Age and Formation Conditions of U Mineralization in the Litsa Area and the Salla-Kuolajarvi Zone ( Kola Region , Russia )

The Kola region (NE of Fennoscandian Shield) has high uranium potential. The most promising structures within the Kola region in respect to uranium enrichment are the Litsa area and the Salla-Kuolajarvi zone. The principal objective of the present study was to define sequence and timing of uranium deposition within these areas. Isotopic (U-Pb and Rb-Sr) exploration of the rocks from Skal’noe and Dikoe U occurrences of the Litsa area and Ozernoe occurrences of the Salla-Kuolajarvi zone was carried out. As it follows from isotopic dating, the principal stages of uranium mineralization had taken place 2.3–2.2, 1.75–1.65, and 0.40–0.38 Ga ago, simultaneously with the stages of alkaline magmatism in the Kola region, which provided the uranium input. Uranium mineralization was related to hydrothermal and metasomatic events under medium to low temperature of ~550 ◦C at 2.3 Ga to ~280 ◦C at 0.4 Ga.


Introduction
On the Fennoscandian Shield in Russia, the areas of high uranium potential are the Onezhskaya structure and Ladoga zone in Karelia, and several structures of the Kola region [1,2].The most promising structures within the Kola region in respect to uranium enrichment are the Litsa area in the northwestern part of the Murmansk region, and the Salla-Kuolajarvi zone at the boundary with Northern Karelia ([3], Figure 1).
The Paleozoic uranium mineralization is the most significant and well-studied one [4,5]).
The Archean age of uranium mineralization in Skal'noe and Dikoe areas of 2700 ± 50 Ma [4] is based on U-Pb age of uraninite from vein granitoids, obtained by Isotope Dilution Thermal Ionization mass-spectrometry (ID TIMS method).The oldest U-Pb age obtained for uraninite from the Dikoe ore occurrence by secondary ion mass-spectrometry (SIMS) SHRIMP II method is 1825 ± 20 Ma [5].U-Pb (ID TIMS) uraninite age for the Polyarnoe area is 2165 ± 42 Ma [4].This is supported by an age of 2185 ± 81 Ma, obtained by the SHRIMP II method for uraninite of Namvara occurrence of the same mineralization type [5].Uraninite from the Cheptjavr occurrence was dated at 1771 ± 34 Ma (SIMS CAMECA IMS-3F [5]), yet uranium mineralization of the Cheptjavr occurrence was believed to be of the same age as the Polyarnoe one [4].U-Pb (ID TIMS) age of pitchblende is 375 ± 18 Ma for the Polyarnoe area, 370 ± 20 Ma for the Litsevskoe area [4] and 455 ± 6 Ma (SHRIMP II) for the Dikoe area [5].Uranium-bearing metasomatites of this age are concentrated in the intersections of tectonic zones with transverse faults, where Paleozoic alkaline, alkaline-magnesian metasomatites and lamprophyre dykes also occur [4].
Thus, we have four stages of uranium mineralization distinguished in [4] with three of them confirmed by SIMS dating [5].Tables of analytical isotope data are published neither in [4] nor in [5], which makes it impossible to estimate the quality of analyses.The first stage of uranium mineralization remains the most doubtful one since the Archean gneisses and plagiogranites of the region with low U content can hardly be a source for the oldest U mineralization.
The principal objective of the present study was to define a sequence of events and timing of the uranium deposition within the Litsa area and Salla-Kuolajarvi zone.Mineralogical and isotopic (U-Pb and Rb-Sr) study of the rocks from Skal'noe and Dikoe ore occurrences of the Litsa area and Ozernoe occurrence in the Salla-Kuolajarvi zone was carried out.

