nepheline (grey) and arfvedsonite (dark). From: Hildegard Wilske

Pulaskite with large rounded microcline augen in a groundmass aegirine. From: Hildegard Wilske

Pulaskite with large rounded microcline augen in a groundmass aegirine. From: Hildegard Wilske

Bibliography

• Sjöqvist, A.S.L., Cornell, D.H., Andersen, T., Erambert, M., Ek, M., and Leijd, M. (2013). Three compositional varieties of rare-earth element ore: Eudialyte-group minerals from the Norra Kärr alkaline complex, southern Sweden. Minerals, 3(1), pp.94–120.
• Thesis (2015). Axel S.L. Sjöqvist: Agpaitic rocks of the Norra Kärr alkaline complex: Chemistry, origin, and age of eudialyte-hosted zirconium and rare-earth element ore.
• Saxon, M., Leijd, M., Forrester, K., and Berg, J., (2015): Geology, mineralogy, and metallurgical processing of the Norra Kärr heavy REE deposit, Sweden. In: Simandl, G.J. and Neetz, M., (Eds.), Symposium on Strategic and Critical Materials Proceedings, November 13-14, 2015, Victoria, British Columbia. British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2015-3, pp. 97-107.
• Tasman metals Ltd. Project (http://www.tasmanmetals.com/s/Norra-Karr.asp)