Metamorphic Conditions and Raman Spectroscopic Carbonaceous Material Thermometry of Host Schists of Olympiada and Eldorado Gold Deposits (Yenisey Ridge, Russia)

: Metamorphic processes play a key role in forming orogenic gold deposits. In this paper, we present new evidence that host schists of the two largest gold deposits of the Yenisey ridge (Russia) Olympiada and Eldorado underwent a single stage of metamorphism in contrast to surrounding blocks. This metamorphism is of moderate thermal gradient and belongs to the Barrovian type, which is typical for the collisional event in the time range 800–850 Ma. The new Ar/Ar age data presented in this paper and the review of magmatic and metamorphic events and ore-forming processes indicate that the most productive stage (gold-sulﬁde-quartz) correlates well in time with the regional metamorphism of the Barrovian type. This indicates that metamorphic processes can have a crucial role in forming gold deposits of the Yenisey ridge. Carbonaceous material thermometry indicates a wide range of obtained temperatures around 90–150 ◦ C around the mean temperature for each sample. The highest temperatures are close to the peak metamorphic temperatures estimated by garnet-biotite thermometry.


Introduction
Metamorphic processes play a key role in forming orogenic gold deposits [1]. The genesis of many gold deposits with polygenic history, which included both metamorphic and magmatic processes, often remains debated. The controversy is due to difficulties in the interpretation of geochronological data for overlapping magmatic and metamorphic processes of different ages. The Yenisey ridge (Russia) is one such region with polygenic history. It is a Neoproterozoic accretionary-collisional orogen, extending for 700 km along the western margin of the Siberian craton situated between the craton to the east and West Siberian basin to the west. Most gold deposits occur in the lower part of the Sukhoi Pit Group, represented by clastic, carbonaceous, and calcareous clastic rocks of the Korda, Gorbilok, and Uderei Formations [2].
We present in this paper new data on pressure-temperature parameters of metamorphism of host schists of the two largest gold deposits of the Yenisey ridge, Olympiada and Eldorado and its tectonic and time correlation with metamorphic events around the whole orogen of the Yenisey ridge. New Ar-Ar data on ore formation of the Eldorado gold deposit allows correlating the ore-forming processes with Barrovian type collisional metamorphism.

Geology of Deposits
The Olympiada deposit occurs closer to the Tatarka fault. The deposit is hosted in metamorphosed rocks of the Korda Formation. From top to base, these are carbonaceous quartz-mica schist, foliated marble, quartz-mica-carbonate schist, and biotite-muscovitequartz schist (Figure 2a). The chemical composition of the rocks ranges from aluminous to carbonate-rich, with minor variations in ferromagnesian content. All rocks are hydrothermally altered to different degrees and contain sulfide mineralization. The principal host rocks are quartz-mica-carbonate (two mica-quartz-calcite) schist and marble. In the underlying quartz-mica (quartz-garnet-mica) and overlying quartz-mica-carbonaceous (carbonaceous silicate) schists, mineralization occurs only near the contacts with carbonate-bearing and carbonate rocks.
Sedimentary rocks are regionally metamorphosed at low amphibolite facies conditions [25,26]. Mineral assemblages are garnet-muscovite-biotite, developed after siltstone. The area of the Olympiada deposit has a local zone of dynamothermal metamorphism and alteration, characterized by distinct mineral assemblages, and spatially restricted to the long-lived tectonic zone along the mineralized horizon. The highest temperature associations of metapelites in the tectonic sliver correspond to the garnet zone (garnet + quartz + biotite ± plagioclase), with sporadic fibrolite-sillimanite and kyanite. Higher pressure rocks correspond to the barrovian type of metamorphism, with mineral assemblages of chloritoid + biotite ± garnet (in metapelite) and margarite + quartz (in metamarl).  [11,14,24].

