STAGES IN FORMATION OF URANIUM MINERALIZATION IN THE SALLA-KOULAJARVINSKAYA ZONE (NORTHERN KARELIA): GEOLOGICAL AND ISOTOPE GEOCHRONOLOGICAL DATA
On the base of U-Pb, Rb-Sr and Sm-Nd isotopic data it is shown that formation of uranium mineralization in the Paleoproterozoic Salla-Koulajarvinsky belt (Northern Karelia) was the long-lasted multi-stage process - developed over more than 1 Ga: from Paleoproterozoic up to Paleozoic. The first stage - 1.75 Ga ago corresponds to the Svekofennian metamorphic event - regional albitization. The process was dated with Rb-Sr (isochronic age of albitites is 1754 ± 39 Ma) and U-Pb methods (age of rutile is 1756 ± 8 Ma). At this stage, with the lower limit of temperature 400-450 °C, conditions were favorable for mobilization and migration of uranium, not for its deposition in minerals. The second stage, 1.62 Ga, was the time of alteration of rocks at the regressive stage of Svekofennian metamorphic event, when carbonate and chlorite rocks were formed after albitites. The age of this stage was estimated according to ThO <sub>2</sub>,UO <sub>2</sub>, and PbO contents in uraninite as 1627 ± 42 Ma. Probably deposition of uraninite took place at this stage at temperature not higher than 300-350 °C. The final - third stage 385 Ma ago corresponds to the Paleozoic tectonic activation and formation of Caledonian alkaline intrusions. Uranium minerals were probably re-deposited at this stage, U-Pb age of brannerite is 385 ± 2 Ma.