Composition and Structure of Zircon from Hydrothermal Uranium Occurrences of the Litsa Ore Area (Kola Region, Russia)

Zircon grains from various metagranitoids (plagioand monzo-granites, gneisses, metasomatic rocks, and pegmatoid veins) from the Skal’noe and Dikoe sites of the Litsa uranium ore area (Kola Region, Russia) were studied in order to reconstruct the sequence and timing of events in the area and to observe effects of hydrothermal process related to uranium mineralization on structure and composition of zircon. Individual zircon grains were studied by means of laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS), ion microprobe and Raman spectroscopy. Isotopic LA–ICPMS data for the Skal’noye and Dikoe ore occurrences suggest the following age sequence of events in the area: intrusion of plagiogranites—2829 ± 12 Ma, formation of magmatic protolith of gneisses—2781 ± 17 Ma, metamorphism of plagiogranites—2636 ± 34 Ma; intrusion of monzogranites and pegmatoid veins—2549–2526 Ma, hydrothermal event with uranium input—2276 ± 21 Ma, last metamorphism of plagioand monzo-granites—1892–1696 Ma. Ore-bearing rocks in the area are pegmatoid veins and quartz–feldspar metasomatites which contain uraninite. During a 2.3 Ga hydrothermal process, newly formed zircon rims grew simultaneously with the precipitation of uraninite in the veins and metasomatites. These rims are characterized by high U and rare earth elements (REE) contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U ratios (0.1–0.007) and a flat LREE-enriched pattern, in some cases inherited from minerals, dissolved during a hydrothermal event (magmatic plagioclase and probably monazite). Hydrothermal zircon rims grew with partial dissolution of the magmatic zircon, as evidenced by the rounded and curved shapes of zircon cores. The degree of alteration caused by hydrothermal events depends on the uranium content in the pre-existing zircon. The effects of zircon alteration and newly formed zircon composition reflect the redistribution of uranium in rocks.


Introduction
Zircon is a widely used geochronometer due to its stability under many geological processes and its ability to retain radiogenic lead (see, e.g., [1,2]). However, natural zircon crystals often show complex secondary internal textures visible in cathodoluminescence and backscattered electron imaging [3], which indicate the presence of recrystallization/alteration processes [4][5][6][7][8]. The zircon solubility increases in the presence of water, CO 2 -and chloride-rich fluids, alkaline solutions and

Geological Setting of the Litsa Area
The Litsa uranium ore area is located in the NW part of the Kola region at the intersection of Titovka-Uraguba and Litsa-Araguba tectonic zones, restricted by N-S and E-W striking faults, which determined a mosaic-block structure of the area (Figure 1a,b). About 40 uranium ore occurrences were discovered in the area [30]. A major part of the Litsa area belongs to the Central Kola Terrane composed of repeatedly metamorphosed 2.9-2.8 Ga gneisses of the Kola Group and 2.8-2.7 Ga tonalite-trondhemite-granodiorite (TTG) and granite-gneiss rocks [31]. Most ore occurrences of the Litsa area are confined to the zones of variable permeability, such as deep faults and shear zones with inherent ductile and brittle deformations of rocks, which provide favorable conditions for hydrothermal and metasomatic processes leading to uranium concentration.
The Skal'noe and Dikoe sites are the largest ore occurrences with the 2.6-2.3 Ga REE-Th-U mineralization [32]. The rocks within the sites are represented by migmatized biotite, garnet-biotite and two-mica gneisses of the Kola Group more strongly deformed in the Dikoe area, and plagiogranites intercalated with amphibole gneisses, amphibolite bodies, and cross-cutting gabbro-dolerite dykes. Most rocks have concordant tectonic boundaries, complicated by zones of cataclasis and schistosity.
Fields of pegmatoid granitoid veins and lenticular bodies of quartz-feldspar metasomatites (up to 250 m length and up to 15 m thick) with a maximum uranium content of 0.2% were revealed in the area [30,33]. Granitoid veins have sharp (intrusive) contacts and are subconcordant with the strongly deformed host gneiss. Quartz-feldspar metasomatites differ from vein pegmatoid granitoids by the structure heterogeneity, more prominent manifestation of recrystallization processes, and indistinct contacts. According to drilling data, the lens-like and nest-like character of the uranium mineralization is preserved to a depth of 500 m [30].
Here we present the most characteristic samples to track zircon alteration under the influence of a hydrothermal fluid: granites, U-bearing pegmatoid veins and metasomatic rocks, and host gneisses (Figure 1b,c).