Geology of Deposits
The Olympiada deposit occurs closer to the Tatarka fault. The deposit is hosted in metamorphosed rocks of the Korda Formation. From top to base, these are carbonaceous quartz-mica schist, foliated marble, quartz-mica-carbonate schist, and biotite-muscovitequartz schist (Figure 2a). The chemical composition of the rocks ranges from aluminous to carbonate-rich, with minor variations in ferromagnesian content. All rocks are hydrothermally altered to different degrees and contain sulfide mineralization. The principal host rocks are quartz-mica-carbonate (two mica-quartz-calcite) schist and marble. In the underlying quartz-mica (quartz-garnet-mica) and overlying quartz-mica-carbonaceous (carbonaceous silicate) schists, mineralization occurs only near the contacts with carbonatebearing and carbonate rocks.
Sedimentary rocks are regionally metamorphosed at low amphibolite facies conditions [25,26]. Mineral assemblages are garnet-muscovite-biotite, developed after siltstone. The area of the Olympiada deposit has a local zone of dynamothermal metamorphism and alteration, characterized by distinct mineral assemblages, and spatially restricted to the long-lived tectonic zone along the mineralized horizon. The highest temperature associations of metapelites in the tectonic sliver correspond to the garnet zone (garnet + quartz + biotite ± plagioclase), with sporadic fibrolite-sillimanite and kyanite. Higher pressure rocks correspond to the barrovian type of metamorphism, with mineral assemblages of chloritoid + biotite ± garnet (in metapelite) and margarite + quartz (in metamarl).
The Tyrada and Chirimba massifs of Ayakhta complex (761±8 Ma [27]) are the nearest granitic intrusions to the Olympiada deposit ( Figure 2a). The gravity data suggest that all of them are parts of a single batholith [28]. Both massifs are similar in composition, mostly consisting of granodiorite (phase 1) and coarse-grained porphyritic biotite and amphibole-biotite granite of normal alkalinity (main phase 2). These massifs contain numerous internal small stocks, dikes, leucogranite veins, fine-grained granite, and aplite (phase 3).
More than 50 estimates for the age of mineralization exist for the Olympiada deposit based on the K-Ar, 40 Ar/ 39 Ar, Rb-Sr, Sm-Nd, and Re-Os techniques. On the basis of this data a number of main ore-forming stages have been distinguished. The age of the nonauriferous quartz-mica-sulfide mineral association is estimated as 817.1 ± 6.3 to 808.4 ± 7.7 Ma. The age of the main productive quartz-gold-arsenopyrite-pyrrhotite association is 803 ± 6.1 to 758 ± 6.0 Ma, with a prevalence of 795 to 784 Ma. The age estimates of the late quartz-gold-antimony association range from 795.2 ± 5.8 to 660 ± 19 Ma [28].
The Eldorado deposit is much less studied than the Olympiada deposit. It is located near the Ishimba fault ( Figure 1). The deposit is situated in the Gorbilok formation of the Sukhoi pit group. The host rocks are garnet-biotite-muscovite schists metamorphosed at low-amphibolite facies conditions. The nearest magmatic rocks are about 15 km away from the deposit and belong to the Kalama granitoid massif of age about 860-880 Ma [13,29]. Twenty-two ore bodies are found in the deposit areas. They form 4 groups ( Figure 2b) and are located in subparallel quartz veins with arsenopyrite. Age estimates for this deposit are very restricted. The age of quartz-gold-arsenopyrite stage is estimated as 795 Ma and gold-polysulfide stage as 780 Ma [7]. The age around 790 Ma was obtained by Gibsher et al. [30] for the host metamorphic schists. These were interpreted as the age of the local dynamometamorphism. The Tyrada and Chirimba massifs of Ayakhta complex (761±8 Ma [27]) are the nearest granitic intrusions to the Olympiada deposit ( Figure 2a). The gravity data suggest that all of them are parts of a single batholith [28]. Both massifs are similar in composition, mostly consisting of granodiorite (phase 1) and coarse-grained porphyritic biotite and amphibole-biotite granite of normal alkalinity (main phase 2). These massifs contain numerous internal small stocks, dikes, leucogranite veins, fine-grained granite, and aplite (phase 3).
More than 50 estimates for the age of mineralization exist for the Olympiada deposit based on the K-Ar, 40 Ar/ 39 Ar, Rb-Sr, Sm-Nd, and Re-Os techniques. On the basis of this data a number of main ore-forming stages have been distinguished. The age of the nonauriferous quartz-mica-sulfide mineral association is estimated as 817.1 ± 6.3 to 808.4 ± 7.7 Ma. The age of the main productive quartz-gold-arsenopyrite-pyrrhotite association is 803 ± 6.1 to 758 ± 6.0 Ma, with a prevalence of 795 to 784 Ma. The age estimates of the late quartz-gold-antimony association range from 795.2 ± 5.8 to 660 ± 19 Ma [28].
The Eldorado deposit is much less studied than the Olympiada deposit. It is located near the Ishimba fault ( Figure 1). The deposit is situated in the Gorbilok formation of the Sukhoi pit group. The host rocks are garnet-biotite-muscovite schists metamorphosed at low-amphibolite facies conditions. The nearest magmatic rocks are about 15 km away from the deposit and belong to the Kalama granitoid massif of age about 860-880 Ma [13,29]. Twenty-two ore bodies are found in the deposit areas. They form 4 groups ( Figure  2b) and are located in subparallel quartz veins with arsenopyrite. Age estimates for this deposit are very restricted. The age of quartz-gold-arsenopyrite stage is estimated as 795 Ma and gold-polysulfide stage as 780 Ma [7]. The age around 790 Ma was obtained by Gibsher et al. [30] for the host metamorphic schists. These were interpreted as the age of the local dynamometamorphism.

