Fluid inclusions in quartz from rocks of different ages and with different uranium contents were studied by microthermometry, gas chromatography, and Raman spectroscopy in three areas with uranium mineralization (Skal’noe, Polyarnoe, and Dikoe) in the Litsa ore district of the Kola Peninsula. The studies have revealed differences in the homogenization temperatures, composition of water–salt systems, and salinity of primary and secondary fluid inclusions in the rocks of these areas. Primary fluid inclusions in the rocks of the Skal’noe and Dikoe areas are characterized by a water–salt system with K and Mg (seldom, Na) chlorides and salinity of 0.2 to 13.9 wt.% NaCl equiv. Secondary inclusions are of higher salinity, up to 19.5 wt.% NaCl equiv., and contain Ca chlorides. Primary inclusions in the rocks of the Polyarnoe area show a more complex salt composition with K, Mg, Ca, and, probably, Li chlorides, with salinity varying from 1.7 to 10.4 wt.% NaCl equiv. Salinity of secondary inclusions is higher, up to 16.9 wt.% NaCl equiv. The gas component of fluid in the rocks of the Skal’noe and Dikoe areas is almost pure CO2 (99.3–100 mol.%); the presence of N2 (1.1 mol.%) and CH4 (0.4 mol%.) was revealed in few inclusions only. In the Polyarnoe area, only water inclusions with N2 were found in the rocks. Thus, the fluids in the rocks of the three areas have mostly a CO2–H2O composition with the presence of K, Mg, and Ca chlorides and, seldom, a water–salt composition with the presence of Na salts only. The increased salinity of secondary inclusions and their more complex composition (CO2 with N2 and CH4 impurities) indicate that the fluid became more reducing and might be responsible for the formation of uranium mineralization.

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