The Gold Mineralization of the Baranyevskoe Au-Ag Epithermal Deposit in Central Kamchatka

: The Baranyevskoe Au-Ag epithermal deposit of low-sulﬁdation (LS) type is located on the Kamchatka Peninsula in the Neogene-Quaternary Central Kamchatka Volcanic Belt, where Au-bearing quartz veins are usually accompanied by veinlet stockworks. Two economic associations are typical of the Baranyevskoe deposit. The ﬁrst corresponds to gold-pyrite-quartz association with low-grade native gold (521–738 ‱ ) intergrown with pyrite. Some accessory Au-Ag minerals within the early association were also identiﬁed: acanthite AgS 2 , hessite AgTe 2 , lenaite Ag(Fe,Cu)S 2 , petzite Ag 3 AuTe 2 , utenbogardite Ag 3 AuS 2 and unnamed Ag-Sb-As sulfosalts. The former Au-Ag minerals were most likely formed in the temperature range of 320–330 ◦ C based on the study of arsenopyrite thermometers and ﬂuid inclusions. The second, a gold-sulfosalt-quartz association, includes high-grade native gold (883-941 ‱ ) in intergrowth with chalcopyrite. Cuprous phases (bornite, chalcocite, heerite, native copper, Cu-Zn solid solutions), Bi-rich sulfosalts (aikinite PbCuBiS 3 , emplectite CuBiS 2 , witticenite Cu 3 BiS 3 ), stannoidite Cu 8 Fe 3 Sn 2 S 12 , mawsonite Cu 6 Fe 2 SnS 8 ), Au-bearing galena, Te-free and Bi-rich tetrahedrite-tennantite represent this association. Fluid inclusions in gold-sulfosalt-quartz association are characterized by homogenization temperature ranging from 226 to 298 ◦ C, and salinity from 0.4 to 1.2 wt. % NaCl eq.

All these Au-Ag deposits, according to the classification of Corbett [12], belong to the low-sulfidation (LS) or quartz-adularia type, except for the recently described Maletoyvayam deposit, which belongs to the high-sulfidation (HS) or quartz-alunite type [13][14][15]. The Baranyevskoe Au-Ag epithermal deposit is of the LS type and estimated to be formed by near-neutral pH fluids [16,17]. It is located in the Kamchatka Peninsula, on the left bank of the Baranye stream (right tributary of the Balkhach River), approximately 60 km from the Milkovo village. The deposit was discovered in 1972 and, ever since, it has been explored by geologists of different mining companies (Koryakgeoldobycha CJSC, Kamchatka Gold OJSC, Kamchatka Gold Exploration LLC, etc.). The reserves of gold at the Baranyevskoe deposit are reported to be greater than 30 metric tons with an average grade of about 9 g/t [18]. The composition of ores has been studied previously, and three main mineral associations were described: gold-quartz-carbonate-adularia, gold-ore stockworks and quartz combs or "brushes" [19]; the composition of gold was compared with other deposits The stratigraphy of the Baranyevskoe ore field includes tuff of intermediate and mafic composition, basalt, trachyandesite, tuffaceous sandstone, tuffaceous siltstone (lower stratigraphical level); and effusive rocks and tuff of intermediate and mafic composition, andesite, basalt (upper structural level) ( Figure 2). Ages of deposits of the Zolotoe ore field are: Kungurtsevskoe-21 Ma [20], Zolotoe-in the interval of 21.3-17.0 Ma, whereas the Baranyevskoe deposit, located in Late Miocene-Pliocene rocks, has an age interval of 3.9-2.4 Ma according to the K-Ar method [27]. The stratigraphy of the Baranyevskoe ore field includes tuff of intermediate and mafic composition, basalt, trachyandesite, tuffaceous sandstone, tuffaceous siltstone (lower stratigraphical level); and effusive rocks and tuff of intermediate and mafic composition, andesite, basalt (upper structural level) ( Figure 2). Ages of deposits of the Zolotoe ore field are: Kungurtsevskoe-21 Ma [20], Zolotoe-in the interval of 21.3-17.0 Ma, whereas the Baranyevskoe deposit, located in Late Miocene-Pliocene rocks, has an age interval of 3.9-2.4 Ma according to the K-Ar method [27].
The Baranyevskoe deposit consists of a system of vein-veinlet and stockwork (veinletdisseminated) ore-bearing structures in the zone of deep northeastern faulting ( Figure 3). The Rusty ore zone is located along the central fault NE-SW and is accompanied by zones of abundant apophyses: the "Central", "Southern" and "Hanging", along with others branches from the axial fault in the hanging wall [18].
Quartz veins are accompanied by veinlet-disseminated stockworks, on top of which there is a 200-m interval of rocks with high profusion of weakly mineralized (less than 1 ppm Au) veinlets of carbonate and zeolite-carbonate compositions. On the other hand, the stockwork includes the rich vein-disseminated gold mineralization with an Au concentration up to 20 ppm, accompanied by metasomatic associations: pyrite-hematite-magnetitesericite (alunite)-quartz in the central part of the stockwork and pyrite-sericite-illite-quartz at the periphery. Vuggy silica is common at deeper levels of stockworks [18]. The proportion of mineralized veinlets overlapping disseminated mineralization increases to the southwest. Quartz veins in the hanging wall are also accompanied by Au-bearing metasomatites comprising quartz, adularia, hydromica, carbonate and clay minerals. Thus, the main ore bodies (zones) of the Baranyevskoe deposit, up to 20 m thick, are composed of: (a) thick quartz veins, (b) disseminated-vein halos, and (c) sulfidized hydrothermalmetasomatic rocks. Consequently, the ore bodies exhibit a ribbon-like shape with a length up to 1500 m, in which gold is unevenly distributed.

