Porphyry-Related Metamorphosed Au-Ag and Cu-Mo Deposits in the Precambrian of the Fennoscandian Shield

: The Pellapahk Cu-Mo and Oleninskoe Au-Ag deposits in the western segment of the Russian Arctic in the Kolmozero–Voronya greenstone belt are considered two parts of an Archean (2.83–2.82 Ga) porphyry-epithermal system, probably the oldest one deﬁned in the Fennoscandian Shield. Formation of the Oleninskoe Au-Ag deposit at the epithermal stage of the system is indicated by the spatial and genetic relationships with the sills of granite porphyry, the geochemical association of ore elements (Au, Ag, Cu, Pb, Sb, As), an Au/Ag ratio of <0.2, and the multiplicity of silver mineralization with different Ag, Cu, Pb, Sb sulfosalts. The geological–structural characteristics of the Oleninskoe and the Pellapahk, i.e., their location in a shear zone, the morphology and size of ore bodies, the scale of the deposits, and the intensity and zoning of rock alteration, do not oppose this model. Mineralized rocks of the Pellapahk Cu-Mo and Oleninskoe Au-Ag deposits were amphibolite metamorphosed in the Neoarchean and again in the Paleoproterozoic. Structures of sulﬁde melt crystallization formed in the ores during metamorphism, those are ﬁne intergrowths of galena, argentotetrahedrite, pyrargyrite, pyrrhotite, ullmannite, stutzite, and other mineral phases of low-melting-point metals such as Ag, Cu, Pb, Sb, As, Bi. deposit, multiphase ﬁne aggregates consist of Bi, Pb, Sb, and As minerals. Other signs of metamorphism of the ore in the Pellapahk deposit are recrystallization of chalcopyrite with re-deposition along cleavage in kyanite, and recrystallization of molybdenite along cleavage in muscovite. Late metamorphism of the ore in the Oleninskoe deposit is indicated by the absence of chlorite and the scarcity of carbonate in altered amphibolite.


Introduction
Porphyry deposits are the major sources of Cu, Mo, and Re in the world [1]; they also provide significant amounts of Ag, Au, and some other metals. These deposits are the products of large hydrothermal systems that developed due to magmatic-hydrothermal phenomena in and around intermediate-to-felsic intrusions, emplaced at relatively high levels in the crust (1-6 km below the paleosurface) [1,2]. Metal-rich fluids exsolve from the shallow-crustal intrusive complexes and alter and mineralize the upper parts of the causative intrusions and the surrounding country rocks. These may be simple systems, consisting of a single intrusion with an associated alteration halo and a high-temperature ore, or more complicated systems of numerous intrusions with overlapping stages of alteration and ore deposition that formed over a wide range of temperatures [3].
Rock crystallization occurs due to pressure quenching, generating the diagnostic porphyritic texture of the mineralizing intrusions. Episodic brittle failure and fluid release from the crystallizing magmas produce a multistage vein stockwork that hosts the bulk of the ore [4]. Mineralization styles include stockwork veins, hydrothermal breccias, disseminations in the groundmass and, rarely, wall-rock replacements.
Epithermal fluids can potentially dissolve significant amounts of gold and silver from porphyry deposits, because the solubility of precious metals, such as aqueous bisulfide complexes, increases with decreasing temperatures when aqueous H 2 S contents remain high [5]. Late-stage processes commence the final stage of porphyry ore formation and the formation of peripheral Ag-Au ore deposits.

Geological Setting of the Kolmozero-VoronyaBelt
The Kolmozero-Voronya greenstone belt separates the Murmansk block from the Kola Province of the Fennoscandian Shield ( Figure 1A). The belt has an approximately 140 km strike length and a width of up to 10-12 km.
The history of the formation of the belt, as described in [20], includes the following events: formation of an island arc at 2.87-2.83 Ga; a break of 50 million years, and subsequent accretion of the island arc to the Murmansk continent; formation of an accretionary orogen (2.78-2.76 Ga); probable collapse of the orogen with formation of post-orogenic (or possibly anorogenic) granite intrusions in the central and southern parts of the belt (2.74-2.72 Ga). The total time of the belt development was about 150 Ma [20].
The structure of the belt is considered a monoclinal set of thrust sheets, with volcanics showing indications of oceanic and island arc magmatism [21,22]. The Archean supracrustal sequences of the belt comprise volcanics of tholeiite-komatiite and daciteandesite-basalt series (the Kolmozero series), and subordinate sedimentary rocks: pelites and sandstones (at the basement of the Kolmozero series and the Porosozero series) ( Figure 1B,C). The total thickness of the cross-section is 1800-2000 m.
Supracrustal sequences in the northwestern part of the Kolmozero-Voronya belt were intruded by: • Gabbro, pyroxenite, and peridotite co-magmatic to the Kolmozero series volcanics; • Quartz porphyry intrusions of the gabbrodiorite-diorite-granodiorite-granite series (the U-Pb age is 2.82-2.83 Ga, more details are given below); • Plagiomicrocline and tourmaline granites (the U-Pb age of zircon from the tourmaline granite is 2451 ± 60 Ma [23]); • Rare metal and tourmaline pegmatite veins (the U-Pb age of microlite from the Cs-Li pegmatite Vasin-Mylk deposit is 2454 ± 8 Ma [24]); • More than one generation of dolerite and picrite porphyry dykes of Proterozoic and possibly Paleozoic age. The rocks of the Kolmozero-Voronya belt were metamorphosed twice under conditions of the lower amphibolite facies: in the Neoarchean (2.7-2.8 Ga) and Paleoproterozoic (1.9-1.8 Ga) [25,26]. The PT-parameters of the early metamorphism were T~600 • C, P = 3-4 kbar. At the late stage of metamorphism, the temperature was a little lower (530 • C on the average), but pressure was higher (~5.5 kbar) [26]. In the northwestern part of the belt, the mineral associations of the late metamorphic stage only partly replaced the early stage associations (paramorphoses of kyanite after andalusite, decomposition of cordierite in the outer parts of the grains (Figure 2), re-crystallization of biotite and muscovite).

