Review of the geology and the apatite distribution of the carbonatite in the Lillebukt Alkaline Complex, Stjernøy Northern Norway

Abstract

This report gives an overview of the geology, the history of investigations and all previous data (now in digital form) on the distribution of P2O5 and apatite in the Lillebukt Alkaline Complex (LAC) and some selected adjacent areas. The LAC is situated on the southern central part of the island Stjernøy in the Altafjord 45 km NW of Alta. The complex comprises an area of about 13 km2 where the main rocks: hornblende clinopyroxenite, alkalisyenite and carbonatite occur in a crudely concentric pattern. In addition, large intrusions of nephelinesyenite occur in the southern part of the complex. All these rocks are post-dated by nepheline syenite pegmatites and by several generations of mafic dykes. We have a data set of 280 bulk rock analyses. The bulk of the samples are from the carbonatite and the hornblende clinopyroxenite, the two rock units in which significant contents of apatite are found. The carbonatites (218 analyses) have an average P2O5 content of 2.33%, with a maximum value of 6.45%, but only 23 samples have levels above 6% P2O5. The pyroxenites (56 analyses) have an average P2O5 content of 2.35 and a maximum value of 13.49%. 4 samples have P2O5 above 8%. All our data combined give an average P2O5 of 2.38% equal to 5.62 weight % of apatite. In well-exposed and well-sampled areas of about 300x300 meters, the spatial distribution of samples gives an average P2O5 of 3.00% in the carbonatite. The contents of heavy metals such as As and Cd are below their detection limits of 10ppm.\rWe believe that our dataset is sufficiently large and representative that it gives accurate information on the P2O5 and apatite contents of the relevant rocks within the Lillebukt Alkaline Complex on Stjernøy. A CD with the total analytical data is included together with this report.