Figure 2.
Geological-structural diagram of the Balkhach volcano-tectonic structure and respective A-B cross-section modified after [18,26].
The Baranyevskoe deposit consists of a system of vein-veinlet and stockwork (veinlet-disseminated) ore-bearing structures in the zone of deep northeastern faulting ( Figure 3). The Rusty ore zone is located along the central fault NE-SW and is accompanied by zones of abundant apophyses: the "Central", "Southern" and "Hanging", along with others branches from the axial fault in the hanging wall [18]. Geological-structural diagram of the Balkhach volcano-tectonic structure and respective A-B cross-section modified after [18,26].  Quartz veins are accompanied by veinlet-disseminated stockworks, on top of which there is a 200-m interval of rocks with high profusion of weakly mineralized (less than 1 ppm Au) veinlets of carbonate and zeolite-carbonate compositions. On the other hand, the stockwork includes the rich vein-disseminated gold mineralization with an Au concentration up to 20 ppm, accompanied by metasomatic associations: pyrite-hematitemagnetite-sericite (alunite)-quartz in the central part of the stockwork and pyrite-sericiteillite-quartz at the periphery. Vuggy silica is common at deeper levels of stockworks [18]. The proportion of mineralized veinlets overlapping disseminated mineralization increases to the southwest. Quartz veins in the hanging wall are also accompanied by Aubearing metasomatites comprising quartz, adularia, hydromica, carbonate and clay minerals. Thus, the main ore bodies (zones) of the Baranyevskoe deposit, up to 20 m thick, are composed of: (a) thick quartz veins, (b) disseminated-vein halos, and (c) sulfidized hydrothermal-metasomatic rocks. Consequently, the ore bodies exhibit a ribbon-like shape with a length up to 1500 m, in which gold is unevenly distributed.

Samples
The rock samples for this study were collected from an open pit in the Rusty, Central and Hanging zones, the underground tunnels of the Northern Zone and outcrops of the Southern Zone. Most of the samples gathered are characterized by the gold-pyrite-quartz association, which is the most widespread in the Central zone where metasomatic rocks are superimposed onto the gold-pyrite-quartz bearing veins. The most abundant mineral within this assemblage, pyrite, is sometimes replaced by hematite, whereas native gold may be found intergrown with pyrite. The role of the gold-sulfosalt-quartz association increases toward the northeastern direction of the deposit, and is more representative for the Northern zone. The gold-sulfosalt-quartz association is formed by samples of sericiteadularia-quartz composition with fragments of host-rock andesites, which are characterized, as a rule, by crustified-breccia textures (Figure 4a). Disseminated sulfides tend to form ribbon-like areas, due to the irregular flow of hydrothermal solutions. Additionally, native gold is found in paragenetic intergrowths with chalcopyrite ( Figure  4b).

Samples
The rock samples for this study were collected from an open pit in the Rusty, Central and Hanging zones, the underground tunnels of the Northern Zone and outcrops of the Southern Zone. Most of the samples gathered are characterized by the gold-pyrite-quartz association, which is the most widespread in the Central zone where metasomatic rocks are superimposed onto the gold-pyrite-quartz bearing veins. The most abundant mineral within this assemblage, pyrite, is sometimes replaced by hematite, whereas native gold may be found intergrown with pyrite. The role of the gold-sulfosalt-quartz association increases toward the northeastern direction of the deposit, and is more representative for the Northern zone. The gold-sulfosalt-quartz association is formed by samples of sericite-adularia-quartz composition with fragments of host-rock andesites, which are characterized, as a rule, by crustified-breccia textures ( Figure 4a). Disseminated sulfides tend to form ribbon-like areas, due to the irregular flow of hydrothermal solutions. Additionally, native gold is found in paragenetic intergrowths with chalcopyrite ( Figure 4b).

The Basic Ore Minerals
Pyrite is the most abundant sulfide at the Baranyevskoe deposit. Some analyses of pyrite (more than 30%) contain As as a minor element. As-bearing pyrite forms intermittent growth zones with As-free pyrite grains (Figure 5a,b). This oscillatory zoning observed in some pyrite grains and sulfosalts of epithermal deposits [28,29] is due to fluctuations in the characteristics of the hydrothermal fluid during the growth of minerals.