The Litsa Area
The Litsa area is located in the northwestern part of the Kola Region in the conjunction zone of the Central-Kola and Murmansk terranes.Lying at the intersection of Titovka-Uraguba and Litsa-Araguba tectonic zones bounded by N-S and E-W striking faults, the Litsa area has a complex mosaic-block structure (Figure 1a,b).Structural control is of crucial importance for the spatial distribution of the uranium mineralization in the area.Most of the Litsa ore occurrences are localized in zones of variable permeability, such as deep faults and shear zones with inherent ductile and brittle deformations, which provide favorable conditions for uranium concentration.The largest part of the Litsa area belongs to the Central Kola terrane composed of repeatedly metamorphosed 2.9-2.8Ga gneisses of the Archean Kola Group, and orthometamorphic 2.8-2.7 Ga tonalite-trondhjemite-granodiorite (TTG) and granite-gneiss rocks [8].The geodynamic evolution of the Litsa area as part of the Kola terrane began in the Late Archean and included several tectonic-magmatic cycles.
The Skal'noe and Dikoe areas are the largest ore occurrences of the REE-Th-U mineralization type of supposed Archean age (Figure 1a,b).The Skal'noe area lies within the Titovka-Uraguba tectonic zone, while Dikoe is located at the intersection of the Titovka-Uraguba and Litsa-Araguba tectonic zones (Figure 1b).The areas are composed of migmatized biotite, garnet-biotite, two-mica gneisses of the Kola Group strongly deformed in the Dikoe area, and plagiogranites with intercalates of amphibole gneisses, amphibolite bodies, and gabbro-diabase dykes.
Uranium anomalies were detected in fault-related fold zones (1 × 5 and 1.5 × 6 km) in strongly deformed gneisses where veins of pegmatoid granites and quartz-feldspathic metasomatites are developed.The uranium mineralization was traced by drilling to a depth of 400 m [4].The average uranium content in the Late Archean gneisses and granitoids is mainly below 2.5 ppm.The maximum content of U in vein pegmatoid granites and metasomatites is up to 0.4 wt.% in the Skal'noe area and up to 0.2 wt.% in the Dikoe area [4,5].
Our study was aimed to the Ozernoe occurrence of U-Mo ores [3], located in the eastern flank of the belt (Figure 1c).Geology of the Ozernoe area, as well as mineral composition of the ore, was described in details earlier [3].Albitite and associated chlorite-albite, carbonate-albite, dolomite, and quartz metasomatites form an echelon structure of lenses of the size up to 10 × 90 m in plagioamphibolites-metamorphosed gabbroids.Carbonate-albite and dolomite metasomatites form small veins or bodies of irregular shape up to 20 cm thick in the central part of albitite lenses.Quartz and dolomite-quartz metasomatites were formed at the final stage of the alteration processes.
Albitite and associated carbonate and chlorite metasomatites are highly radioactive, which is provided by high uranium content reaching up to 0.12 wt.%, registered in carbonate-albite and carbonate metasomatites.
Metasomatites of the Ozernoe occurrence, as well as the host plagioamphibolites, displays some extent of sulfide and oxide mineralization.Altered plagioamphibolites contain pyrite, chalcopyrite, ilmenite, and magnetite.In albitite, hematite and rutile substitute magnetite and ilmenite, respectively.Both albitite and carbonate metasomatites contain molybdenite, marcasite, uraninite and brannerite.

Rock Samples
Rocks for the present study were sampled in two most representative early U ore occurrences of the Litsa area: Dikoe and Skal'noe, which belong to REE-Th-U type of mineralization (Figure 1a,b) and from albitites of the Ozernoe occurrence (Figure 1c).

Skal'noe Occurrence
Samples were collected from plagiogranites with varying degrees of schistocity.Sample KT-1massive plagiogranite.The rock consists of plagioclase (60%), quartz (35%), and biotite (3-5%); and microcline, muscovite, and chlorite (<1%).Accessory phases are represented by apatite, zircon, and iron-oxides.Some biotite crystals are showing alteration to chlorite.This is a massive fine-to medium-grained rock with hypidiomorphic texture and with areas of granoblastic, which occupy at least 10% of the total rock.The secondary minerals are represented by muscovite and microcline.Microcline is observed in areas of granulation, rarely replacing plagioclase.