Materials and Methods
Samples of garnet-biotite schists from the Eldorado and garnet-biotite, garnet-staurolite and biotite-muscovite schists from the Olympiada deposits were taken. Samples

Materials and Methods
Samples of garnet-biotite schists from the Eldorado and garnet-biotite, garnet-staurolite and biotite-muscovite schists from the Olympiada deposits were taken. Samples from the Olympiada deposit were taken from well core which intersects ore bodies. Samples from the Eldorado deposit were taken from the quarry. All schists are enriched in carbonaceous material which forms thin lumpy inclusions in rock-forming minerals: garnet, biotite, muscovite, quartz, chlorite, and chloritoid ( Figure 3).  (Table 1). Biotite contains 1.88 wt.% of TiO2 and xFe (Fe/Fe+Mg) = 0.63 (Table 2). Plagioclase contains xAn = 0.25-0.37 with a decrease in the content of the anorthite component towards the Organic maturity was determined in selected samples on polished sections about 0.2 mm thick, cut perpendicular to the foliation. For the acquisition of the Raman spectra, a Horiba Jobin Yvon LabRam HR confocal Raman spectrometer equipped with a frequency-doubled Nd-YAG laser (100 mW, 532.2 nm) and diffraction gratings of 1200 and 1800 grooves/mm, a Peltier-cooled, slow-scan, ccD matrix-detector and an Olympus bX 40 microscope were used at the Institute of Geology and Mineralogy SB RAS. To obtain a better signal to noise ratio five scans with an acquisition time of 30 sec in the 700-2000 cm −1 (first-order) and 2200-3200 cm −1 (second-order) region are summed. From 40 to 80 spectra were recorded for each sample. the measured first-order bands of the raman spectra were the D1 [3] or D band [31] at~1350 cm −1 ; the G [3] or O band [31] at ca. 1580 cm −1 , the D2 band at~1610 cm −1 , and the D3 band at~1500 cm −1 . The second-order bands were recorded at~2450 cm −1 ,~2700 cm −1 (s1 band) and~2900 cm −1 (s2 band, [3]). The peak position, area, and peak width (full width at half maximum-FWHM) of the bands were determined using the computer program Fityk 1.3.1. The R1 ratio is calculated as D1/G peak intensity ratio and the R2 ratio is given as D1/(G + D1 + D2) peak area ratio [3].
Minerals for 40 Ar/ 39 Ar isotopic-geochronologic studies were separated from rocks using conventional techniques of magnetic and density separation. Samples of monomineralic fractions and biotite MCA-11 (OSO no. 129-88, which was used as the mineral monitor) were wrapped in Al foil, placed into a quartz ampoule, which was then pumped out and welded. Biotite MCA-11 (prepared at the Fedorovskii All-Russia Institute of Mineral Resources in 1988 as a K/Ar standard for K/Ar dating) was certified as a 40 Ar/ 39 Ar monitor with the use of the muscovite Bern 4m and biotite LP-6 internationally certified standards [32]. The integral age of biotite MCA-11 was assumed to correspond to the calibration results: 311.0 ± 1.5 Ma. The quartz ampoules with samples were irradiated in the Cd-coated channel of a reactor (BBP-K type) at the Tomsk Polytechnic Institute. The gradient of the neutron flux did not exceed 0.5% of the sample size. The experiments on the stepwise heating were carried out in a quartz reactor with an external heater. The blank for 40Ar (10 min at 1200 • C) was no higher than 5 × 10 −10 ncm 3 . Ar was purified using Ti and ZrAl SAES getters. The isotopic composition of Ar was measured on a Noble Gas 5400 (Micromass, United Kingdom) mass spectrometer. The errors quoted below in the text, table, and figures correspond to ±1σ.