The Basic Ore Minerals
Pyrite is the most abundant sulfide at the Baranyevskoe deposit. Some analyses of pyrite (more than 30%) contain As as a minor element. As-bearing pyrite forms intermittent growth zones with As-free pyrite grains (Figure 5a,b). This oscillatory zoning observed in some pyrite grains and sulfosalts of epithermal deposits [28,29] is due to fluctuations in the characteristics of the hydrothermal fluid during the growth of minerals. The concentration of As in pyrite varies from grain to grain, reaching up to 7.4 wt. % (Figure 5c) while no other trace elements have so far been identified in the pyrite of the Baranyevskoe deposit (Table 1).

Figure 4.
Sample of sericite-adularia-quartz composition with crustified-breccia texture containing disseminated sulfide and sulfosalts (a) and visible gold on an enlarged fragment of the sample (b) (sample Bar_2-1, Rusty zone).

The Basic Ore Minerals
Pyrite is the most abundant sulfide at the Baranyevskoe deposit. Some analyses of pyrite (more than 30%) contain As as a minor element. As-bearing pyrite forms intermittent growth zones with As-free pyrite grains (Figure 5a,b). This oscillatory zoning observed in some pyrite grains and sulfosalts of epithermal deposits [28,29] is due to fluctuations in the characteristics of the hydrothermal fluid during the growth of minerals. The concentration of As in pyrite varies from grain to grain, reaching up to 7.4 wt. % (Figure 5с) while no other trace elements have so far been identified in the pyrite of the Baranyevskoe deposit (Table 1).

Figure 5.
Back-scattering electron mode (BSE)-scanning electron microscope (SEM) images of As-containing pyrite grains (a,b) and the negative correlation S versus As in pyrite of Baranyevskoe deposit (c).
The composition in non-oxidized samples varies from low-grade to high-grade gold with a gap in the isomorphic series in the range of Au 74-88 at. %. Some gold grains from the Rusty and Central zones, most of the grains from the Hanging zone and half of the gold grains from the Northern zone consist of native gold and electrum with a range of fineness of 521-738‱ ( Figure 6, based on data in Table 2), which are characteristic of the gold-pyrite-quartz association of the early ore-forming stage. A significant part of gold grains from the Northern zone and some grains from the Hanging zone correspond to high-grade native gold (883-941‱) ( Figure 6, based on data in Table 2), typical of the gold-sulfosalt-quartz association. Electrum, "küstelite", and the high-grade native gold are representative in the Southern zone from sediments below of quartz veins and in oxidized ores. Oxidized ores of Southern and Central zones also contain the finest gold (920-280‱) ( Figure 7, Table 3). Thus, Au-Ag solid solutions of different compositions are involved in the formation ores in different-stage in almost all zones of the Baranyevskoe deposit.   Table 3).

Gold-Pyrite-Quartz Association
The gold-pyrite-quartz mineralization is more widespread in the central part of the deposit (Rusty zone), where metasomatites are overprinted on veinlets of gold-pyritequartz veins. The low-grade gold (electrum), as a rule, was found as individual grains in concentrates of crushed sample. Such gold grains are inclusions in pyrite, and associated with acanthite Ag2S, petzite Ag3AuTe2 and uytenbogardtite Ag3AuS2 (Figure 8a). In the Central zone, this mineral assemblage is characterized by inclusions of pyrite crystals in gold (Figure 8d,e). Gold is associated with hessite Ag2Te and galena PbS (Figure 8b). The  Table 2).  Figure 6. Variations in the composition of Au-Ag solid solutions from different ore zones of the Baranyevskoe deposit (data from Table 2).  Table 3).

Gold-Pyrite-Quartz Association
The gold-pyrite-quartz mineralization is more widespread in the central part of the deposit (Rusty zone), where metasomatites are overprinted on veinlets of gold-pyritequartz veins. The low-grade gold (electrum), as a rule, was found as individual grains in concentrates of crushed sample. Such gold grains are inclusions in pyrite, and associated with acanthite Ag2S, petzite Ag3AuTe2 and uytenbogardtite Ag3AuS2 (Figure 8a). In the Central zone, this mineral assemblage is characterized by inclusions of pyrite crystals in gold (Figure 8d,e). Gold is associated with hessite Ag2Te and galena PbS (Figure 8b). The gold-pyrite-quartz mineralization in the Southern zone is featured by the development of unusual composition in acanthite, with traces of Sb, As and Se, and sometimes Au: (Ag, Au)2(S,Sb,As,Se), which is found as individual grains in ocher-clay samples (Figure 8h). The silver sulfosalts Ag10(Sb,As)S5 and Ag17(Sb,As)2(S,Se)10 composition are found both in the Southern and Central zones in the gold-pyrite-quartz mineralization ( Table 4). The replacement of acanthite by lenaite Ag(Fe,Cu)S2 occurs (Figure 8i). Sulfides in intergrowth with gold are sometimes replaced by Fe-oxide (Figure 8c).   Table 3).  The gold-pyrite-quartz mineralization is more widespread in the central part of the deposit (Rusty zone), where metasomatites are overprinted on veinlets of gold-pyrite-quartz veins. The low-grade gold (electrum), as a rule, was found as individual grains in concentrates of crushed sample. Such gold grains are inclusions in pyrite, and associated with acanthite Ag 2 S, petzite Ag 3 AuTe 2 and uytenbogardtite Ag 3 AuS 2 (Figure 8a). In the Central zone, this mineral assemblage is characterized by inclusions of pyrite crystals in gold (Figure 8d,e). Gold is associated with hessite Ag 2 Te and galena PbS (Figure 8b). The gold-pyrite-quartz mineralization in the Southern zone is featured by the development of unusual composition in acanthite, with traces of Sb, As and Se, and sometimes Au: (Ag, Au) 2 (S,Sb,As,Se), which is found as individual grains in ocher-clay samples (Figure 8h). The silver sulfosalts Ag 10 (Sb,As)S 5 and Ag 17 (Sb,As) 2 (S,Se) 10 composition are found both in the Southern and Central zones in the gold-pyrite-quartz mineralization ( Table 4). The replacement of acanthite by lenaite Ag(Fe,Cu)S 2 occurs (Figure 8i). Sulfides in intergrowth with gold are sometimes replaced by Fe-oxide (Figure 8c).