Analytical Methods
Mineralogical and isotope analyses were performed in the Geological Institute, Kola Science Center, Russian Academy of Sciences.Minerals for the analysis were prepared using magnetic separation in a Frantz separator and density separation in heavy liquids with subsequent handpicking of minerals under a binocular microscope.Monazite and uraninite grains were embedded in epoxy mounts and polished.The back-scattered electron (BSE) images were studied on a LEO-1450 Scanning Electron Microscope equipped with an XFlash-5010 Bruker energy dispersive spectrometer (GI KSC RAS, Apatity, Russia) with QUANTAX 200 software.
U-Pb (ID-TIMS) analysis was used to date monazite, uraninite, brannerite, and rutile.Digestion of minerals and U and Pb extraction followed method of Krogh [12].Lead and uranium concentration were determined using the mixed 208 Pb + 235 U tracer.The measurements were performed on MI-1201T and Finnigan MAT 262 mass spectrometers.Pb and U were loaded together on outgassed single Re filaments with H 3 PO 4 and silica gel.The temperatures of measurement were 1300 • C for Pb and 1500 • C for U.The laboratory blanks were 0.3 ng for Pb and 0.01 ng for U.The Pb isotope ratios were corrected for mass fractionation with a factor of 0.18% per amu (for MI-1201T) and 0.11% per amu (for Finnigan MAT-262) based on repeat analyses of the standard NBS SRM 982 [13].The U analyses were corrected for mass fractionation with a factor of 0.08% and 0.003% per amu for MI-1201T and Finningan MAT-262, respectively, based on repeated analyses of the NBS U 500 standard.Reproducibility of the U-Pb ratios was estimated as 0.7% from the analysis of standard zircon IGFM-87 (Ukraine) at 95% confidence level.
Rb-Sr dating of whole rocks and rock-forming minerals (biotite, muscovite, K feldspar, plagioclase, apatite) was carried out via MI-1201T mass-spectrometer using Ta filaments.The Rb and Sr contents were determined by isotopic dilution method.The Sr isotope ratios were normalized to the NISTSRM-987 value of 0.71034 ± 0.00026.The uncertainties of the 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios do not exceed ±0.04% and ±0.5% (2σ), respectively.The laboratory blanks were 2.5 ng for Rb and 1.2 ng for Sr.All calculations were made with the program Isoplot/Ex 3.70 [14].