Petrography and Mineralogy
Three samples from the well core of Olympiada deposit were taken for detailed study: two samples of garnet-biotite (oc-40 and oc-99) and one of the garnet-staurolite-chloritoid schists (oc-24).
The mineral assemblage of garnet-biotite schists is Grt + Bt + Chl + Pl + Qz. The sample oc-40 has a distinct laminated structure with alternating laminae of a dominantly quartzfeldspar-mica composition containing a small amount of carbonaceous matter and layers enriched with the carbonaceous matter. Garnet is concentrated in certain layers, where it forms idiomorphic porphyroblasts with a sectorial internal structure. The folded structure and the development of schistosity at an angle to the bedding are characteristic of the rock (Figure 3a Table 1). The content of almandine, pyrope, and grossular increases to the edges of grains, while the content of spessartine decreases ( Figure 4)       The host rocks of the Eldorado deposit are rather homogenous in composition. Generally, it is garnet-biotite schist with the mineral assemblage Grt+Bt+Ms+Chl+Pl+Qz. They have lepidoblastic texture with garnet porhyroblasts about 1.2 mm in diameter. Two samples of schists were studied (19-el-2 и 19-el-5).  (Table 3).
Garnet-staurolite-chloritoid schist (oc-24) has a schistose structure. The rock is significantly enriched in the carbonaceous matter, which occurs in the form of inclusions in garnet, staurolite, chloritoid, chlorite, and micas (Figure 3c,d). In garnet, mainly the edge parts of the grains are enriched with the carbonaceous matter. In staurolite and chloritoid, it is concentrated in the central parts. A sectorial structure is often noted in garnet. Chloritoid, staurolite, and biotite are partially replaced by secondary chlorite.  (Table 3).
The host rocks of the Eldorado deposit are rather homogenous in composition. Generally, it is garnet-biotite schist with the mineral assemblage Grt + Bt + Ms + Chl + Pl + Qz. They have lepidoblastic texture with garnet porhyroblasts about 1.2 mm in diameter. Two samples of schists were studied (19-el-2 и19-el-5).

P-T Parameters of Metamorphism
Temperature estimates have been made with a garnet-biotite geothermometer [33] and a Ti-in-Bt geothermometer [34,35], pressure has been estimated with a garnet-biotiteplagioclase geobarometer [36]. For P-T calculations composition of garnet and plagioclase rims and biotite from the rocks matrix were taken. These minerals do not form reaction textures with each other and do not indicate other evidence of disequilibrium.
Ten other samples of garnet-biotite schists were taken from the same well core. They have mineral composition and compositions of rock-forming minerals similar to those for schists described above. We do not show here its detailed description, obtained P-T parameters fall into the range: Grt-Bt [33]-540-570 • C, Ti-in-Bt [34]-560-590 • C, pressure 6.3-7.4 kbar (Table 4). Table 4. P-T parameters of metamorphism of garnet-biotite schists of the Olympiada deposit.

Chlorite Thermometry
Mineral microtextures indicate that chlorite in samples from the Olympiada deposit replaces minerals of the peak metamorphic assemblage: biotite, staurolite, and chloritoid ( Figure 3). For the temperature estimates, chlorite thermometry has been applied. Both empirical and semi-empirical approaches were used for chlorite geothermometry, and results obtained are summarized in Table 3. Among the empirical methods, four different geothermometers of Kranidiotis and MacLean [37] (KM), Jowett [38] (J), McDowell and Elders, [39] (ME) and Cathelineau and Nieva [40] (CN) were used. The highest temperature (390-410 • C) was indicated by thermometers of Jowett [38] and McDowell and Elders [39] for samples oc-24, oc-99 and 19-el-2 and slightly less (350-370 • C) for the sample oc-40. Calibrations CN and KM indicate lower temperature in the range 310-370 • C.
The semi-empirical approach of Bourdelle et al. [41] was also used as it is best suited for low temperature chlorites (<350 • C). Individual T-R 2+ -Si diagrams (where T-temperature, R 2+ -sum of divalent cations, Si [apfu]) were plotted and temperatures were graphically estimated ( Figure 5). Most of the chlorites in T-R 2+ -Si diagram plot above 350 • C isotherm. Since, the optimal reliability of this thermometer is up to 350 • C only, this method could not estimate the upper temperature limit of chlorites but it clearly indicates that for all chlorite the upper temperature limit is higher than 350 • C.

