Geological characteristics of the Sokli carbonatite complex, Finland

Abstract

The nepheline syenite deposit mined by Norsk Nefelin is situated within a Caledonian province of basic and ultrabasic rocks near the northern tip of Norway. The deposit lies near the southern shore of the island of Stjernoy and crops out between 250 and 700 m above sea level. At the surface the nepheline syenite is lens shaped, 1,700 m in length with a maximum width of 300 m, and covers an area of about 270,000 m 2 . The wall rocks to the northeast are carbonatites and nepheline-rich gneisses and to the southwest amphibole-rich rocks.Two major facies of nepheline syenite, biotite type and hornblende-pyroxene type, have been distinguished. Both types contain perthite (average 56 modal %) and nepheline (average 34 modal %) with minor plagioclase, calcite, clinopyroxene, hornblende, biotite, sphene, and magnetite. In the biotite type the main mafic is biotite (2.5-6.0 modal %); in the hornblende type, common hornblende (0.3-2.5 modal %). The hornblende-nepheline syenite contains 0.2-1.2 modal % sphene, whereas this mineral is rare in the biotite type. Corundum and zircon are present in both facies but are rare; their frequency in the product is subject to routine control.The nepheline syenite consists of anhedral-subhedral feldspars; the other minerals are interstitial. Nepheline crystals contain feldspar relicts. The texture may suggest a metasomatic origin of the nepheline syenite in its present form.Geochemical studies show that Fe 2 O 3 and CaO concentrations are lowest in an elongate zone close to the contact. In the same zone, SiO 2 and Na 2 O contents are at a maximum.A marked foliation in the nepheline syenite is caused by a crystallographic orientation of perthite. In the center and the southeast end of the body the foliation dips 70 degrees SW; toward the northwest the dip steepens to 70 degrees NE.Mafic dikes a few centimeters to several meters in thickness cut the nepheline syenite; their volume is estimated at 6 to 7 percent of the mass.Along fault zones the nepheline syenite is partly altered to zeolite, prehnite, kaolin, etc.