The Litsa Area
The Skal'noe and Dikoe areas contain two major types of mineralization: monazite-zirconuraninite and molybdenite-thorite-uraninite [4].Mineral composition of samples from pegmatoid veins show that the first association is represented by monazite (35%), apatite (30%), zircon (25%), and uraninite (10%).Uraninite occurs as intergrowths with apatite and zircon (Figure 2a).Uraninite seems to be the later phase since it overgrows intergrowths of monazite and apatite.Monazite and apatite definitely grew together from viscous melt because they do not have crystallographic faces and form clusters of monazite with apatite domains and apatite inclusions.Then these clusters were overgrown with uraninite grain.Zircon form independent well faceted crystals.Galgenbergite Ca(Ce,La) 2 (CO 3 ) 4 •H 2 O and anglesite rims develop between uraninite and plagioclase (Figure 2b).Rare small inclusions of thorite (about 10 µm) were found within monazite grains.Molybdenite forms thin rims on uraninite grains and together with galenite fills fractures in uraninite.
U-Pb analysis of monazite from pegmatoid veins (samples TK-20 and TK-22) of the Dikoe uranium occurrence have shown that three monazite fractions yielded near concordant upper intercept age of 2549 ± 7 Ma (MSWD = 0.13) with a lower intercept at 204 ± 55 Ma (Table 1, Figure 3).U-Pb dating of uraninite was carried out for two pegmatoid veins of the same outcrop (samples TK-19 and TK-20).The uraninite grains were extracted from the epoxy mount after electron microscope analysis.A regression line through five points defines an upper intercept age of 2276 ± 21 Ma with a lower intercept at 402 ± 88 Ma (Table 1, Figure 3).High mean square weighted deviation (MSWD) of 12 shows chemical variation of uraninite grains.Rare small inclusions of thorite (about 10 µm) were found within monazite grains.Molybdenite forms thin rims on uraninite grains and together with galenite fills fractures in uraninite.
U-Pb analysis of monazite from pegmatoid veins (samples TK-20 and TK-22) of the Dikoe uranium occurrence have shown that three monazite fractions yielded near concordant upper intercept age of 2549 ± 7 Ma (MSWD = 0.13) with a lower intercept at 204 ± 55 Ma (Table 1, Figure 3).U-Pb dating of uraninite was carried out for two pegmatoid veins of the same outcrop (samples TK-19 and TK-20).The uraninite grains were extracted from the epoxy mount after electron microscope analysis.A regression line through five points defines an upper intercept age of 2276 ± 21 Ma with a lower intercept at 402 ± 88 Ma (Table 1, Figure 3).High mean square weighted deviation (MSWD) of 12 shows chemical variation of uraninite grains.°С-closure temperature for an Rb-Sr biotite system [15].An age of 1701 Ma corresponds to the development of secondary minerals at hydrothermal-metasomatic impact.
Mineral Rb-Sr isochrons (Ap-Pl-WR-Bt) for plagiogranites from Skal'noe area (sample KT-1 and KT-2) defined two ages: 1902 ± 21 Ma and 2370 ± 25 Ma (Table 2, Figure 4b).Older age of 2370 ± 25 Ma most likely has resulted from dating of coarser-grained biotite fraction where retention of Sr is better due to larger size of crystals.Rb-Sr dating of minerals from the same vein (sample TK-20) defined an age of 1964 ± 21 Ma for primary biotite and whole rock and of 1701 ± 15 Ma for secondary muscovite, plagioclase and microcline (Table 2, Figure 4a).The first age is probably a cooling age at temperature of about 350 • C-closure temperature for an Rb-Sr biotite system [15].An age of 1701 Ma corresponds to the development of secondary minerals at hydrothermal-metasomatic impact.
Mineral Rb-Sr isochrons (Ap-Pl-WR-Bt) for plagiogranites from Skal'noe area (sample KT-1 and KT-2) defined two ages: 1902 ± 21 Ma and 2370 ± 25 Ma (Table 2, Figure 4b).Older age of 2370 ± 25 Ma most likely has resulted from dating of coarser-grained biotite fraction where retention of Sr is better due to larger size of crystals.
Minerals 2018, 8, x FOR PEER REVIEW 7 of 12 microcline (Table 2, Figure 4a).The first age is probably a cooling age at temperature of about 350 °С-closure temperature for an Rb-Sr biotite system [15].An age of 1701 Ma corresponds to the development of secondary minerals at hydrothermal-metasomatic impact.Mineral Rb-Sr isochrons (Ap-Pl-WR-Bt) for plagiogranites from Skal'noe area (sample KT-1 and KT-2) defined two ages: 1902 ± 21 Ma and 2370 ± 25 Ma (Table 2, Figure 4b).Older age of 2370 ± 25 Ma most likely has resulted from dating of coarser-grained biotite fraction where retention of Sr is better due to larger size of crystals.

The Salla-Kuolajarvi Zone
Uraninite is associated with marcasite, molybdenite, melonite, and rarely altaite, which fill fractures and inclusions or form micrograins at the boundary of uraninite with silicates and carbonates (Figure 5a).Xenomorphic grains of brannerite were found in a form of a chain at the boundary of carbonate metasomatite and albitite (Figure 5b).Brannerite grains are X-ray amorphous, the mineral was identified by X-ray diffraction method after annealing and thereby regaining crystalline structure.