•
Rare metal and tourmaline pegmatite veins (the U-Pb age of microlite from the Cs-Li pegmatite Vasin-Mylk deposit is 2454 ± 8 Ma [24]); • More than one generation of dolerite and picrite porphyry dykes of Proterozoic and possibly Paleozoic age.
The rocks of the Kolmozero-Voronya belt were metamorphosed twice under conditions of the lower amphibolite facies: in the Neoarchean (2.7-2.8 Ga) and Paleoproterozoic (1.9-1.8 Ga) [25,26]. The PT-parameters of the early metamorphism were T~600°C, P=3-4 kbar. At the late stage of metamorphism, the temperature was a little lower (530°C on the average), but pressure was higher (~5.5 kbar) [26]. In the northwestern part of the belt, the mineral associations of the late metamorphic stage only partly replaced the early stage associations (paramorphoses of kyanite after andalusite, decomposition of cordierite in the outer parts of the grains (Figure 2), re-crystallization of biotite and muscovite).  [27]. (B) Schematic geological map of the northwestern part of the Kolmozero-Voronya greenstone belt, modified from reference [28]. (C) Schematic stratigraphic column of the belt.  [27]. (B) Schematic geological map of the northwestern part of the Kolmozero-Voronya greenstone belt, modified from reference [28]. (C) Schematic stratigraphic column of the belt. Three main fault systems are defined in the belt ( Figure 1B) [29][30][31]. Deep faults of a NW direction along the boundaries of the belt comprise the first system. The second sys tem unites strike-slip faults and shears of a NW up to latitudinal direction. These faults separate volcanic-sedimentary rock series or cut the stratigraphic boundaries at an acute angle of 5-15°. The faults of the second system can be traced for a few kilometers; the shears control zones of rock alteration [32]. The third fault system includes faults of a NE up to sub-meridional direction, which divide the belt into a number of blocks displaced for hundreds of meters. The faults of the third system are often marked by dolerite and picrite porphyry dykes ( Figure 1).

Mineral Deposits Associated with the Granite Porphyry Intrusions
More than 20 intrusions of the gabbrodiorite-diorite-granodiorite-granite porphyry series are located in the NW part of the belt. The rocks of mafic and intermediate compo sition form mainly small differentiated intrusions with the long axis less than 1000 m More acidic rocks (granodiorite and granite porphyry) can form bigger intrusions up to 8 km long and 1.5 km wide ( Figure 1). The biggest intrusion at Pellapahk hosts a Cu-Mo deposit. Granite porphyry also forms sills in metavolcanics and metasediments in the cen tral part and northern flank of the belt (Figure 1). The Oleninskoe gold-silver deposit is associated with granite porphyry sills in the Oleny Ridge amphibolite.

The Pellapahk Cu-Mo Porphyry Deposit
The Pellapahk granite porphyry intrusion is of elongated form, orientated according to the general strike of the belt: in the southeast, it splits into a number of thin granite porphyry sills ( Figure 1B). The granite porphyry intrudes mafic and intermediate volcan ics of the Kolmozero series and high-alumina primary sedimentary rocks of the Po rosozero series. Xenoliths of the high-alumina gneiss were noted in the southern part o the intrusion. In the north, the granite porphyry intrusion is cut by younger intrusions o tourmaline plagiomicrocline granite. The intrusions and the surrounding country metavolcanic-sedimentary rocks are crossed by numerous dykes of pyroxenite, dolerite picrite porphyry, and by pegmatite veins (Figures 3 and 4).
The age of zircon from the Pellapahk granite porphyry, defined by the U-Pb method is 2828±8 Ma [10]; i.e., the granite porphyry crystallized at the late stage of formation o Three main fault systems are defined in the belt ( Figure 1B) [29][30][31]. Deep faults of a NW direction along the boundaries of the belt comprise the first system. The second system unites strike-slip faults and shears of a NW up to latitudinal direction. These faults separate volcanic-sedimentary rock series or cut the stratigraphic boundaries at an acute angle of 5-15 • . The faults of the second system can be traced for a few kilometers; the shears control zones of rock alteration [32]. The third fault system includes faults of a NE up to sub-meridional direction, which divide the belt into a number of blocks displaced for hundreds of meters. The faults of the third system are often marked by dolerite and picrite porphyry dykes (Figure 1).

Mineral Deposits Associated with the Granite Porphyry Intrusions
More than 20 intrusions of the gabbrodiorite-diorite-granodiorite-granite porphyry series are located in the NW part of the belt. The rocks of mafic and intermediate composition form mainly small differentiated intrusions with the long axis less than 1000 m. More acidic rocks (granodiorite and granite porphyry) can form bigger intrusions up to 8 km long and 1.5 km wide ( Figure 1). The biggest intrusion at Pellapahk hosts a Cu-Mo deposit. Granite porphyry also forms sills in metavolcanics and metasediments in the central part and northern flank of the belt (Figure 1). The Oleninskoe gold-silver deposit is associated with granite porphyry sills in the Oleny Ridge amphibolite.

