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Elena Afanasieva, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
Vitaly Mikhailov, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
Claude Caillat, AREVA NC (France)
Michel Cuney, CREGU (France)
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The analysis of spatial-temporal patterns of uranium and complex mineralization distribution in the Baltic Shield shows that the highest levels of uranium accumulation and the great variety of geological types of uranium mineralization appeared in later epochs of the ore formation (Svecofennian, Riphean, Vendian-Paleozoic). Blocks and particular structures subjected to polychronous tectonomagmatic and tectonothermal activation and hosting regional structural-stratigraphic unconformity zones are the most enriched in uranium. These blocks underwent intensive lithophile differentiation and polychronous granite formation completed in the generation of high-radioactive granite massifs.
In the Baltic Shield, the Svecofennian megablock corresponds to the above-mentioned conditions of ore location. Of the greatest importance is its southern part, which underwent intensive Riphean tectonomagmatic activation, which resulted in the emergence of a belt of uranium-enriched rapakivi granite and a series of superimposed Riphean depressions (Pasha-Ladoga, Muhos, Pori); the pre-Riphean Karku unconformity-type uranium deposit is located in the Pasha-Ladoga depression. The Dvina-Kandalaksha Graben (Belomorian megablock), where a number of uranium ore showings are known, requires more detail exploration for uranium deposits of the pre-Riphean unconformity type.
The joint zone of the Baltic Shield and the Russian Plate is also promising for uranium. Several uranium deposits and ore showings located in the basal horizon of the Vendian cover over the massifs of uranium-enriched leucocratic granitoids and assigned to the pre-Vendian unconformity type (Slavyanka, Ratnitskoe etc.) occur there. A large group of uranium-ore targets associated with Ordovician dyctionemous shale also occur in the southern slope of the Baltic Shield.
In addition to unconformity-type deposits, abundant ore showings of other types are of prognostic interest. Ore targets associated with albitite and carbonate-albite metasomatite, sulphide-quartz-carbonate veins, volcanogenic assemblages, activation granitoids are purely uranium. Uranium-precious-metal-vanadium deposits in fold-fault zones, uranium-thorium-rare-metal ore showings in quartz-feldspathic metasomatite and pegmatite, phosphorus-uranium ore showings in skarn-like rocks, uranium-bearing rare metal-rare earth mineralization in alkali metasomatite are assigned to the group of complex ore targets.
Main perspectives for increasing the uranium resource potential in the Baltic Shield are related to the pre-Riphean structural-stratigraphic unconformity zones of the Pasha-Ladoga, Dvina-Kandalaksha grabens, fold-fault zones in Early Proterozoic depressions with black shale of the Onega, Imandra-Varzuga and other depressions and with activation granitoids. The uranium potential of the pre-Vendian structural-stratigraphic unconformity widespread in the joint area of the Baltic Shield and the Russian Plate requires further prognostic-metallogenic study.
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