Gold-Sulfosalt-Quartz Association
Native gold, belonging to this association, is intergrown with chalcopyrite, which is by far the most common mineral. Sample Bar-5-2 collected from underground mines of the Northern Zone is a typical example of a gold-sulfosalt-quartz association (Figure 9). The main mineral phase associated with gold is emplectite CuBiS 2 (up to 50 µm), which occurs intergrown with gold or included in chalcopyrite in their joint paragenesis (Figure 9b,c,e-g,). Native gold within this mineral assemblage is always high-grade with a concentration of Ag no greater than 12 at. %.

Gold-Sulfosalt-Quartz Association
Native gold, belonging to this association, is intergrown with chalcopyrite, which is by far the most common mineral. Sample Bar-5-2 collected from underground mines of the Northern Zone is a typical example of a gold-sulfosalt-quartz association (Figure 9). The main mineral phase associated with gold is emplectite CuBiS2 (up to 50 μm), which occurs intergrown with gold or included in chalcopyrite in their joint paragenesis ( Figure  9b,c,e,f,g,). Native gold within this mineral assemblage is always high-grade with a concentration of Ag no greater than 12 at. %.
For the Northern and Hanging zones, emplectite is characterized by containing Pb around 2-3 wt % and sometimes Sb up to 1.81 wt %. Mawsonite Cu6Fe2SnS8 is also often found within this association (Figure 9a,e,g). Additionally, there is tetrahedrite of unusual composition, in which the Bi concentration either prevails over As-(Cu,Fe,Zn)12(Sb,Bi,As)4S13-or completely replaces it ( Table 5, No. 31, 32). The tetrahedrite in Bar-5-2 sample contains more than 4 wt % of Zn. Secondary minerals replacing the chalcopyrite are typically covellite CuS and geerite Cu8S5 (Figure 9c-d). Aikinite PbCuBiS3 is as well a normal-assemblage mineral of the gold-sulfosalt-quartz association in the Northern and Hanging zones ( Table 5). The gold-sulfosalt-quartz association in the Rusty zone ( Figure 10) is distinguished by the presence of copper-bearing (4.93 wt % of Cu) electrum, the composition of which corresponds to the formula Au0.70Ag0.17Cu0.13, or more simply, Au7Ag2Cu, as well as bornite ( Figure 10 a-b). The tetrahedrite in Bar-2-1 sample is also Bi-rich (Table 5), but its concentration is lower than that of As. Another typomorphic Bi-rich mineral, wittichenite Cu3BiS3, instead of emplectite (CuBiS2), is characteristic of this association in the Rusty zone (Figure 10e-h).  For the Northern and Hanging zones, emplectite is characterized by containing Pb around 2-3 wt. % and sometimes Sb up to 1.81 wt. %. Mawsonite Cu 6 Fe 2 SnS 8 is also often found within this association (Figure 9a,e,g). Additionally, there is tetrahedrite of unusual composition, in which the Bi concentration either prevails over As-(Cu,Fe,Zn) 12 (Sb,Bi,As) 4 S 13 -or completely replaces it ( Table 5, No. 31, 32). The tetrahedrite in Bar-5-2 sample contains more than 4 wt. % of Zn. Secondary minerals replacing the chalcopyrite are typically covellite CuS and geerite Cu 8 S 5 (Figure 9c-d). Aikinite PbCuBiS 3 is as well a normal-assemblage mineral of the gold-sulfosalt-quartz association in the Northern and Hanging zones ( Table 5). The gold-sulfosalt-quartz association in the Rusty zone ( Figure 10) is distinguished by the presence of copper-bearing (4.93 wt. % of Cu) electrum, the composition of which corresponds to the formula Au 0.70 Ag 0.17 Cu 0.13 , or more simply, Au 7 Ag 2 Cu, as well as bornite (Figure 10a,b). The tetrahedrite in Bar-2-1 sample is also Bi-rich (Table 5), but its concentration is lower than that of As. Another typomorphic Bi-rich mineral, wittichenite Cu 3 BiS 3 , instead of emplectite (CuBiS 2 ), is characteristic of this association in the Rusty zone (Figure 10e-h).