The Pellapahk Cu-Mo Porphyry Deposit
The Pellapahk granite porphyry intrusion is of elongated form, orientated according to the general strike of the belt: in the southeast, it splits into a number of thin granite porphyry sills ( Figure 1B). The granite porphyry intrudes mafic and intermediate volcanics of the Kolmozero series and high-alumina primary sedimentary rocks of the Porosozero series. Xenoliths of the high-alumina gneiss were noted in the southern part of the intrusion. In the north, the granite porphyry intrusion is cut by younger intrusions of tourmaline plagiomicrocline granite. The intrusions and the surrounding country metavolcanic-sedimentary rocks are crossed by numerous dykes of pyroxenite, dolerite, picrite porphyry, and by pegmatite veins (Figures 3 and 4). rock, with quartz and oligoclase phenocrysts up to 2 mm in size. The accessory miner are titanite, ilmenite, rutile, tourmaline, apatite, and scheelite.The sulfides are pyrite, p rhotite, chalcopyrite, rare sphalerite and pentlandite.  Granite porphyry of the Pellapahk intrusion belongs to the calc-alkaline series (Ta 1, Figure 5). Chemical composition of the rock corresponds to metaluminous I-type gr    Granite porphyry of the Pellapahk intrusion belongs to the calc-alkaline series (Table 1, Figure 5). Chemical composition of the rock corresponds to metaluminous I-type granite [33,34]. The age of zircon from the Pellapahk granite porphyry, defined by the U-Pb method, is 2828 ± 8 Ma [10]; i.e., the granite porphyry crystallized at the late stage of formation of the volcanic-sedimentary sequence, before the main metamorphic event (2770 ± 40 Ma) [25].
Granite porphyry of the Pellapahk intrusion belongs to the calc-alkaline series (Table 1, Figure 5). Chemical composition of the rock corresponds to metaluminous I-type granite [33,34].    REE spectra (Figure 6), drawn with data from Table 2 and Table 5, are similar to the REE spectrum of I-type granite, formed under conditions of subduction and island arc [34].   The zone of intense rock alteration has a total thickness of more than 700 m and lows the contact of the granite porphyry with high-alumina gneiss ( Figure 3). The gene zoning of the alteration is the following (from north to south): (0) slightly altered gran porphyry; (1) quartz-microcline rock; (2) muscovite schist; (3) muscovite-kyanite (or/a andalusite) schist; (4) altered (gedrite, cordierite) high-alumina schist, and (5) non-alte biotite-andalusite schist ( Figure 3). Rocks in zones (1)-(3) formed after granite porphy and in (4), after the high-alumina schist (5) of the Porosozero series.
Alteration processes caused the decomposition of plagioclase and microcline, the formation of high-alumina minerals andalusite and kyanite (Figure 2A), and the replacement of biotite by muscovite in the granite porphyry. The muscovite and muscovite-kyanite schists have high SiO 2 and K 2 O content, compared to the composition of the primary rock, but are poorer in MgO, CaO, and Na 2 O (Table 1).
All altered rocks contain sulfide mineralization. Cordierite-gedrite rock after highalumina schists contains disseminated pyrrhotite in intergrowths with chalcopyrite, pyrite, rarely sphalerite, arsenopyrite, cobaltite-gersdorfite, and pentlandite; the main oxide minerals are ilmenite and magnetite. Muscovite, muscovite-kyanite schists and quartzmicrocline rocks after granite porphyry contain pyrite mineralization up to 3 vol.% with minor chalcopyrite, pyrrhotite, and sphalerite. Rutile is the main oxide mineral in these rocks and replaces ilmenite.
The Cu-Mo mineralization comprises a stockwork of quartz and quartz-calciteepidote veinlets in the zone of alteration of granite porphyry. The stockwork was traced for 1500-1600 m along the strike; its width varies from 350 to 600 m (Figures 3 and 4). The mineralization was drilled down to a depth of 360 m in the NW part of the stockwork, and probably goes deeper [9].
Geochemical association of the metals in the mineralized altered rocks in the stockwork includes Mo, Cu, Ag, Bi, W, Se, and Sb ( Figure 7, Table 2).The content of Cu, Mo, Bi, and W in the altered rocks is 10 or more times higher than in the primary granite porphyry.
inerals 2021, 11, x FOR PEER REVIEW 9 of 2 schists have high SiO2 and K2O content, compared to the composition of the primary rock but are poorer in MgO, CaO, and Na2O (Table 1). All altered rocks contain sulfide mineralization. Cordierite-gedrite rock after high alumina schists contains disseminated pyrrhotite in intergrowths with chalcopyrite, py rite, rarely sphalerite, arsenopyrite, cobaltite-gersdorfite, and pentlandite; the main oxid minerals are ilmenite and magnetite. Muscovite, muscovite-kyanite schists and quartzmicrocline rocks after granite porphyry contain pyrite mineralization up to 3 vol.% with minor chalcopyrite, pyrrhotite, and sphalerite. Rutile is the main oxide mineral in thes rocks and replaces ilmenite.
The Cu-Mo mineralization comprises a stockwork of quartz and quartz-calcite-epi dote veinlets in the zone of alteration of granite porphyry. The stockwork was traced fo 1500-1600 m along the strike; its width varies from 350 to 600 m (Figures 3 and 4). Th mineralization was drilled down to a depth of 360 m in the NW part of the stockwork and probably goes deeper [9].
Geochemical association of the metals in the mineralized altered rocks in the stock work includes Mo, Cu, Ag, Bi, W, Se, and Sb ( Figure 7, Table 2).The content of Cu, Mo, Bi and W in the altered rocks is 10 or more times higher than in the primary granit porphyry. In the stockwork, the main ore minerals are pyrite, chalcopyrite, and rutile. Sporadi ore minerals are molybdenite, sphalerite, galena, tetrahedrite, pyrrhotite, cubanite, ilmen ite, scheelite. The rare minerals are native bismuth, gudmundite, tennantite, löllingite lillianite, and pentlandite [9,36]. The oxidation zone is not well developed in the deposit but bornite and covellite were rarely noted to replace chalcopyrite in the outer parts of th In the stockwork, the main ore minerals are pyrite, chalcopyrite, and rutile. Sporadic ore minerals are molybdenite, sphalerite, galena, tetrahedrite, pyrrhotite, cubanite, ilmenite, scheelite. The rare minerals are native bismuth, gudmundite, tennantite, löllingite, lillianite, and pentlandite [9,36]. The oxidation zone is not well developed in the deposit, but bornite and covellite were rarely noted to replace chalcopyrite in the outer parts of the grains.
Pyrite is the most abundant sulfide mineral both within the mineralized Cu-Mo stockwork, and outside of it. Idiomorphic grains of pyrite are regularly disseminated in the rock groundmass, but the content of pyrite and the size of its grains (cubic crystals may reach 3 mm) increase in the quartz-carbonate-epidote veinlets.