The Salla-Kuolajarvi Zone
Uraninite is associated with marcasite, molybdenite, melonite, and rarely altaite, which fill fractures and inclusions or form micrograins at the boundary of uraninite with silicates and carbonates (Figure 5a).Xenomorphic grains of brannerite were found in a form of a chain at the boundary of carbonate metasomatite and albitite (Figure 5b).Brannerite grains are X-ray amorphous, the mineral was identified by X-ray diffraction method after annealing and thereby regaining crystalline structure.The three rutile fractions from albitite (sample KP-19) yielded U-Pb upper intercept age of 1757 ± 7 Ma with the lower intercept at 416 ± 14 Ma (Figure 6a; Table 2).Paragenesis of hematite and rutile implies high-oxidation conditions [16], which are known to be unfavorable for uranium deposition, U-minerals therefore, were formed later, probably, at the same time as carbonate metasomatites.
Minerals 2018, 8, x FOR PEER REVIEW 9 of 12 The three rutile fractions from albitite (sample KP-19) yielded U-Pb upper intercept age of 1757 ± 7 Ma with the lower intercept at 416 ± 14 Ma (Figure 6a; Table 2).Paragenesis of hematite and rutile implies high-oxidation conditions [16], which are known to be unfavorable for uranium deposition, U-minerals therefore, were formed later, probably, at the same time as carbonate metasomatites.The uraninite age obtained by CHIME method [17] is 1627 ± 42 Ma.This value corresponds to the formation time of carbonate-quartz and carbonate metasomatites, which controls distribution of uraninite mineralization.This age coincides within the error limits with Rb-Sr age of 1610 ± 30 Ma, obtained for the Mayskoe gold deposit-located 8 km to the east from the Ozernoe, where it was interpreted as the age of metasomatic alteration of rocks [11].
The brannerite (UTi2O6) age of 385 ± 2 Ma was obtained for three brannerite fractions as a lower intercept age (Figure 6a; Table 2).The upper intercept of the discordia line is in a good agreement with the rutile age.
Rb-Sr isochron data for biotite, apatite, albite, and whole rock defines albitite age at 1754 ± 39 (Figure 6b; Table 2) and coincide with rutile age.The dolomite point lies outside of the isochron and confirms the field evidence that carbonate mineralization occurred later.A similar age (1728 ± 39 Ma) was obtained using the Sm-Nd method for quartz-albite-carbonate-amphibole metasomatite from Alim-Kursujarvi occurrence, located 12 km north from the Ozernoe area.

Discussion
According to the obtained isotope data we have the following sequence of events.The monazite age of 2549 ± 7 Ma, obtained from two pegmatoid veins of the Dikoe uranium occurrence, most likely, corresponds to the time of pegmatoid veins crystallization.The rock-samples have a magmatic hypidiomorphic texture, where monazite belongs to magmatic mineral assemblage typical for granites.According to [4] the first mineralization in the Litsa area was described as REE-Th-U in pegmatoid granites and metasomatites.The obtained monazite age shows that REE-Th mineralization (since monazite is the main concentrator of REE and Th) occurred at 2.55 Ga as a result of magmatic crystallization of pegmatoid veins.U-Pb age of uraninite from the same pegmatoid veins determines the first stage of uranium mineralization at 2267 ± 27 Ma, associated with hydrothermal processes.Quite possibly, this uranium mineralization belongs to the same stage as U mineralization in chlorite-albite metasomatites and albitites of the Polyarnoe, Namvara and Cheptjavr occurrences, dated with high uncertainties at 2165 ± 42 Ma [4] and 2185 ± 81 Ma [5].The uraninite age obtained by CHIME method [17] is 1627 ± 42 Ma.This value corresponds to the formation time of carbonate-quartz and carbonate metasomatites, which controls distribution of uraninite mineralization.This age coincides within the error limits with Rb-Sr age of 1610 ± 30 Ma, obtained for the Mayskoe gold deposit-located 8 km to the east from the Ozernoe, where it was interpreted as the age of metasomatic alteration of rocks [11].
The brannerite (UTi 2 O 6 ) age of 385 ± 2 Ma was obtained for three brannerite fractions as a lower intercept age (Figure 6a; Table 2).The upper intercept of the discordia line is in a good agreement with the rutile age.
Rb-Sr isochron data for biotite, apatite, albite, and whole rock defines albitite age at 1754 ± 39 (Figure 6b; Table 2) and coincide with rutile age.The dolomite point lies outside of the isochron and confirms the field evidence that carbonate mineralization occurred later.A similar age (1728 ± 39 Ma) was obtained using the Sm-Nd method for quartz-albite-carbonate-amphibole metasomatite from Alim-Kursujarvi occurrence, located 12 km north from the Ozernoe area.