Concentration of Ore and Chalcogenic Elements
The concentration of ore and chalcogenic elements in samples from different zones of the Baranyevskoye deposit were obtained. The concentration of precious metals is unevenly distributed: tenths of ppm Au in the Southern zone, units of ppm in the Rusty and Central zones, tens of ppm in the Hanging zone, whereas the highest concentrations of hundreds of ppm occur in the Northern zone (Table 6). In general, Au prevails over Ag in all zones (Au/Ag ratios are 2.80-9.91 and Ag/Au ratios are 0.10-3.72) except for the Southern zone, in which the Au/Ag ratios vary in the range of 0.16-0.52 for the quartz vein, and reaches up to 2.35 in the metasomatitic host rock (sample BAR-6-5). Thus, the samples from the tunnels (BAR-5) of the Northern Zone are the richest in Au. Sample BAR-5-1 represents the gold-pyrite-quartz association, while sample BAR-5-2 represents the gold-sulfosalt-quartz association, in which the concentrations of Pb, Sb, Se are increased by one order of magnitude, Cu and Sn by two orders of magnitude, and Bi by four orders of magnitude compared with the content of these elements in samples from other zones. At the same time, the As concentration is not high compared to its anomalous values (1030 ppm) in the gold-pyrite-quartz association of the same zone. The aforementioned distribution of elements is consistent with mineralogical features: Asbearing pyrite in one association and the presence of many Bi-rich and Sn-bearing sulfosalts included in chalcopyrite in another.

Concentration of Ore and Chalcogenic Elements
The concentration of ore and chalcogenic elements in samples from different zones of the Baranyevskoye deposit were obtained. The concentration of precious metals is unevenly distributed: tenths of ppm Au in the Southern zone, units of ppm in the Rusty and Central zones, tens of ppm in the Hanging zone, whereas the highest concentrations of hundreds of ppm occur in the Northern zone (Table 6). In general, Au prevails over Ag in all zones (Au/Ag ratios are 2.80-9.91 and Ag/Au ratios are 0.10-3.72) except for the Southern zone, in which the Au/Ag ratios vary in the range of 0.16-0.52 for the quartz vein, and reaches up to 2.35 in the metasomatitic host rock (sample BAR-6-5). Thus, the samples from the tunnels (BAR-5) of the Northern Zone are the richest in Au. Sample BAR-5-1 represents the gold-pyrite-quartz association, while sample BAR-5-2 represents the gold-sulfosalt-quartz association, in which the concentrations of Pb, Sb, Se are increased by one order of magnitude, Cu and Sn by two orders of magnitude, and Bi by four orders of magnitude compared with the content of these elements in samples from other zones. At the same time, the As concentration is not high compared to its anomalous values (1030 ppm) in the gold-pyrite-quartz association of the same zone. The aforementioned distribution of elements is consistent with mineralogical features: As-bearing pyrite in one association and the presence of many Bi-rich and Sn-bearing sulfosalts included in chalcopyrite in another.

Study on Fluid Inclusions
Fluid inclusions were investigated only for the gold-sulfosalt-quartz association. Small (15-20 µm) primary gaseous and two-phase gas-liquid inclusions contained in quartz were identified. In the central parts of translucent crystals of quartz, there are mainly groups of isometric two-phase inclusions (Figure 11a) whereas gas and two-phase fluid inclusions, on the growth zones of quartz crystals, have an irregular shape (Figure 11b).
Homogenization temperature of fluid inclusions varied in the range of 226-298 • C (Figure 12a). The ice melting in cooled fluid inclusions appeared at temperatures from -0.8 to −0.2 • C ( Table 7). The salinity of fluid inclusions was estimated from 0.4 to 1.2 wt. % NaCl eq. Melting of eutectic in frozen fluid inclusions occurred in the temperature range of −49 to −25 • C.
by eutectic melting at range of −45 to −27 °C. The fluid inclusions in the quartz of this ore association contain NaCl and KCl. Low melting temperatures of eutectics from −49 to −27 C° are characteristic of fluid inclusions in quartz of the gold-sulfosalt-quartz association with high-grade gold (Table 7). With regard to the composition of fluids at the latest stage, in addition to NaCl, there could be admixtures of CaCl2, MgCl2, FeCl2, FeCl3, K2CO3, which may lower the melting point of the eutectic of fluid inclusions [24]. The composition of gas inclusions, apparently, is dominated by water vapor. Figure 11. Microphotographs of quartz from the gold sulfosalt-quartz association: (a)-two-phase inclusions of isometric shape in the center of a transparent quartz crystal (sample Bar-5_2/1). Gaseous and two-phase inclusions of irregular shape on the crystal growth zone are shown in the inset; (b)-paragenesis of quartz, chalcopyrite and sulfosalts (dark) in a quartz vein (sample Bar-5_2/1). Primary two-phase fluid inclusions in a quartz crystal are shown in the inset. Note: n-number of analyses; Th °C-homogenization temperature of fluid inclusions; Teu °C-eutectic temperature (first melting temperature); Tm ice °C-final ice melting temperature; MPa-calculated pressure [22]; eq-equivalent.
The wide range of temperature of homogenization in the fluid inclusions (298-226 °C) along with a narrow range of salinity (0.4-1.2 wt % NaCl eq.) (Figure 12b) may be MPa, and it is mostly close to the pressures of ore formation in the near-surface volcanic-hydrothermal mineral systems. If we take these values for hydrostatic pressure, then we can assume that these ores from the Baranyevskoe deposit were formed at a depth of 260 to 530 m. The temperature and salinity of ore-forming fluids of the late gold-sulfosalt-quartz association varied over a narrow interval; however, an even more complex composition of salts in the ore-forming fluids is assumed at the final stage of gold-sulfosalt-quartz associations: salts that may be expected to be present: NaCl, CaCl2, MgCl2, FeCl2, FeCl3, K2CO3, compared to the composition of fluids (NaCl and KCl) at the initial stage of formation of this association.

