Uraniferous phosphorite and apatite-bearing gneisses in the Proterozoic of Finland

Uranium is known to occur in the Proterozoic of Finland (1) within the Lower Proterozoic/Archaean boundary zone, (2) as strata-bound mineralizations in the Lower Proterozoic (Karelian) quartzites, (3) in albitite and albite diabase dykes cutting across these quartzites, (4) as uraninite-carbonate veins in a Lower Proterozoic (Karelian) greenstone-phyllite formation in northern Finland, (5) in migmatized Lower Proterozoic (Svecofennian) schists in southern Finland, and (6) in metamorphic phosphatic rocks of the Lower Proterozoic (both Karelian and Svecofennian) schist belts. However, the uranium occurrences found so far are too small or of too low grade to be of economic importance for the present. Here, the uranium- and phosphorous-bearing (U-P-type) occurrences at Nuottijarvi, Lampinsaari, and Temo are briefly described, and the characteristics of this type of uranium mineralization are summarized. The U-P occurrences are usually bedded, and the host rocks include fine-grained apatite-quartz-feldspar rock, and apatite-bearing or phosphorite-banded dolomite and skarn rock. Uranium occurs as uraninite crystallites within the apatite grains; it may also be contained in the apatite as a substitute for calcium. The grade of the occurrences is usually 0.03-0.04% U and 3-5% P2O5, and their Th content less than 10 ppm. Radiometric U-Pb dates indicate that the U-P occurrences were most probably deposited 1900-2080 Ma ago and were subjected to regional metamorphism about 1880 Ma ago. The metamorphic grade varies from intermediate greenschist to high amphibolite facies. The U-P occurrences are interpreted to be syngenetic and of marine sedimentary or mixed sedimentary and volcanic origin. It is also suggested that the uraniferous phosphatic rocks and other Proterozoic phosphorite occurrences in Finland and Sweden may belong to a possible Lower Proterozoic phosphogenic province of the Baltic shield. (author)