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results obtained are summarized in Table 3. Among the empirical methods, four different geothermometers of Kranidiotis and MacLean [37] (KM), Jowett [38] (J), McDowell and Elders, [39] (ME) and Cathelineau and Nieva [40] (CN) were used. The highest temperature (390-410 °C) was indicated by thermometers of Jowett [38] and McDowell and Elders [39] for samples oc-24, oc-99 and 19-el-2 and slightly less (350-370 °C) for the sample oc-40. Calibrations CN and KM indicate lower temperature in the range 310-370 °C. The semi-empirical approach of Bourdelle et al. [41] was also used as it is best suited for low temperature chlorites (<350 °C). Individual T-R 2+ -Si diagrams (where T-temperature, R 2+ -sum of divalent cations, Si [apfu]) were plotted and temperatures were graphically estimated ( Figure 5). Most of the chlorites in T-R 2+ -Si diagram plot above 350°C isotherm. Since, the optimal reliability of this thermometer is up to 350 °C only, this method could not estimate the upper temperature limit of chlorites but it clearly indicates that for all chlorite the upper temperature limit is higher than 350°C.  [42]. Isotherms (in 50 °C steps) calculated with Bourdelle et al. [41] geothermometer.

Raman Spectroscopy
Carbonaceous matter (CM) inclusions in garnet, quartz, chloritoid and micas ( Figure  6) were analyzed. From 44 to 79 measurements were made for each sample (Table 5). CM particles were analyzed in situ enclosed within transparent grains in sections orientated perpendicular to the foliation. Raman spectra acquired from samples display sharp and intense G peaks at ~1578 cm -1 , D1 peaks at ~1347 cm -1 , and weakly resolved, broad shoulders of the D1 bands at ~1615 cm -1 indicating the D2 peak (Figure 7). The second-order spectra are characterized by bands at ~ 2693 cm -1 (Table 5). D1 bands at ~1353 cm -1 and G peaks at ca. 1576 cm -1 were measured on the first-order Raman spectra of samples.

Raman Spectroscopy
Carbonaceous matter (CM) inclusions in garnet, quartz, chloritoid and micas ( Figure 6) were analyzed. From 44 to 79 measurements were made for each sample (Table 5). CM particles were analyzed in situ enclosed within transparent grains in sections orientated perpendicular to the foliation. Raman spectra acquired from samples display sharp and intense G peaks at~1578 cm −1 , D1 peaks at~1347 cm −1 , and weakly resolved, broad shoulders of the D1 bands at~1615 cm −1 indicating the D2 peak (Figure 7). The secondorder spectra are characterized by bands at~2693 cm −1 (Table 5). D1 bands at~1353 cm −1 and G peaks at ca. 1576 cm −1 were measured on the first-order Raman spectra of samples.      In the geothermometer of Beyssac et al. [4] metamorphic temperature is calculated as a linear function of the R2 parameter (R2 = D1/(G + D1 + D2) area ratio) and the thermometer is calibrated in the range of 330-650 • C. The calibration of geothermometerby Rahl et al. [43] is based on both R1 (R1 = D1/G high ratio) and R2 parameters and is applicable for temperatures as low as 100 • C. Both of these calibrations were used for temperature estimates and they indicated very similar results (Table 5). Temperatures obtained for each sample fall into a wide range around 100-150 • C around the mean temperature (Table 5, Figure 8). CM inclusions in different minerals (garnet, quartz, muscovite, and chlorite) indicate a large scatter of crystallinity in each mineral phase (Figures S1 and S2).

Ar-Ar Dating
More than 50 estimates for the age of mineralization exist for the Olympiada deposit based on various techniques, whereas the age data on the Eldorado deposit is very restricted.
Gold at the Eldorado deposit mostly resides in quartz rather than in sulfides and is of two generations, associated with carbon-saturated quartz and early sulfides (arsenopyrite and pyrite) or with late complex ore mineralization (sphalerite, chalcopyrite, and galena). Gold of the first generation is invisible, with small particles more strongly deformed and uniformly distributed, which makes up the greatest part of the gold reserves, while the particles of the second generation gold are coarser.   Table 5. Raman spectra position, FWHM and estimated temperature. Notes: R1 and R2 reported here are averages Raman spectra collected per sample. Uncertainties are represented by sta ard errors (SE), which were determined by dividing the standard deviation of the measurements by the square root of number of measurements.  40 Ar/ 39 Ar dating was performed for clarification of ore processes of the Eldorado deposit. For dating a sample of quartz vein with fragments of host garnet-biotite schists was taken. Quartz veins in garnet-biotite schists do not cross schistosity of host rocks and contain its lens-shaped fragments, mean while they are deformed conformal to host schists and often form lenses in it (Figure 9a). No retrograde changes like secondary chlorite were noticed (Figure 9b). Quartz contains inclusions of large arsenopyrite crystals (up to 1 cm) and is cut by a light-green sericite vein, which contains arsenopyrite grains as well (Figure 9c). The concentration of gold in the sample is about 5.3 ppm. For dating of the younger age limit of the ore mineralization, the fine-grained sericite was taken. Obtained plateau includes 58.4% of the total argon and indicates the age 798.6 ± 5.3 Ma (Figure 9d, Table 6). Table 6. Ar-Ar sericite data.