The main productive minerals are chalcopyrite and molybdenite. Chalcopyrite forms irregular grains up to 0.5 mm in the rock mass; it develops in fractures in pyrite grains and along cleavage in kyanite, but much more rarely in andalusite ( Figure 8B).
inerals 2021, 11, x FOR PEER REVIEW 10 of 27 the rock groundmass, but the content of pyrite and the size of its grains (cubic crystals may reach 3 mm) increase in the quartz-carbonate-epidote veinlets.
The main productive minerals are chalcopyrite and molybdenite. Chalcopyrite forms irregular grains up to 0.5 mm in the rock mass; it develops in fractures in pyrite grains and along cleavage in kyanite, but much more rarely in andalusite ( Figure 8B). Molybdenite forms fine flakes (mainly 0.05-0.1 mm, rarely up to 0.5 mm) along cleavage in muscovite or along quartz grain boundaries. Chains of molybdenite flakes cross pyrite crystals, chalcopyrite, and sphalerite grains [36].
Galena and sphalerite are rare in the rock groundmass, but their content significantly increases in the quartz-epidote-calcite veinlets. The rare minerals of As, Sb, and Bi (fahlore, gudmundite, lillianite) are associated mainly with galena.

The Oleninskoe Au-Ag Deposit
The Oleninskoe Au-Ag deposit is located at the northwestern thinning of the Oleny Ridge amphibolite strata ( Figure 1) in a shear zone of northwest strike [37][38][39]. The amphibolite and high-alumina metasedimentary schist host numerous granite porphyry sills with a thickness of 0.1-6.0 m (Figures 9 and 10).
The Oleny Ridge amphibolite borders biotite-andalusite gneiss of the Porosozero series in the south, and a stratum of alternation of amphibolite, biotite-cordierite, and biotite-andalusite gneisses in the north. Both southern and northern contacts are tectonic. Molybdenite forms fine flakes (mainly 0.05-0.1 mm, rarely up to 0.5 mm) along cleavage in muscovite or along quartz grain boundaries. Chains of molybdenite flakes cross pyrite crystals, chalcopyrite, and sphalerite grains [36].
Galena and sphalerite are rare in the rock groundmass, but their content significantly increases in the quartz-epidote-calcite veinlets. The rare minerals of As, Sb, and Bi (fahlore, gudmundite, lillianite) are associated mainly with galena.

The Oleninskoe Au-Ag Deposit
The Oleninskoe Au-Ag deposit is located at the northwestern thinning of the Oleny Ridge amphibolite strata ( Figure 1) in a shear zone of northwest strike [37][38][39]. The amphibolite and high-alumina metasedimentary schist host numerous granite porphyry sills with a thickness of 0.1-6.0 m (Figures 9 and 10).
The Oleny Ridge amphibolite borders biotite-andalusite gneiss of the Porosozero series in the south, and a stratum of alternation of amphibolite, biotite-cordierite, and biotite-andalusite gneisses in the north. Both southern and northern contacts are tectonic.
The latest rocks in the deposit are granite-pegmatite veins, which cut all rocks, including those that are mineralized. The thickest veins can be traced in a NW direction for hundreds of meters ( Figure 9).
The rock in the sills is very fine-grained; the size of plagioclase, quartz grains, and biotite and muscovite flakes in the groundmass is less than 0.05 mm; quartz phenocrysts are <0.5 mm. Phenocrysts of plagioclase were not found. Minerals 2021, 11, x FOR PEER REVIEW 11 of 27 Figure 9. Schematic geological map of the Oleninskoe deposit, modified from references [28,37]. The latest rocks in the deposit are granite-pegmatite veins, which cut all rocks, including those that are mineralized. The thickest veins can be traced in a NW direction for hundreds of meters ( Figure 9).  The latest rocks in the deposit are granite-pegmatite veins, which cut all rocks, in cluding those that are mineralized. The thickest veins can be traced in a NW direction fo hundreds of meters (Figure 9). The age of the granite porphyry sill in the Oleninskoe deposit was defined by a U-Pb geochronological study of zircons. Zircons (sample AK-706) are long-prismatic, idiomorphic and sub-idiomorphic, from 50 to 100 µm in size, of brownish color, and with fine growth zoning suggesting a magmatic origin. The zircon fractions yield a discordia with an upper intercept at 2817 ± 9 Ma (MSWD = 0.54, Table 3, Figure 11) that represents the crystallization age of the granite porphyry sill. The rock in the sills is very fine-grained; the size of plagioclase, quartz grains, and biotite and muscovite flakes in the groundmass is less than 0.05 mm; quartz phenocrysts are <0.5 mm. Phenocrysts of plagioclase were not found.
The age of the granite porphyry sill in the Oleninskoe deposit was defined by a U-Pb geochronological study of zircons. Zircons (sample AK-706) are long-prismatic, idiomorphic and sub-idiomorphic, from 50 to 100 µm in size, of brownish color, and with fine growth zoning suggesting a magmatic origin. The zircon fractions yield a discordia with an upper intercept at 2817 ± 9 Ma (MSWD = 0.54, Table 3, Figure 11) that represents the crystallization age of the granite porphyry sill.  *Isotope ratios are corrected for the blank and common lead. Rho-the correlation coefficient of the 207Pb/235U-206Pb/238U ratios. The chemical composition of the non-altered granite porphyry sills is generally similar to the granite porphyry of the Pellapahk intrusion (Table 4, Figure 5), and the granite porphyry from the Oleninskoe deposit belongs to the series of calc-alkaline I-type granites.
The spectrum of REE in sample AK-706 (Oleninskoe) differs in the lower content of REE ( Figure 6), but the form of the curve is very similar to the one in the REE spectra of the Pellapahk granite-porphyry.