Geosciences 2019, 9, x FOR PEER REVIEW 3 of 14 processes, and indistinct contacts. According to drilling data, the lens-like and nest-like character of the uranium mineralization is preserved to a depth of 500 m [30].
Here we present the most characteristic samples to track zircon alteration under the influence of a hydrothermal fluid: granites, U-bearing pegmatoid veins and metasomatic rocks, and host gneisses (Figure 1b,c).  [33]), illustrating rock varieties composed the area, location of quarts-feldspar metasomatic bodies and veins, and sampling points. Bt-biotite, Tur-tourmaline, Amf-amphibole, Grt-garnet.
Samples TK-20 is a vein pegmatoid granitoid with massive structure, collected from a 20-70 cm thick vein in biotite gneisses. The sample shows a magmatic hypidiomorphic texture and is composed of plagioclase (50%), quarts (10%), microcline (20%), biotite (10%), muscovite (10%), apatite, monazite, uraninite, zircon and sulfides. Magmatic plagioclase forms large grains with thin polysynthetic twins. Some grains have bent twins and wavy extinction, reflecting deformation processes. Plagioclase is recrystallized to form small grains along cracks and twin bands. Quartz and microcline are presented in the fine-grained mass along with magmatic plagioclase. Biotite occurs among recrystallized plagioclase and quartz grains, and is partly replaced by chlorite. Large flakes of secondary muscovite are observed in a fine-grained mass around large plagioclase grains. Uranium minerals (apatite, monazite, uraninite, zircon) mainly occur in biotite.

Analytical Methods
Zircon grains were separated from rock samples, embedded in epoxy mounts, ground down to about half of their thickness and polished. The back-scattered electron (BSE) and cathodoluminescence (CL) images, as well as the qualitative composition of inclusions, were studied using a LEO-1450 scanning electron microscope equipped with an XFlash-5010 Bruker Nano GmbH energy dispersive spectrometer with QUANTAX 200 software at the Geological Institute, Kola Science Center of Russian Academy of Sciences in Apatity, Russia.
Zircon was dated by means of laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) in the University of Tasmania (Australia). Measurements were conducted on an Agilent 7900 quadrupole mass-spectrometer with Resonetics Resolution laser ablation system equipped with a coherent CompexPro 110 ArF Excimer laser with wavelength of 193 nm (10-9 m). Zircon ablation took place in a flow of helium (0.35 L/min) mixed with argon (~1 L/min). A small amount of nitrogen (~1.5 mL/min) was added to the gas stream after the ablation cell to improve sensitivity. Zircons were sampled from 29-32 µm spots using the laser with 5 Hz frequency and 2 J/cm 2 fluence. A detailed analytical method for U-Pb dating of zircon was reported in [34,35]. Individual corrected ratios and ages are reported with 1σ analytical errors (68% confidence), and the uncertainties in the calculated discordia ages are reported with 2σ errors (95% confidence). The Isoplot 3.60 software [36] was used for conventional concordia plots and age calculations.
The chemical composition of zircons including the rare earth elements (REE) was analyzed on a Cameca IMS-4F ion microprobe (France) in the Yaroslavl Branch of the Physical Technological Institute (Yaroslavl, Russia). The primary O 2ion beam spot size was~20 µm. Each analysis was averaged from 5 measurement cycles. The relative analytical errors were 10-15%, and the average detection limits were 10 ppb. The respective procedure is described in [37].
Raman spectroscopy of zircon was conducted in the St. Petersburg Resource "Geomodel" centre using the Raman spectrometer HORIBA Jobin-Yvon LabRAM HR 800, Ar-laser with a wavelength of 514.5 nm and 488 nm, an exposure time of 3 sec, 5 repetitions, and a laser power of 50 mW.

LA-ICP-MS Data
The cathodoluminescence (CL) images show that all zircon grains have a complex internal structure: the cores of various shape and preservation are surrounded by rims, which may be either light or dark in CL ( Figure 2). composition, we calculated the age for the two samples (Figure 3a,b).