Discussion
Gold from the Baranyevskoye deposit is subdivided into two classes: the first includes a series of compositions related to low-grade gold and electrum with a fineness of (52-74 at. % Au) ( Figure 13). Sufficiently large grains (50-100 microns) are, as a rule, in association with pyrite and are characteristic of the early high-temperature stage. Similar compositions have been described for hypogenic gold at the Aginskoye deposit [11]. The second class corresponds to high-grade gold (88-94 at. % Au) (Figure 13), which, together with chalcopyrite and Bi-rich sulfosalts, compose the mineral association of the lowertemperature stage. However, it should be noted that high-grade gold is also rarely found in the early gold-pyrite-quartz association, being represented there only by thin wormlike veins included in low-grade gold (Figure 8g).  Note: n-number of analyses; Th • C-homogenization temperature of fluid inclusions; Teu • C-eutectic temperature (first melting temperature); Tm ice • C-final ice melting temperature; MPa-calculated pressure [22]; eq-equivalent.
Fluid inclusions in quartz from the gold-sulfosalt quartz association are characterized by eutectic melting at range of −45 to −27 • C. The fluid inclusions in the quartz of this ore association contain NaCl and KCl. Low melting temperatures of eutectics from −49 to −27 C • are characteristic of fluid inclusions in quartz of the gold-sulfosalt-quartz association with high-grade gold (Table 7). With regard to the composition of fluids at the latest stage, in addition to NaCl, there could be admixtures of CaCl 2 , MgCl 2 , FeCl 2 , FeCl 3 , K 2 CO 3 , which may lower the melting point of the eutectic of fluid inclusions [24]. The composition of gas inclusions, apparently, is dominated by water vapor.
The wide range of temperature of homogenization in the fluid inclusions (298-226 • C) along with a narrow range of salinity (0.4-1.2 wt.% NaCl eq.) (Figure 12b) may be explained by processes of mixing of hot magmatogenic fluids with cold fresh meteoric waters. The presence of primary gaseous and two-phase fluid inclusions on the growth zones of quartz crystals indicates boiling of the ore-forming fluid. The calculated pressure of homogenization of fluid inclusions in quartz from the late stage of mineralization of the Baranyevskoe deposit varies from 2.6 to 8.3 MPa, and it is mostly close to the pressures of ore formation in the near-surface volcanic-hydrothermal mineral systems. If we take these values for hydrostatic pressure, then we can assume that these ores from the Baranyevskoe deposit were formed at a depth of 260 to 530 m.
The temperature and salinity of ore-forming fluids of the late gold-sulfosalt-quartz association varied over a narrow interval; however, an even more complex composition of salts in the ore-forming fluids is assumed at the final stage of gold-sulfosalt-quartz associations: salts that may be expected to be present: NaCl, CaCl 2 , MgCl 2 , FeCl 2 , FeCl 3 , K 2 CO 3 , compared to the composition of fluids (NaCl and KCl) at the initial stage of formation of this association.