The mineralized zone consists of a number of ore lenses within an area of 900 × 50 m (Figure 9). The zone is orientated according to the general strike of the rocks. The ore lenses are up to 3.5 m thick (1.5 m on average) with the length up to 50 m; they form an echelon-like series of lenticular bodies, cutting general schistosity in the host rocks at an acute angle of 10-15 • (Figure 9). Three lenses are outcropped on the surface, but drilling shows their number is more than three. The mineralization is traced by drilling down to a depth of 200 m, and goes deeper. The thickness of the ore intervals and Au grades increase with the depth [31]. The weighted average gold content is 7.6 g/t; inferred resources are estimated at 10 t Au down to a depth of 100 m [13].
The amphibolite is intensely altered. Diopsidization (diopside-zoisite, diopsideepidote, diopside-epidote-carbonate, diopside-epidote-garnet associations) and biotitization cover the whole mineralized zone, and quartz-arsenopyrite-tourmaline metasomatic rocks are spatially confined to the ore lenses. Granite porphyry is turned to quartzalbite-muscovite, quartz-arsenopyrite-tourmaline, and quartz metasomatic rocks: the latter formed the most recently and very locally (ellipsoidal bodies,~30 × 10 cm) ( Figure 12). All kinds of metasomatic rocks and the zoning of alteration can be seen in lens 2, outcropped with a trench along the strike (Figure 12).
The mineralized zone consists of a number of ore lenses within an area of 90 ( Figure 9). The zone is orientated according to the general strike of the rocks. lenses are up to 3.5 m thick (1.5 m on average) with the length up to 50 m; they echelon-like series of lenticular bodies, cutting general schistosity in the host ro acute angle of 10-15° (Figure 9). Three lenses are outcropped on the surface, bu shows their number is more than three. The mineralization is traced by drilling a depth of 200 m, and goes deeper. The thickness of the ore intervals and Au g crease with the depth [31]. The weighted average gold content is 7.6 g/t; inferred r are estimated at 10 t Au down to a depth of 100 m [13].
The amphibolite is intensely altered. Diopsidization (diopside-zoisite, diop dote, diopside-epidote-carbonate, diopside-epidote-garnet associations) and bio cover the whole mineralized zone, and quartz-arsenopyrite-tourmaline met rocks are spatially confined to the ore lenses. Granite porphyry is turned to quar muscovite, quartz-arsenopyrite-tourmaline, and quartz metasomatic rocks: t formed the most recently and very locally (ellipsoidal bodies, ~30 × 10 cm) (Figur kinds of metasomatic rocks and the zoning of alteration can be seen in lens 2, ou with a trench along the strike (Figure 12).   * High loss on ignition is connected with carbonaceous matter in the rocks. ** The deficit appears due to high content of arsenopyrite mineralization, the samples were assayed for As with ICP-MS, see Table 5. VP-10, AK-706-granite porphyry; AK-710, AK-712, AK-900K, AK-900C, AK-900T-quartz-albite-muscovite metasomatite; AK-707-quartz-arsenopyrite-tourmaline metasomatite; AK-711-quartz metasomatite. n.a. = not assayed. The geochemical association of the metals in the mineralized altered rocks includes Au-As-Ag-Sb-Cu-Pb-W ( Table 5). The concentration coefficient of As, Ag, and Sb in the mineralized rocks is more than 100 if compared to the unaltered granite porphyry ( Figure 13). The highest content of As, Ag, Pb, and Sb was found in quartz-arsenopyritetourmaline rock ( Figure 13, Table 5). Quartz metasomatic rock is not as rich in Pb, Ag, and Sb as quartz-albite-muscovite and quartz-arsenopyrite-tourmaline metasomatites (Table 5), but it is rich in Au ( Table 4). The Au/Ag ratio in the mineralized rocks is <0.2 [29], except the quartz metasomatite. The geochemical characteristics of the rocks correspond well to the composition of mineralization (Table 6). Arsenopyrite, pyrrhotite, and ilmenite are the most abundant ore minerals, which are present in all altered rocks [37,38]. Sporadic ore minerals in biotitizated and diopsidizated amphibolite are chalcopyrite and sphalerite; pentlandite and electrum are rare. Quartz-arsenopyrite-tourmaline and quartz metasomatic rocks in the exocontact zones of granite porphyry contain rich Pb-Ag-Sb-Au mineralization totaling more than 50 mineral phases: 20 of them are the minerals of the precious metals Ag and Au (Tables 6 and 7). Ilmenite in quartz-arsenopyrite-tourmaline and quartz metasomatic rocks is partly (in the outer parts of the grains) or fully replaced by rutile [37,38].
Four types of minerals of the Au-Ag series were recognized in the deposit: 1-electrum 25-32 mas.% Au in association with arsenopyrite, löllingite, and pyrrhotite; 2-electrum 33-47 mas.% Au in intergrowths with galena, dyscrasite, and sulfosalts; 3-gold (78-95 mas.% Au) in quartz; 4-native silver (<7 mas.% Au) in the crust of weathering of the ore [37]. Types 1 and 3 are widespread in the deposit, and 2 and 4 relate only to the quartz-arsenopyritetourmaline rocks rich in Pb-Ag-Sb. Mustard gold was found in the oxidized ore with high Ag-Sb electrum. These mineral aggregates consist of electrum, Fe and Sb oxides, Ag chlorides, and bromides [38].
In the Oleninskoe ores we see fine intergrowths of Ag, Cd, Pb, As, Sb and Te minerals. These multiphase aggregates are up to 0.2 mm in size, and the size of individual micrograins in the intergrowths is <5 µm. The polymineral intergrowths adjoin bigger grains of argentotetrahedrite, pyrrhotite, and galena host sulfides (Figures 14 and 15). Four types of minerals of the Au-Ag series were recognized in the deposit: 1-electrum 25-32 mas.% Au in association with arsenopyrite, löllingite, and pyrrhotite; 2-electrum 33-47 mas.% Au in intergrowths with galena, dyscrasite, and sulfosalts; 3-gold (78-95 mas.% Au) in quartz; 4-native silver (<7 mas.% Au) in the crust of weathering of the ore [37]. Types 1 and 3 are widespread in the deposit, and 2 and 4 relate only to the quartzarsenopyrite-tourmaline rocks rich in Pb-Ag-Sb. Mustard gold was found in the oxidized ore with high Ag-Sb electrum. These mineral aggregates consist of electrum, Fe and Sb oxides, Ag chlorides, and bromides [38].