Cores with thin zoning (points 4, 7 and 14, 16 in the Figures 2) and similar chemical composition-low uranium content (50-245 ppm) and elevated Th-U ratios (0.2-0.6, Table 1) -were calculated together and yielded an upper intercept U-Pb age of 2829±12 Ma. The dark cores (points 1 and 8) with moderate U content (586-602 ppm) have a U-Pb age of 2692±15 Ma. The bright luminescent rim (point 2) with high U content (1372 ppm) has a 207 Pb/ 206 Pb age of 2608±3 Ma. Gray in CL rims (points 6,9,10,11,12,15) have similar luminescence and zoning but different chemical composition and age. Rims with low Th content of 5-11 ppm and Th-U ratio of 0.01 (points 9, 10, 15) have an upper intercept age of 2636±34 Ma. The rim at point 6 has the same chemical composition, but a younger 207 Pb/ 206 Pb age of 2341±4 Ma. Rims with high U content of 1381-1404 ppm (points 11, 12) yielded a very discordant U-Pb date of 1892±20 Ma (Figure 3a).
Zircon in the sample of monzogranite (KT-3) is represented by large brown crystals. Around the irregularly shaped core there are altered areas dark in CL and light gray areas with growth zoning (points 17 and 18) for which a date of 2526±19 Ma was obtained (Figure 3c), though U-Pb data are highly discordant. The age of the altered dark area is 1696±4 Ma. All zircon domains are rich in uranium (1653-2410 ppm) with a low Th-U ratio of 0.01-0.08 (Table 1).  Zircon in the sample of plagiogranite KT-1 is represented by corroded crystals, with the core of irregular shape. The most ancient date (2958 ± 4 Ma) was obtained at the point 3 and not manifested anywhere else in our samples. A similar age (2910 ± 21 Ma) was obtained for the zircon from the gneisses of the Kola series [38]. The sample KT-2 contains best preserved zircon grains. Since both plagiogranite samples were taken at the same outcrop and are similar in mineral and chemical composition, we calculated the age for the two samples (Figure 3a,b).  (Table 1). Oscillatory zoning rims (21, 23) and individual fine-zoned zircon grains (points 20, 27) with moderate to high U content (319-844 ppm) yielded an upper intercept U-Pb age of 2781±17 Ma (Figure 3d). There is a more ancient dark area of zircon with a 207 Pb/ 206 Pb age of 2817±3 Ma (point 25). Cores with thin zoning (points 4, 7 and 14, 16 in the Figure 2) and similar chemical composition-low uranium content (50-245 ppm) and elevated Th-U ratios (0.2-0.6, Table 1) -were calculated together and yielded an upper intercept U-Pb age of 2829 ± 12 Ma. The dark cores (points 1 and 8) with moderate U content (586-602 ppm) have a U-Pb age of 2692 ± 15 Ma. The bright luminescent rim (point 2) with high U content (1372 ppm) has a 207 Pb/ 206 Pb age of 2608 ± 3 Ma. Gray in CL rims (points 6,9,10,11,12,15) have similar luminescence and zoning but different chemical composition and age. Rims with low Th content of 5-11 ppm and Th-U ratio of 0.01 (points 9, 10, 15) have an upper intercept age of 2636 ± 34 Ma. The rim at point 6 has the same chemical composition, but a younger 207 Pb/ 206 Pb age of 2341 ± 4 Ma. Rims with high U content of 1381-1404 ppm (points 11, 12) yielded a very discordant U-Pb date of 1892 ± 20 Ma (Figure 3a). Zircon in the sample of monzogranite (KT-3) is represented by large brown crystals. Around the irregularly shaped core there are altered areas dark in CL and light gray areas with growth zoning (points 17 and 18) for which a date of 2526 ± 19 Ma was obtained (Figure 3c), though U-Pb data are highly discordant. The age of the altered dark area is 1696 ± 4 Ma. All zircon domains are rich in uranium (1653-2410 ppm) with a low Th-U ratio of 0.01-0.08 (Table 1).
The sample KT-5 of muscovite-biotite gneiss contains short prismatic zircon grains with oscillatory zoning. The bright core (points 22, 26) with low U (62-97 ppm) and high Th-U ratios (0. 44 (Figure 3d). There is a more ancient dark area of zircon with a 207 Pb/ 206 Pb age of 2817 ± 3 Ma (point 25).