Discussion
According to the obtained isotope data we have the following sequence of events.The monazite age of 2549 ± 7 Ma, obtained from two pegmatoid veins of the Dikoe uranium occurrence, most likely, corresponds to the time of pegmatoid veins crystallization.The rock-samples have a magmatic hypidiomorphic texture, where monazite belongs to magmatic mineral assemblage typical for granites.According to [4] the first mineralization in the Litsa area was described as REE-Th-U in pegmatoid granites and metasomatites.The obtained monazite age shows that REE-Th mineralization (since monazite is the main concentrator of REE and Th) occurred at 2.55 Ga as a result of magmatic crystallization of pegmatoid veins.U-Pb age of uraninite from the same pegmatoid veins determines the first stage of uranium mineralization at 2267 ± 27 Ma, associated with hydrothermal processes.Quite possibly, this uranium mineralization belongs to the same stage as U mineralization in chlorite-albite metasomatites and albitites of the Polyarnoe, Namvara and Cheptjavr occurrences, dated with high uncertainties at 2165 ± 42 Ma [4] and 2185 ± 81 Ma [5].
The Rb-Sr data obtained for pegmatoid vein of the Dikoe area and plagiogranites of the Skal'noe area have shown 1964 ± 21 and 1902 ± 21 Ma ages for primary magmatic minerals (biotite, apatite and plagioclase).These data probably represent cooling ages, since the crystallization age of pegmatoid veins and plagiogranites are 2.55 (monazite age) and 2.8 Ga [8], correspondingly.The closure temperature for an Rb-Sr biotite system is of about 350 • C [15] and any high-temperature event can disturb it.The thermal event of this time can be 1.94 Ga intrusion of the Kaskel'javr granitoid complex (Figure 1b) [18].The older age of 2370 ± 25 Ma obtained for apatite, plagioclase, and coarser-grained biotite fraction probably reflects the first hydrothermal event of rocks at 2.3 Ga, which is masked by subsequent events in other samples.The temperature of formation of uranium metasomatites in vein pegmatoid granitoids is estimated at 500-550 • C using mineral geochemistry [19].An Rb-Sr system of biotite, which essentially determines isochrons, shows that since 1964-1902 Ma, the temperature did not exceed 350 • C.
An age of 1701 ± 15 Ma (Rb-Sr) for secondary muscovite, plagioclase and microcline from pegmatoid vein corresponds to a hydrothermal-metasomatic event.This stage was also manifested in the Salla-Kuolajarvi zone where it resulted in albitite formation (1757 ± 7 Ma U-Pb rutile age).The closure temperature for U-Pb rutile system is 400-450 [20], Rb-Sr isochron for biotite, apatite, albite, and whole rock defines the same age of 1754 ± 39 Ma.Thus it is not a cooling age but a real age of albitite formation at temperature not higher than 350 • C-the closure temperature for an Rb-Sr biotite system.
The brannerite formation at 385 ± 2 Ma in the Ozernoe area was related to Paleozoic hydrothermal processes.Uranium mineralization of this age is widely developed in the Litsa area as well, where it is represented by a pitchblende formation [4,5].The influence of Paleozoic processes is also reflected in the lower Discordia intercepts for U-Pb uraninite and monazite data.
Analysis of the chemical composition of rocks of the Litsa area [5] shows that uranium content is low in the Archean gneisses and plagiogranites (1-2.5 ppm), and a bit higher in the proterozoic granites (Kaskel'javr and Litsa-Araguba complexes)-3-5 ppm.At the same time, as it was mentioned in [4], the uranium content as well as amount of other incompatible elements increases dramatically in the host gneisses and plagiogranites in areas of concentration of uraniferous pegmatites and metasomatites.It means that the formation of uranium concentrations is not associated with uranium redistribution from the host rocks, and we need to look for a U source outside the host area.
Discussing the uranium mineralization of the Litsa area, it must be kept in mind that the Litsa area is located in close proximity to the Pechenga structure, and the processes of ore formation in the region should not be considered separately [21].Thus, a formation of 2.3-2.2Ga subalkaline volcanics of the Pirtijarvi suite [22,23] within the Pechenga structure might have triggered the oldest stage of Litsa U mineralization.
The next stage of mineralization described by [4] as "Th-U mineralization at 1850-1750 Ma in quartz-albite-microcline and quartz-microcline metasomatites" seemed to consist of two different types connected with different regional processes.The first stage is dated by 1825 ± 20 Ma uraninite from quartz-feldspar metasomatites of the Dikoe area [5].Brannerite ores of 1.83 Ga within Litsevskoe occurrence are reported in [21] but without isotopic data.The source of the uranium-enriched fluid of this age could be the mantle melts, which gave rise to the foidolite-carbonatite series of the Proterozoic Gramyaha-Vyrmes massif of 1884 ± 6 Ma [24], whose initial magmas are similar in composition to the volcanics of the fourth suite of the Pechenga structure.
The next stage took place 1.75-1.65Ga ago-uraninite formed in the quartz-albite-microcline metasomatites of the Litsa area (Beregovoe [4] and Cheptjavr [5] occurrences) and in the carbonate-chlorite metasomatites of the Salla-Kuolajarvi area.This was the period of last magmatic activity in the region, when emplacement of 1.77-1.75Ga granitoids of the Litsa-Araguba complex [25]; granite veins of similar age and composition in the basement of the Pechenga structure and 1.71 Ga lamprophyre dykes had taken place [8].This stage was accompanied with extensive hydrothermal activity, widely manifested in the Pechenga structure [26] and throughout the Kola region, which was inferred from Rb-Sr and K-Ar data [8].According to our Rb-Sr data the temperature of U ore deposition during this period did not exceed 350-300 • C.
The last (Paleozoic) 0.40-0.38Ga stage of uranium mineralization was associated with the formation of U-bearing albite-hydromica-chlorite metasomatites typical for the Litsa area [1,4,5].Activation of hydrothermal and metasomatic processes at this time was connected with the Paleozoic 408-360 Ma alkaline magmatism (alkaline and nepheline syenite intrusions with carbonatites) [27].The temperature of this stage according to mineral geochemistry [19] was 280-220 • C.

