Accessory mineral paragenesis and U potential of late orogenic potassic granites of southern Finland

A 100-150 km-wide zone of Paleoproterozoic basement, extending for ~500 km from SW Finland to Russia, represents one of the most U-Th-K enriched areas in Fennoscandia. The major U-Th-K rich rocks correspond to several generations of highly potassic granites. They were emplaced into Paleoproterozoic metavolcanic and metasedimentary rocks belonging to a presumed arc complex formed at ~1.90-1.87 Ga and accreted to central Finland during the Svecofennian orogeny at ~1.88 Ga. The most radioactive granites were emplaced between 1.85-1.79 Ga and are considered as late-orogenic. They are leucocratic and assumed to derive from low degree of partial melting of supracrustal rocks during a low-P, high-T metamorphic event. They usually form relatively thin undulating sheets, dikes, and pods and commonly exhibit gradual transition with migmatized enclosing rocks, rarely form small granite plutons. Their grain size varies from fine grain to pegmatitic. They are composed of K-feldspar, quartz, and plagioclase in highly variable proportions together with biotite (< 5 vol. %), ± muscovite, ± cordierite, ± garnet.
Abundance and paragenetic association of magmatic U-bearing accessory minerals are highly variable from one intrusion to another and include: zircon, apatite, ± monazite, ± uraninite, ± uranothorite, ± allanite. Yttrocolumbo-tantalite only occurs in one granite. Other accessories are ilmenite, magnetite, molybdenite and rarely tourmaline. Alteration minerals: sericite, chlorite, albite, epidote, hematite, and rare coffinite and LREE-Th carbonates are in minor amounts. Primary uraninite is partly leached (either hydrothermally or by weathering). Zircons can be strongly metamict and Zr, Si and Hf are variably substituted by Al, P, Ca, Y and U in domains crosscutting the magmatic zonation of the crystals, monazite is sometime corroded and allanite may be altered to Th-REE carbonates.
The composition of the granites varies from plagioclase-rich to K-feldspar-rich through eutectic granite compositions as generally observed in migmatitic domains. Locally quartz leaching (episyenitization) with strong albitization occurs. Most granites are weakly peraluminous (1.04 < A/CNK < 1.10), a few with garnet and cordierite schlierens may reach an A/CNK value of 1.55. Their Fe, Ti, and Mg contents are low. Their feldspar-related trace element contents vary largely (Ba: 45-1700, Rb: 85-290, Sr: 40-230 ppm). Their variable U (6-790 ppm), Pb (13-766 ppm), Th (1.6-1350 ppm), Zr (20-1010 ppm), Y (6 ~ 130 ppm), total REE (56-3060 ppm) contents reflect their extremely variable abundance in accessory minerals. Most samples have low Th/U (≤ 1) indicating strong U enrichment relative to the average crustal Th/U ratio of 3.5. The late orogenic potassic granites of southern Finland share many characteristics with Rössing alaskites, but accumulations large enough to form an economic deposit remain to be discovered. They also represent good U sources for vein type deposits.