Precambrian basement gneisses at Orrefjell occur in a north-south trending mountain north of the Precambrian Salangsdalen tectonic window in Troms. The Orrefjell alaskite pegmatite and the Precambrian gneiss, which it intrudes, are overlain by a series of relatively thin flat-lying Caledonian nappes. The uranium mineralization, bosted by the alaskite pegmatite, is interpreted to represent late stage differentiates and late magmatic fluids of the alaskite pegmatite, enriched in uranium together with trace elements
like Nb, Y, Ce and the REE. A Rb-Sr whole-rock isotope study of the pegmatite failed to define an isochron, but the errorchron points towards a Middle Proterozoic magmatic event. The high 87Sr/86sr
initial ratio (0. 735) suggests that the pegmatite was derived by anatexis from continental crustal material. U-Pb isotope studies of uraninite from the mineralized zone yielded an age of 1745 Ma, suggesting a Svecofennian age of the mineralization as well as the alaskite pegmatite. The obtained ages and the general geology of the area indicate that the Precambrian rocks at Orrefjell constitute a separate Precambrian basement window, although it cannot be excluded that it represents a detached slice, separated from the rocks of the Salangsdalen window by a subhorizontal thrust.