One of the zircon grains from Mu-Bt gneisses (sample RK-473) is a fragment of zoned crystal with high contents of U (948-1090 ppm) and REE (1475-1802 ppm) (Figure 4b, left image, Table 2). Its REE pattern has higher LREE ((Sm/La)n=1.1-1.0) than typical "magmatic" zircon, despite fine zoning and potassium feldspar-biotite-quartz association of mineral inclusions, typical of granite. The second grain has a zoned core and fractured rim. Both U and REE contents (426-662 ppm and 279-1166 ppm, respectively) are lower than those in the first grain. Distribution of REE reveals a HREE-enriched pattern ((Lu/Gd)n=33-38) both in the core and in the rim.    Zircon from metasomatic rock (TK-473A) has zoned cores, light in BSE and structureless dark rims (Figure 4 c). The first core (left image) has moderate U content (535 ppm), while the core and rim of the second grain have very high U content up to 6760 ppm ( Table 2). Content of REE in all points is rather high (1360-3270 ppm). Low U core (point 2) has a LREE-depleted pattern with (Sm/La)n=6 with negative Eu anomaly, while other domains are enriched both in HREE and LREE (~10 4 × and 10 3 × chondrite, respectively) and have a "cup-shaped" REE pattern with positive Eu-anomaly, uncharacteristic for zircon [39].
Zircon from the pegmatoid vein (sample TK-20), being generally similar to zircon from metasomatites, has a more complex structure. Cores, internal fractured rims, and external unstructured rims were distinguished here (Figure 4d). Inclusions in the cores and inner fractured rims are represented by the potassium feldspar-biotite-apatite-quartz association, which suggests the growth of zircon from the melt during crystallization of pegmatite veins. All zircon domains, apart from core 7 with (Lu/Gd)n = 2.5, exhibit a flat REE pattern with (Lu/Gd)n ranging 0.2-0.9. The cores have an increased content of uranium (1180-2588 ppm) and thorium (250-1715 ppm), and the rims have a very high content of uranium (2688-6560 ppm), suggesting simultaneous formation of rims and uranium minerals. The REE content in the rims (3291-8760 ppm) is also the highest of all samples (Table 2).

Raman Spectroscopy
Raman spectroscopy is very instrumental for studying radiation-induced metamictization of zircon at the micro level [41]. Raman spectrum of crystalline zircon consists of several bands: 202, 214, 225, 357, 393, 439, 974 and 1008 cm −1 with the most intense bands at 1008, 439 and 357 cm −1 , which correspond to the internal vibration of SiO 4 tetrahedrons, while the three bands at 202, 214 and 225 cm −1 are external modes, corresponding to vibrations involving movements of SiO 4 tetrahedrons and Zr ions [42,43].
Zircon from plagiogranite (sample KT-1) is represented by grains with growth zoning (Figure 5a). The central domain (point 13) exhibits Raman spectrum with typical zircon Raman modes, though with broader bands, with decreased total Raman intensity and bands shifted towards smaller wavenumbers, corresponding to disturbances of zircon structure [41]. The uranium-rich rim (point 14) displays even more reduced and shifted Raman peaks, which reflects increased damage to the structure. The rim exhibits additional bands at 163, 635, 782 and 1048 cm −1 . The occurrence of extra Raman bands near 162, 509, 635 cm -1 is attributed to Zr-rich clusters of ZrO polyhedra or to the presence of a submicrometric non-crystalline Zr-O phase [44]. The extra Raman band at 785 cm -1 appears to be caused by Si-rich phases, formed along with ZrO clusters as a result of the strong distortion of zircon crystal lattice [44].  Zircon from muscovite-biotite gneiss (sample  shows Raman zircon peaks in areas with growth zoning (point 4, 7, Figure 5b), but with lower intensity of bands. The 1008 cm -1 band shifts to 997 cm −1 and the signal at 975 cm −1 disappears. In recrystallized areas (points 2, 3, 5) the signal from the 1008 cm −1 band also disappears, which indicate the appearance of an amorphous phase [45].
High-U rims and remnant of core in zircon from metasomatite (sample TK-473A) show wide low Raman peaks indicating strong damage of crystal lattice (Figure 5c). The Raman spectra has no 975 and 1008 cm -1 signals, but has peaks at 164, 501-506, and 778-781 cm −1 , reflecting amorphous state of the substance with ZrO and Si-rich domains [44,45]. Zircon from muscovite-biotite gneiss (sample  shows Raman zircon peaks in areas with growth zoning (point 4, 7, Figure 5b), but with lower intensity of bands. The 1008 cm -1 band shifts to 997 cm −1 and the signal at 975 cm −1 disappears. In recrystallized areas (points 2, 3, 5) the signal from the 1008 cm −1 band also disappears, which indicate the appearance of an amorphous phase [45].
High-U rims and remnant of core in zircon from metasomatite (sample TK-473A) show wide low Raman peaks indicating strong damage of crystal lattice (Figure 5c). The Raman spectra has no 975 and 1008 cm -1 signals, but has peaks at 164, 501-506, and 778-781 cm −1 , reflecting amorphous state of the substance with ZrO and Si-rich domains [44,45].