Discussion
Gold from the Baranyevskoye deposit is subdivided into two classes: the first includes a series of compositions related to low-grade gold and electrum with a fineness of (52-74 at. % Au) ( Figure 13). Sufficiently large grains (50-100 microns) are, as a rule, in association with pyrite and are characteristic of the early high-temperature stage. Similar compositions have been described for hypogenic gold at the Aginskoye deposit [11]. The second class corresponds to high-grade gold (88-94 at. % Au) (Figure 13), which, together with chalcopyrite and Bi-rich sulfosalts, compose the mineral association of the lowertemperature stage. However, it should be noted that high-grade gold is also rarely found in the early gold-pyrite-quartz association, being represented there only by thin worm-like veins included in low-grade gold (Figure 8g). Similar textures were previously described at the Aginskoye deposit [11]. According to the authors, such high-grade gold is secondary in origin, formed during the oxidation of hypogenic gold by meteoric waters at the stage of hypergenesis. However, we consider that the formation of high-grade gold in the Baranyevskoe deposit is mainly due to a change in the physicochemical conditions and the composition of hydrothermal solutions. Andreeva and Kudaeva [20], who previously studied the typomorphism of gold in the Balkhach ore cluster, noted that the grain size and fineness increase within the transition from quartz-carbonate-adularia stockwork rocks to carbonate rocks and quartzcarbonate-adularia veins. On the other hand, silver content in gold is considered as an indicator of the temperature regime during the formation of ores [31]. The composition of native gold evolves from very high-grade gold to electrum and Hg-bearing gold in the Western Tuva deposits (Russia) [32] as opposed to what is described in this study. Similar textures were previously described at the Aginskoye deposit [11]. According to the authors, such high-grade gold is secondary in origin, formed during the oxidation of hypogenic gold by meteoric waters at the stage of hypergenesis. However, we consider that the formation of high-grade gold in the Baranyevskoe deposit is mainly due to a change in the physicochemical conditions and the composition of hydrothermal solutions. Andreeva and Kudaeva [20], who previously studied the typomorphism of gold in the Balkhach ore cluster, noted that the grain size and fineness increase within the transition from quartz-carbonate-adularia stockwork rocks to carbonate rocks and quartz-carbonate-adularia veins. On the other hand, silver content in gold is considered as an indicator of the temperature regime during the formation of ores [31]. The composition of native gold evolves from very high-grade gold to electrum and Hg-bearing gold in the Western Tuva deposits (Russia) [32] as opposed to what is described in this study. Electrum is associated with pyrite at the Valunistoye deposit (Chukotka), and high-grade gold is found in the form of rims and veins in electrum, occurring later in the paragenesis [33], as in our case. The Au-Ag-Cu system with reference to [34] is shown in [35]. Composition-temperature ranges in this isotherm reflect metastable equilibria. On one side of the triangle, there is a wide row of Au-Ag solid solutions below 300 • C isotherm. That is, alloys of different compositions can crystallize at the same temperature. Therefore, at temperatures corresponding to hydrothermal conditions, opposite trends of the variation in Ag content, in gold grains, are observed for different deposits.
Pyrite is one of the most important indicator minerals in studying the features of the genesis in ore deposits. The As concentration in the pyrite of the Baranyevskoe deposit (7.37 wt. %) exceeds the As in pyrite of numerous deposits, including the values in the Kumroch deposit, where pyrite contains up to 6.79 wt. % As [36]. For instance, the content of As in pyrite at the Zaozigou Gold Deposit (Central China) is 4.1 wt. % [37], it is 4.5 wt. % in pyrite of the Roudný deposit, Bohemian Massif [38], while As in pyrite from El Valle gold deposit (Spain) is the most abundant (up to 9.5 wt. % As) [39]. Therefore, it becomes evident that the solid solution of Au is dominated by arsenian pyrite in all these deposits. However, high As concentrations are characteristic only for certain areas of zoned pyrite. Compositional zoning of pyrites in gold deposits reflects the chemical evolution of ore bearing fluids. The relatively high activity of As and Au during crystallization of the early generation of pyrite allowed As-bearing pyrite to be precipitated as a consequence. This is consistent with the extremely high As concentration in the samples of the goldpyrite-quartz association in the Northern Zone ( Table 6). The As-bearing pyrites were formed at temperatures of at least 320-330 ºC, based on arsenopyrite thermometers and fluid inclusion data [38]. The variable amount of As in grains of pyrite reflects changes in physicochemical conditions (T, f S 2 , f O 2 , pH) and the composition of fluids, which, at the same time, determine the appearance of a concentration gradient on the pyrite growth surface [40]. Pyrite and gold of low-grade composition in the early association are genetically linked with the Ag-Au minerals: acanthite, hessite, lenaite, petzite, utenbogardtite and Ag-sulfosalts: Ag 10 (Sb,As)S 5 and Ag 17 (Sb,As) 2 (S, Se) 10 .
An increase in copper concentration during the development of the ore-forming system led to the formation of a later gold ore association with the leading role of chalcopyrite, as well as other cuprous phases (bornite, chalcocite, geerite, native copper and Cu-Zn solid solutions). Simultaneously, there was an increase in the fineness of gold from 700-800‱ in the early association up to 900-950‱ in the late one, as well as an increase in the concentration of bismuth (leading to the formation of emplectite CuBiS 2 or wittichenite Cu 3 BiS 3 ); the concentration of tin in the ore-forming system was also increased, forming mawsonite Cu 6 Fe 2 SnS 8 .