Heating
Step 40  and often form lenses in it (Figure 9a). No retrograde changes like secondary chlorite were noticed (Figure 9b). Quartz contains inclusions of large arsenopyrite crystals (up to 1 cm) and is cut by a light-green sericite vein, which contains arsenopyrite grains as well ( Figure  9c). The concentration of gold in the sample is about 5.3 ppm. For dating of the younger age limit of the ore mineralization, the fine-grained sericite was taken. Obtained plateau includes 58.4% of the total argon and indicates the age 798.6±5.3 Ma (Figure 9d, Table 6).

P-T Conditions of Metamorphism
Conventional thermometry data indicate that the metamorphism of garnet-biotite and garnet-staurolite schists occurred at temperatures of about 530-570 • C and pressure mainly around 6-7 kbars and they were very similar for both deposits. Similar P-T parameters were obtained in earlier works (see review of Sazonov et al. [2]) and they correspond to the P-T parameters of Barrovian metamorphism estimated in the Mayakon area [20] ( Figure 10).
Late chlorite is typical for schists from the Olympiada deposit; relatively high temperatures obtained by chlorite thermometry (more than 350 • C) indicate that chlorite was formed probably due to low-temperature metamorphism, but not in a hydrothermal environment.
Typical prograde metamorphic zoning in garnets with decreasing spessartine and increasing almandine from core to the rims and almost homogenous in grossular content indicates a single stage of prograde metamorphism. The rocks do not have any evidence of early HT/LP metamorphism which was mentioned in the nearest area [20] or the grade of early metamorphism was not higher than greenschist facies. This shows that the early stage of metamorphism described over the whole YR [44] probably has a patchy distribution.
Detailed studying of metamorphic processes of Transangarian part of YK was performed along Tatarka thrust (Figure 1, review [44] and references therein). Fe-and Al-rich metapelitic schists of the Korda formation show petrological evidence of two superimposed metamorphic events. An early middle Proterozoic event produced low-pressure, andalusite-bearing assemblages at c. 3.5-4 kbar and 540-560 • C. During a subsequent late Proterozoic event, a moderate-pressure (Barrovian-type), regional metamorphic overprint produced kyanite-bearing mineral assemblages that replaced minerals formed in the low-pressure event [21]. Due to these two metamorphic events, a specific chemical zoning with sharp increasing of grossular component from core to the rims develops in garnet grains [20,21].
Conventional thermometry data indicate that the metamorphism of and garnet-staurolite schists occurred at temperatures of about 530-570 °C mainly around 6-7 kbars and they were very similar for both deposits. Simi eters were obtained in earlier works (see review of Sazonov et al. [2]) and th to the P-T parameters of Barrovian metamorphism estimated in the May ( Figure 10).
Late chlorite is typical for schists from the Olympiada deposit; relati peratures obtained by chlorite thermometry (more than 350 °C) indicate th formed probably due to low-temperature metamorphism, but not in a hyd vironment. Figure 10. Pressure-Temperature diagram with parameters of metamorphism of th (rectangulars) [20] and Olympiada and Eldorado schists (crosses). Grey arrow sho of schists of the Mayakon area from early HT/LP metamorphism to the late Barro morphism [20].
Typical prograde metamorphic zoning in garnets with decreasing s increasing almandine from core to the rims and almost homogenous in gro indicates a single stage of prograde metamorphism. The rocks do not have of early HT/LP metamorphism which was mentioned in the nearest area [2 of early metamorphism was not higher than greenschist facies. This shows stage of metamorphism described over the whole YR [44] probably has a p tion.
Detailed studying of metamorphic processes of Transangarian part o formed along Tatarka thrust (Figure 1, review [44] and references therein). F metapelitic schists of the Korda formation show petrological evidence of posed metamorphic events. An early middle Proterozoic event produced andalusite-bearing assemblages at c. 3.5-4 kbar and 540-560°C. During a s  [20] and Olympiada and Eldorado schists (crosses). Grey arrow shows the PTpath of schists of the Mayakon area from early HT/LP metamorphism to the late Barrovian-type metamorphism [20].
The early stage considered to be occurred as a result of the orogeny during late Mesoearly Neoproterozoic. This is supported by earlier U-Th-Pb, Rb-Sr, and K-Ar data on granite-gneiss domes (1100-950 Ma) and the more recent single-zircon (U-Pb SHRIMP II) and 40/39Ar dating of metapelites, metabasites, and rapakivi granites (1140-870 Ma) [13,22,45]. The age estimates around c. 1050 Ma, were obtained by U-Th-Pb dating of xenotime inclusions in the core of zoned garnet grains [21]. The age of the second (Barrovian-type) metamorphic episode is estimated in the range from 850 to 800 Ma. Age estimates are based on CHIME monazite dating [21] and much Ar-Ar data on biotite from metamorphic schists around the whole YK [13,[20][21][22]30].
This review indicates the strong correlation of Barrovian-type metamorphism with the time interval 800-850 Ma in the YK collisional orogen. Mineral assemblages and compositional zoning in garnets from both Eldorado and Olympiada deposits indicate that the rocks underwent only one episode of metamorphism with PT-parameters the thermal gradient of Barrovian type of metamorphism and do not have evidence of the early stage. In the light of regional studies we can suggest that the age around 840 Ma obtained for host schists of Eldorado deposit should be considered as an age of a single metamorphic event, but not a local dynamometamorphism as was suggested by Gibsher et al. [30] and that there is no evidence of early metamorphism with the age around 1050.