In the Oleninskoe ores we see fine intergrowths of Ag, Cd, Pb, As, Sb and Te minerals. These multiphase aggregates are up to 0.2 mm in size, and the size of individual micrograins in the intergrowths is <5 µm. The polymineral intergrowths adjoin bigger grains of argentotetrahedrite, pyrrhotite, and galena host sulfides (Figures 14 and 15).  We interpret the origin of these multiphase grains as a product of sulfide melt crystallization [40]. It is known that in the case of temperature increases, partial melting of sulfides begins when the whole rock is solid, before the melting of silicate phases ("sulfide anatexis"). A low melting temperature is characteristic of Zn, Ga, Ag, Cd, In, Hg, Tl, Pb, As, Sb, Bi, Se, and Te sulfides, and the first melt appears in the systems Ag-Pb-S, Pb-Sb-S, Ag-Sb-S, PbS-CuS-Sb2S3 and other triple and pseudotriple systems in the temperature interval 300-500°C [39]. In the case of metamorphism under conditions of amphibolite- Four types of minerals of the Au-Ag series were recognized in the deposit: 1-electrum 25-32 mas.% Au in association with arsenopyrite, löllingite, and pyrrhotite; 2-electrum 33-47 mas.% Au in intergrowths with galena, dyscrasite, and sulfosalts; 3-gold (78-95 mas.% Au) in quartz; 4-native silver (<7 mas.% Au) in the crust of weathering of the ore [37]. Types 1 and 3 are widespread in the deposit, and 2 and 4 relate only to the quartzarsenopyrite-tourmaline rocks rich in Pb-Ag-Sb. Mustard gold was found in the oxidized ore with high Ag-Sb electrum. These mineral aggregates consist of electrum, Fe and Sb oxides, Ag chlorides, and bromides [38].
We interpret the origin of these multiphase grains as a product of sulfide melt crystallization [40]. It is known that in the case of temperature increases, partial melting of sulfides begins when the whole rock is solid, before the melting of silicate phases ("sulfide anatexis"). A low melting temperature is characteristic of Zn, Ga, Ag, Cd, In, Hg, Tl, Pb, As, Sb, Bi, Se, and Te sulfides, and the first melt appears in the systems Ag-Pb-S, Pb-Sb-S, Ag-Sb-S, PbS-CuS-Sb 2 S 3 and other triple and pseudotriple systems in the temperature interval 300-500 • C [39]. In the case of metamorphism under conditions of amphibolitegranulite facies (T = 500-800 • C), partial melting of low-melting-point sulfides begins in the rocks, and later, when the temperature decreases, the sulfide melt crystallizes and forms multiphase intergrowths of sulfides, sulfosalts, tellurides, and selenides. These structures of sulfide melt crystallization were described in the ores of many polymetal and gold deposits in metamorphic complexes, for example, Broken Hill (Australia) [39], Montauban [41] and Hemlo (Canada) [42,43], and Bleikvassli (Norway) [44].
The rocks of the Oleninskoe Au-Ag deposit were metamorphosed under conditions of lower amphibolite facies at 500-600 • C [26]. This temperature was high enough to start melting Pb, Ag, Cu, and Sb sulfides. The signs indicating the origin of the multiphase aggregates as a product of crystallization of sulfide melt are the following: multiphase composition of the grains, where minerals of the low-melting-point metals Pb, Ag, Sb, and Te are necessarily present; absence of signs of replacement of the earlier formed minerals by the later ones; drop-like or roundish irregular forms of the aggregates, which adjoin a larger grain of host sulfide; multiphase sulfide filling of fractures in quartz rock mass. These points fully correspond to the signs of sulfide melt crystallization in sulfide ores as they are given in [39].

Fluid Composition
Total salinity of fluid in the inclusions in quartz from the studied deposits varies from medium to high (Table 8) [46]. Quartz from the Pellapahk mineralized rocks shows high salinity of fluid inclusions of 341 g/kg H 2 O (Table 8). In the Oleninskoe deposit, total salinity varies, depending on the metasomatic rocks: in quartz-albite-muscovite and quartz metasomatites it is moderate at~130 g/kg H 2 O, and in quartz-arsenopyrite-tourmaline rock it is even more than 400 g/kg H 2 O. The latter corresponds well to the data obtained earlier for the same rocks [47]. Ca and Na (Ca > Na) are the two main cations in the fluids in quartz from the Olenickoe gold-silver deposit; potassium and magnesium content is much lower, especially in quartz metasomatic rock. In the Pellapahk Cu-Mo deposit (Table 8), alkaline metals (sodium and potassium) are predominant: together they make up 85% of mole shares of cations in the inclusions in quartz.
The anion composition of the fluids is also different. In the Oleninskoe deposit, it is characterized by a prevalence of Cl − over SO 4 2− (in mole shares). The mole share of Cl reaches 95% in quartz-albite-muscovite rock. HCO 3 − content is low. In the Pellapahk deposit, the mole share of Cl − is relatively low: 43% of the anions, close to the one of SO 4 2− (41%); HCO 3 − accounts for 15%. The Na/Cl ratio in the inclusions varies from 0.33 to 0.55 in quartz from the Oleninskoe deposit. This indicates a significant role of Ca and Mg chlorides in the fluids. The (Na + K)/Cl ratio in quartz from the Pellapahk deposit is 1.97. This means that sodium and potassium sulfates and carbonates are present in the fluid inclusions.

Discussion
The fact that the intrusions, which host the Pellapahk and the Oleninskoe deposits, refer to one and the same intrusive complex, can be proved by the age of the rocks and by their geochemical characteristics.