Discussion
According to U-Pb dating of magmatic zircon represented by cores with thin growth zoning, the crystallization of plagiogranites in the area occurred 2829 ± 12 Ma ago. Grains with oscillatory zoning from biotite-muscovite gneiss with U-Pb age of 2781 ± 17 Ma appear to the next magmatic event in the area. This zircon has low Th and low to moderate U contents (with low Th-U ratio), reflecting chemical composition of the parental rock [46]. A further zircon growth resulted in the crystallization of low-U rims 2636 ± 34 Ma and high-U rims 1892 ± 20 Ma ago. The results obtained are in a good agreement with the data published for the rocks of the northwestern part of the Kola-Norwegian block [47,48]. The low U and Th (50-245 and 15-129 ppm, respectively) zircon cores from plagiogranites retained crystallographic forms and the magmatic REE pattern.
Crystallization of monzogranites occurred at 2526 ± 19 Ma according to the age of zircon domains with growth zoning and high U concentrations. Coeval pegmatoid veins were crystallized 2549 ± 7 Ma ago (U-Pb monazite age [32]).
The pegmatoid vein (sample TK-20), besides zircon, contains monazite, uraninite, and apatite, which are in close association with each other, but formed at different stages of the rock evolution. Magmatic zircon with elevated U content (1200-2600 ppm) crystallized in veins along with monazite and apatite. Unaltered magmatic zircon core (point 7- Figure 4d) is depleted in LREE, which is consistent with joint growth with monazite [49].
An increase in the uranium content in zircon cores in veins and metasomatites over 1000 ppm resulted in the damaged crystal structure and makes zircon less stable. None of the high-U cores have a crystallographic shape. The small size of the cores when compared to the bulk of the crystal, their rounded shape and curved boundaries, sometimes with the shadow of the former prismatic crystal ( Figure 6) testifies to dissolution of primary zircon grains. Such a picture can correspond to the metasomatic replacement of cores during the rim growth according to the "dissolution-reprecipitation" mechanism [4,5,50], with probable redistribution of trace elements in the bulk of a crystal [50].
The cup-shaped REE pattern of zircon domains in metasomatic rocks (TK-473) with positive Eu-anomaly and high LREE could be inherited during hydrothermal alteration of rocks with partial dissolution of monazite and magmatic plagioclase. A similar process was described in [51], where it was shown that during metasomatic reactions, newly formed minerals can inherit the REE pattern from the replaced ones.
the bulk of a crystal [50].
The cup-shaped REE pattern of zircon domains in metasomatic rocks (TK-473) with positive Eu-anomaly and high LREE could be inherited during hydrothermal alteration of rocks with partial dissolution of monazite and magmatic plagioclase. A similar process was described in [51], where it was shown that during metasomatic reactions, newly formed minerals can inherit the REE pattern from the replaced ones. Raman spectroscopy confirms that the damage of the crystal lattice increases with the uranium content in zircon, which is reflected by the change of shape and position of individual Raman active modes: a decrease in the total spectrum intensity, band broadening, and peak shifts toward lower Raman spectroscopy confirms that the damage of the crystal lattice increases with the uranium content in zircon, which is reflected by the change of shape and position of individual Raman active modes: a decrease in the total spectrum intensity, band broadening, and peak shifts toward lower wave numbers. After the formation of rims 2.3 Ga ago, the temperature did not rise above 300-350 • C, when radiation damage of the zircon structure was annealed [41].
Magmatic zircon from plagiogranites with low U and Th contents (50-245 and 15-129 ppm, respectively) retains crystallographic forms and the magmatic REE pattern. Magmatic zircon from pegmatoid veins and metasomatites with elevated U content (1200-2600 ppm) is partly dissolved during rim formation, has no crystallographic forms and is preserved as rounded cores. This means that an increase in the uranium content over 1000 ppm leads to a distortion of the crystal structure and makes zircon less stable.
Zircon rims, formed simultaneously with uraninite during the hydrothermal process, are characterized by high U and REE contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U rations (0.1-0.007), flat LREE enriched patterns and a high content of "non-formula" elements: Al, Ca, and Fe. The hydrothermal zircon REE pattern can reflect that of the minerals that were replaced during hydrothermal event.
Studies of the composition, REE patterns and Raman spectra of zircon from sites with uranium mineralization have shown that the degree of alteration of zircon from granitoids in areas with hydrothermal activity depends on the uranium content in the zircon. The processes of zircon alteration reflect the redistribution of uranium in the rocks.