Carbonaceous Material Thermometry
Previous studies [46] have shown that a large range of crystallinity is characteristic of carbonaceous matter of organic origin, in contrast to that precipitated from the fluid phase. The temperature obtained on the basis of CM Raman spectra falls in a wide range of about 100-150 • C around the mean temperature for each sample, which suggests varying degrees of graphitization for CM particles in the sample. A large scatter of CM crystallinity inside a single mineral phase indicates that it does not depend on the structural position in the rock. This range exceeds the dispersion of the points for the calibration of the CM thermometer ±50 • C suggested by Beyssac et al. [3]. As no different morphological types of CM were found in samples, this heterogeneity may be due to heterogeneity of the carbonaceous matter or different orientation of microdomains inside a single CM particle. The influence of the mineral matrix or composition of the metamorphic fluids is not excluded [47].
Comparison of data obtained by conventional and CM thermometry shows that the Grt-Bt temperature estimates are generally close to the highest temperatures obtained by CM thermometry (Figure 8) and most CM temperature estimates are much lower than data of Grt-Bt thermometry. This is most pronounced for samples from the Olympiada deposit (Figure 8a-c).
These data indicate that thermometry using Raman spectroscopy of carbonaceous material is a reliable tool for temperature estimates in the temperature range 500-600 • C, but it can only be used if there is a sufficiently large number of analyzes.

Ar-Ar Dating
The sericite vein that was used for Ar-Ar dating cuts the host gray quartz vein and trapped fragments of biotite schist. Arsenopyrite grains are located in both quartz and sericite veins (Figure 9), which implies sericite growth simultaneously or later than arsenopyrite. This is consistent with the Ar-Ar date of 798.6 ± 5.3 Ma as an age of quartzgold-arsenopyrite mineralization or its younger age limit. These data are similar to the age of quartz-gold-arsenopyrite stage (795 Ma) and gold-polysulfide stage (780 Ma) obtained earlier by Sazonov et al. [7].