Chemical composition of the unaltered rocks from the deposits does not differ significantly: all of them belong to the calc-alkaline series of metaluminous granites with moderate alkalinity, and the fields of rock composition overlap in the classification diagrams ( Figure 5).
The REE spectra of granite porphyry from the studied deposits are of the same form, characterized by enrichment in light REE and the absence of the Eu anomaly ( Figure 6). This form of spectrum is typical for I-granites formed in zones of subduction and island arcs [34], and differs significantly from the REE spectra of tourmaline plagiomicrocline anorogenic granites from the intrusions located north of the Pellapahk (Figure 1) [48].
In the Phanerozoic belts, the porphyry intrusions form at a depth of 1-6 km and produce magmatic porphyry-epithermal ore systems, which occur in near-surface conditions. These systems show a specific vertical and lateral zoning in rock alteration and in the distribution of the mineralization, from deep to shallow horizons (and from the center to the edge) (Mo, Cu)-(Cu)-(Pb, Zn)-(Au, Ag, As, Sb); the zoning can be full or fragmentary [2,40,49,50]. Similar systems formed in the Precambrian, or even in the Archean, but the probability they were preserved and have not been eroded since that time is low.
If we consider the Pellapahk and the Oleninskoe deposits as parts of an Archean porphyry-epithermal system, then the Pellapahk Cu-Mo deposit is located at its centre, and the Oleninskoe (Au, Ag, As, Sb) is a distal epithermal deposit in relation to the Pellapahk deposit.
Porphyry Cu deposits in the Phanerozoic are known to display a consistent, broadscale alteration zoning that comprises, centrally from the bottom upward, zones of sodiccalcic, potassic, chlorite-sericite, sericitic, and advanced argillic (quartz-pyrophyllite, quartz-alunite, quartz-kaolinite) alteration [2,4]. In the Pellapahk deposit, we see zones of quartz-microcline (potassic alteration), quartz-muscovite (sericitic zone), and quartzandalusite-muscovite (advanced argillic zone) metasomatic rocks (Figures 2 and 3). This zoning generally corresponds that described for the Phanerozoic deposits, but the altered rocks of the Pellapahk deposit were later metamorphosed: sericite was replaced by muscovite, and alumina silicates of the argillic zone (pyrophyllite, alunite, kaolinite) by andalusite during the Neoarchean metamorphism of lower amphibolite facies. Subsequently, during the Paleoproterozoic metamorphism, kyanite formed paramorphoses after andalusite. The origin of the quartz-muscovite-andalusite schists of the Pellapahk deposit as altered and later metamorphosed volcanic rocks was first published by Glagolev [51], who studied petrography of the Kolmozero-Voronya belt in early 1970s when the Cu-Mo mineralization was unknown.
Generally, the Pellapahk granite porphyry was Si-K altered, and Na, Mg, Ca, and Fe were partly removed from the rock (Table 1). Sodium was taken away from the porphyry system, but Mg, Ca, and Fe were re-deposited in the zone of alteration of high-alumina gneiss in the mineral forms of gedrite and cordierite [36].The zone of gedrite-cordierite alteration is 100-150 m thick and follows the contact with the altered granite porphyry (part of it is shown in Figure 3).
The metamorphism of the mineralized rocks in the Pellapahk deposit can be confirmed by findings of multiphase grains with structures of sulfide melt crystallization, which in the Pellapahk deposit are not as frequent as in the Oleninskoe. Multiphase fine aggregates from the Pellapahk deposit consist of minerals of Pb, Ag, Bi, and Zn galena, argentotetrahedrite, lillianite [45], or minerals of Cu, Sb, and As tetrahedrite and tennantite, löllingite, and chalcopyrite ( Figure 16). It is interesting to note formation of löllingite and the absence of arsenopyrite in the rocks with intense pyrite dissemination.
Minerals 2021, 11, x FOR PEER REVIEW 2 alteration is 100-150 m thick and follows the contact with the altered granite porp (part of it is shown in Figure 3). The metamorphism of the mineralized rocks in the Pellapahk deposit can be firmed by findings of multiphase grains with structures of sulfide melt crystalliz which in the Pellapahk deposit are not as frequent as in the Oleninskoe. Multiphas aggregates from the Pellapahk deposit consist of minerals of Pb, Ag, Bi, and Zn g argentotetrahedrite, lillianite [45], or minerals of Cu, Sb, and As tetrahedrite and te tite, löllingite, and chalcopyrite ( Figure 16). It is interesting to note formation of löll and the absence of arsenopyrite in the rocks with intense pyrite dissemination. Other signs of metamorphism of the ore in the Pellapahk deposit are recrystalliz of chalcopyrite with deposition along cleavage in kyanite, which replaces andalusite ures 2 and 8), and recrystallization of molybdenite along cleavage in muscovite.
The fact that large amounts of pyrite are preserved within the Pellapahk depo dicates that high f(S2) conditions must have prevailed during metamorphism.
The following distinctive features show metamorphism of the mineralized ro the Oleninskoe deposit. First, there are numerous findings of multiphase sulfide g with structures of sulfide melt crystallization [45], described in Section 4.1.2. Then metamorphism is indicated by the mineral composition of the altered rocks, where rite is absent, calcite is very rare, and actinolite-hornblende and diopside are abun Chlorite and carbonates are typical minerals (together with quartz) in altered amphib but during amphibolite metamorphism, when H2O and CO2 are removed from the Other signs of metamorphism of the ore in the Pellapahk deposit are recrystallization of chalcopyrite with deposition along cleavage in kyanite, which replaces andalusite (Figures 2 and 8), and recrystallization of molybdenite along cleavage in muscovite.
The fact that large amounts of pyrite are preserved within the Pellapahk deposit indicates that high f (S 2 ) conditions must have prevailed during metamorphism.