Primary galena from hydrothermal deposits has been shown to contain anomalous and significant levels of Bi, Ag, Te, Se, Sb, Cu, Tl and Zn [41]. Increased concentrations of Bi and Ag indicate the dissolution of the proportion of AgBiS 2 in galena [42]. At the Baranyevskoe deposit, galena contains only traces of Au (up to 2.02 wt. %), and no other minor elements were found. It is assumed that a gold-bearing variety of galena contains probably microscopic or nano-inclusions of gold or gold-bearing-minerals (https://www. mindat.org/min-26564.html, 11 October 2021). Au-bearing galena from the Baranyevskoe deposit is, however, found as inclusions in Au-Ag alloys. Then, it could be argued that gold content was measured from the matrix during the analysis. Firstly, however, the galena inclusions are large enough (10-20 microns) for correct analysis; secondly, they look absolutely homogeneous (Figure 8b); and thirdly, Ag is absent in the analysis of galena, and it should have been captured together with Au. Therefore, Au-containing galena is a feature of the gold-pyrite-quartz association.
The tetrahedrite (Cu 10 (Fe,Zn) 2 Sb 4 S 13 )-tennantite (Cu 10 (Fe,Zn) 2 As 4 S 13 ) solid-solution series (fahlore) is common, and widespread in Au-Ag epithermal ore deposits around the world [43]. The role of sulfosalts in gold deposits is significant, since they are closely associated with native gold, and their study is considered to be very important in identifying the nature of gold mineralization. The rather significant variability in composition makes fahlores a useful indicator of ore-forming processes and fluid compositions during their development [44].
Tetrahedrite of the Baranyevskoe deposit contains significant amounts of bismuth, which in some cases may dominate over arsenic Cu 12 (Sb,Bi,As) 4 S 13 . A similar and even richer in bismuth (up to 22.17 wt. % Bi) tetrahedrite was described earlier in Schwarzwald ore district with 1.83 atoms per formula unit (apfu) Bi [44] compared to Baranievskoye, where 0.20-1.10 apfu Bi, based on 4 total apfu (Sb+As+Bi) was established. However, the Birichest sulfosalts are also Pb-bearing [45], whereas, in the herein investigated association, it lacks Pb-Bi fahlores. Only one grain of sulfosalt containing lead (Cu 3 Fe 3 PbS 7 ) was found. A feature of the gold-sulfosalt-quartz association of the Baranyevskoe deposit is the presence of the Te-free tetrahedrite-tennantite series, while Te-rich fahlore (goldfieldite) is characteristic in numerous epithermal deposits [46], including the gold-forming stages of the Ozernovskoe and Aginskoe epithermal deposits in Kamchatka. Moreover, at the Aginskoe deposit, the Te-rich minerals (AuTe 2 , PbTe, Ag 2 Te, Ag 3 AuTe 2 ) are rather common, while pyrite is subordinate [8,9,11]. The presence of Bi-rich and, simultaneously, Te-poor varieties of sulfosalts is considered a typomorphic feature of the gold-sulfosalt-quartz association.
The increasing role of bismuth at the late gold stage (gold-sulfosalt-quartz association) expressed by the crystallization of bismuth-rich minerals-emplectite CuBiS 2 , wittichenite Cu 3 BiS 3 , tetradymite Bi 2 Te 2 S, sulfosalt (Cu,Fe)Bi 5 Te 5 S 3 -is consistent with the data of the chemical analysis of the ores from this association ( Table 6). Sulfosalts of copper, iron and tin-mawsonite Cu 6 Fe 2 SnS 8 and stannoidite Cu 8 Fe 3 Sn 2 S 12 -are also characteristic of the gold-sulfosalt-quartz association. These minerals indicate some enrichment in tin at a late stage in the development of the ore-forming system. Mawsonite Cu 6 Fe 2 SnS 8 has been identified in association with pyrite and tetrahedrite in a vein orebody, and its formation is related to interactions during the substitution of the tin-bearing famatinite by tetrahedrite [47]. The gold-sulfosalt-quartz association identified in this study is in many respects similar to that from the Kairagach gold deposit, Uzbekistan, which is characterized by a Au-Sn-Bi-Se-Te geochemical profile; namely, it is comparable to the third generation of ore mineralization of Kairagach gold deposit: Bi-sulfosalts as well as native gold of high fineness, tetrahedrite-annivite series. These are characterized by high (up to 9 wt. %) content of Bi [48].
The microthermometry study on fluid inclusions in quartz with the most abundant dissemination of chalcopyrite and sulfosalts grains (gold-sulfosalts-quartz association) revealed in the quartz aggregate, temperatures corresponding to a range of 299-226 • C. Similar temperatures of homogenization of primary inclusions were reported for the quartz of the Aginskoe deposit (LS type) 230-280 • C [9], and were also established for the Rodnikovoe and Asachinskoe deposits in South Kamchatka [6,49,50]. In addition, at the IS type field (Cesme Hafez, Iran) [51], homogenization temperatures of primary inclusions are within the same range (140-280 • C). At the same time, it is well compatible with many HS type deposits, e.g., Maletoyvayam (255-245 • C) [15].
The salinity range can be strongly influenced, on one hand, by mixing with meteoric water (dilution), or, on the other hand, by boil-off (concentrating) [52,53]. In general, the salinity of LS type deposits shows a wide range of values. The mineralization in LS type crystallizes, as a rule, from relatively dilute brines <5 wt. % NaCl eq. Aginskoye deposit is no greater than 2 wt. % NaCl equiv. [9], as in the Juliet field (LS type), at the Okhotsk-Chukotka volcanic belt, the salinity of the inclusions is around 1.2-5.6 wt. % NaCl eq. [54]. However, data from fluid inclusions, in quartz, associated with the main gold stage in HS type deposits-Mt Carlton, Lepanto, Agan, Mt Carlton, NE Australia, Danchenkovskoe, and Maletoyvayam [15,[55][56][57][58]-similarly indicate salinities up to 4.5 wt. % NaCl eq. In