Conclusions
The earliest stage of uranium mineralization in the Litsa area occurred 2267 ± 27 Ma ago according to U-Pb age of uraninite from vein pegmatoid granitoids of the Dikoe area.The magmatic age of these veins is 2549 ± 7 Ma based on U-Pb monazite dating.Monazite as the main concentrator of REE and Th probably defines the development of the oldest REE-Th mineralization in the area.
Isotopic U-Pb and Rb-Sr data both obtained in this study and published earlier show that the principal stages of uranium mineralization in the Litsa area and Salla-Kuolajarvi zone had taken place 2.3-2.2,1.75-1.65,and 0.40-0.38Ga ago.
Comparison of the sequences of endogenous events in the Litsa area and Salla-Kuolajarvi zone with those in adjacent structures in the time span from 2.5 to 0.4 Ga allows us to link the stages of uranium mineralization to the regional cycles of endogenous activity, accompanied by the mantle alkaline magmatism in the Kola region, which provided the uranium input.
Uranium mineralization was related to hydrothermal events under medium to low temperatures from 550-500 • C at 2.27 Ga to 350-300 • C at 1.75-1.65Ga and 280-220 • C at 0.40-0.38Ga.A question of why uranium was concentrated in the Litsa area should be the subject of a future study.

Figure 3 .
Figure 3. U-Pb isotope data for monazite and uraninite from vein pegmatoid granitoids of the Dikoe occurrence of the Litsa area.

Figure 3 .
Figure 3. U-Pb isotope data for monazite and uraninite from vein pegmatoid granitoids the Dikoe occurrence of the Litsa area.

Figure 3 .
Figure 3. U-Pb isotope data for monazite and uraninite from vein pegmatoid granitoids of the Dikoe occurrence of the Litsa area.

Table 1 .
U-Pb data for uranium minerals from vein pegmatoid granitoids of the Dikoe ore-occurrence (Litsa area) and albitites of the Ozernoe occurrence (Salla-Kuolajarvi zone).Sr dating of minerals from the same vein (sample TK-20) defined an age of 1964 ± 21 Ma for primary biotite and whole rock and of 1701 ± 15 Ma for secondary muscovite, plagioclase and

Table 2 .
Rb-Sr data for rocks and minerals of uranium occurrences in the Litsa area and the Salla-Kuolajarvi belt.

Table 2 .
Rb-Sr data for rocks and minerals of uranium occurrences in the Litsa area and the Salla-Kuolajarvi belt.