Correlation of the Mineralization Periods with the Main Tectonomagmatic Events
Correlation of the mineralization periods with the main tectonomagmatic events [13,14,22,27,28,30] in the region shows the following. In the early Neoproterozoic (1050-950 Ma), the terrigenous strata of the Sukhoi Pit Group were deformed and metamorphosed as a result of orogeny. These processes were most intense in the Tatarka-Ishimba fault system. In the earlier, syncollisional, period of this stage, the formation of Teya-type granite-gneiss domes was accompanied by extensive pegmatization [13]. The late collisional period (880-860 Ma) was marked by the formation of K-Na granitoid plutons of the Kalama type (Teya-Eruda complex) [13,27]. Premineral metasomatites formed in a large contact aureole, in the zone where these plutons influenced enclosing black shale-terrigenous strata. They host Au fields and deposits, which formed later.
Barrovian-type metamorphism took place at~860-800 Ma [21,23]. It is genetically related to thrusting and the final stage in the evolution of a collisional orogen. Two following stages of metamorphism were distinguished [23]. The early stage of collisional metamorphism with the peak around 860-850 Ma was related to mostly westward thrusts of blocks of Siberian craton, the late collisional stage around 800 Ma-with eastward thrusts of the Central Angara terrane to the Siberian craton [21].
The formation of quartz-vein zones (Sovetskoe, Eldorado, and other deposits) (830-820 Ma) correlates well with that of thrust nappes. The paragenetic relationship between quartz veining and thrust nappes is confirmed by the fact that quite complex synfolding veins have obvious features of metamorphic bodies [48,49]. The late collisional metamorphism (802-798 Ma) caused by eastward movements of blocks in the zone of high order splays developed at this stage [23]. The formation of bimodal dike swarm dated at~790 Ma is related to Neoproterozoic extension along the western margin of Siberian Craton and mark the upper age limit of collisional metamorphism In the Late Neoproterozoic, rift and intraplate magmatism was most intense and frequent in the Tatarka-Ishimba fault system. Here, we distinguish four stages in the formation of rift structures, accompanied by intraplate magmatism at 780, 750, 700, and 670-650 Ma. The formation of gold-sulfide ores (720-711 Ma) parallels the initiation and evolution of rift structures (Uvolga, Indola, Talovka grabens, Teya-Chapa trough) (720-700 Ma) as well as intraplate subalkaline granitoid and alkaline magmatism (Kutukas and Gurakhta granitoid complexes, 690-700 Ma; Zakhrebetnaya and Middle Tatarka complexes, 700-710 Ma). The time of Au-Sb mineralization correlates with the pre-Vendian orogeny, the initiation of fault grabens infilled with the Chapa Group sediments, and the emplacement of alkalic-ultramafic intrusions (670-650 Ma). The latest postmineral alterations of mineral matter may be related to the tectonomagmatic processes manifested in syenite-alkali syenite (Middle Vorogovka complex) and subalkalic granitoid (Tatarka massif) intrusions with an age of 630-620 Ma.
This review indicates that the most productive quartz-gold-arsenopyrite stage correlates well in time with the collisional stage (800-850 Ma) and formation of barrovian-type metamorphic complexes over the whole YK (Figure 11), where as the closest magmatic events occurred much earlier (Teya-Eruda complex) or much later (Ayahta complex). This implies that metamorphic processes can play a key role in forming YK gold deposits.

Conclusions
(1) The host rocks of both Olympiada and Eldorado gold deposits are metamorphosed during the single episode of Barrovian-type metamorphism at similar temperatures 530-570°C. The rocks do not indicate any traces of the early HT/LP metamorphism Figure 11. The time correlation scheme of main magmatic and metamorphic events with mineralization stages of gold deposits of the Yenisey Ridge.

Conclusions
(1) The host rocks of both Olympiada and Eldorado gold deposits are metamorphosed during the single episode of Barrovian-type metamorphism at similar temperatures 530-570 • C. The rocks do not indicate any traces of the early HT/LP metamorphism observed in other areas of the YK. The growth of the late chlorite after peak metamorphic minerals occurred at temperatures higher than 350 • C probably during retrograde metamorphism. (2) Carbonaceous material thermometry indicates a wide range of obtained temperatures around 90-150 • C for each sample. The highest temperatures are close to the peak metamorphic temperatures estimated by garnet-biotite thermometry. We guess that thermometry using Raman spectroscopy of carbonaceous material is a reliable tool for temperature estimates, but it can only be used if there is a sufficiently large number of analyzes. (3) The new 39/40Ar age data on ore-forming processes of the Eldorado deposit around 796 Ma are very close to the age of the Barrovian-type metamorphism estimated in the area under study and around the whole Yenisey ridge. This may indicate their genetic affinity. (4) The review of magmatic and metamorphic events and ore-forming processes of Yenisey ridge indicate that the most productive ore stage (gold-sulfide-quartz) have a good time correlation with the regional metamorphism of Barrovian type. This indicates that metamorphic processes can play a key role in forming of gold deposits of the Yenisey ridge.

Conflicts of Interest:
The authors declare no conflict of interest.