The following distinctive features show metamorphism of the mineralized rocks in the Oleninskoe deposit. First, there are numerous findings of multiphase sulfide grains with structures of sulfide melt crystallization [45], described in Section 4.1.2. Then, late metamorphism is indicated by the mineral composition of the altered rocks, where chlorite is absent, calcite is very rare, and actinolite-hornblende and diopside are abundant. Chlorite and carbonates are typical minerals (together with quartz) in altered amphibolite, but during amphibolite metamorphism, when H 2 O and CO 2 are removed from the rocks, chlorite is replaced by amphiboles [52,53], and the content of carbonates reduces.
Then, pyrite in the Oleninskoe deposit is sporadic, and pyrrhotite is the most abundant sulfide mineral. This can be a result of metamorphism: pyrrhotite replaces pyrite under the conditions of amphibolite metamorphism [53,54].
The composition of the fluid inclusions in quartz varies depending on the character of rock alteration: in the Pellapahk deposit, the fluids are alkaline (Na, K) and SO 4 2− rich, and in the Oleninskoe deposit, they are Ca-Mg-Cl dominant.
Mineralogical and geochemical features of the Oleninskoe deposit indicate its genesis with high oxidizing fluids, which could be generated by a magmatic source [55]. The spatial and genetic relationship of mineralization with the sills of granite porphyry, the geochemical association of metals, an Au/Ag ratio of <0.2, and the multiplicity of silver mineralization with different Ag, Cu and Pb sulfosalts are all characteristics that enable the classification of the Oleninskoe deposit as a sub-epithermal deposit located in the vicinity of a big granite porphyry intrusion; the Pellapahk intrusion being the most probable. The geological-structural characteristics of the Oleninskoe deposit-its position in a shear zone, the morphology and size of ore bodies, the scale of the deposit, the intensity and zoning of host rocks alteration-do not oppose this model.
If the Oleninskoe and Pellapahk deposits are the parts of the Pellapahk-Oleninskoe porphyry-epithermal system, then the age of primary mineralization is close to the time of the formation of granite porphyry at 2.83-2.81 Ga. The deposits were later lower amphibolite metamorphosed together with the hosting volcanic-sedimentary rocks in the Neoarchean (T = 550-600 • C, P = 3-4 kBar) and in the Paleoproterozoic (T~530 • C, P~5.5 kBar) [26], and the mineralized rocks demonstrate signs of metamorphism of the ore.
Mineral deposits, associated with granite porphyry intrusions and dykes, are well known in the Fennoscandian and other Precambrian shields around the world. In the Fennoscandian Shield, we can mention the Cu porphyry (with gold) deposit of Aitik in Norbotten, Sweden [56], the molybdenum porphyry deposit ofLobash and neighboring gold deposit Lobash-1 in the Central Karelia [57,58], the minor gold deposits of Taloveis and Falaley in the Kostamuksha greenstone belt, Western Karelia [59], Mo-W with gold occurrences of Yalonvaara and Hatunoya in Southern Karelia [60], etc. The deposits and occurrences in Karelia formed not at the time of crystallization of the porphyry intrusions, but at the stage of the Paleoproterozoic (or rarer, Neoarchean) regional metamorphism [61]. The geological setting, morphology and size of the ore bodies, geochemical characteristics, and mineralogy show these deposits belonging to the orogenic genetic class [62]. In the Kolmozero-Voronya belt, the Nyalm-1 deposit is considered an orogenic gold deposit, associated with a granodiorite porphyry intrusion [63,64].
Some deposits in the Fennoscandian Shield are classified as metamorphosed epithermal: those are the gold deposit of Kutemajarvi in the Tampere belt in South Finland, and the silver (with base metals) deposit of Taivaljarvi in the Tipasjarvi belt in Central Finland.
The Kutemajarvi gold deposit comprises eight tube-like ore bodies, located 500 m south from the Pukala monzogranite-porhyry tonalite intrusion in the intermediate metavolcanics. The intrusion and the deposit are of the Paleoproterozoic age of 1.90-1.88 Ga [65].
The Taivaljarvi is an Archean stratiform Ag-Zn-Pb deposit, consisting of four mineralized horizons formed by fluids of magmatic origin, which migrated along shear zones in felsic pyroclastic rocks (rhyolite tuffs) [66]. The average gold content in the deposit is 0.29 g/t. The age of the rocks hosting the mineralization is 2.83-2.75 Ga [66].
Thus, the Pellapahk-Oleninskoe porphyry-epithermal system is the oldest system with complex Cu-Mo and Au-Ag mineralization in the Fennoscandian Shield. Distinctive geochemical and mineralogical features distinguish the Oleninskoe deposit from all other gold deposits in Fennoscandia.

Conclusions
Gabbro-diorite-granodiorite-granite porphyry intrusions formed at the late stages of the formation of the volcanic-sedimentary sequences of the Kolmozero-Voronya greenstone belt. The age of the intrusions is 2.83-2.82 Ga. Cu-Mo, Au-Ag, and Au deposits are associated with the quartz porphyry series.
The Pellapahk Cu-Mo and the Oleninskoe Au-Ag deposits are considered two parts of the Pellapahk-Oleninskoe porphyry-epithermal system: the Cu-Mo deposit in a big granite porphyry intrusion makes its central part, and the gold-silver deposit is located in the flank where it is associated with granite porphyry sills.
The following distinctive features indicate the sub-epithermal origin of the Au-Ag mineralization in the Oleninskoe deposit: the spatial and genetic relationships with the sills of granite porphyry, the geochemical association of ore elements, an Au/Ag ratio of <0.2, and the multiplicity of silver mineralization with different Ag, Cu, Pb, and Sb sulfosalts. The geological-structural characteristics of the Oleninskoe deposit (i.e., its location in a shear zone, the morphology and size of ore bodies, the scale of the deposit, the intensity and zoning of host rocks alteration) do not oppose this model.
The Pellapahk-Oleninskoe porphyry-epithermal ore system is probably the oldest one in the Fennoscandian Shield. Mineralized rocks of the Pellapahk and Oleninskoe deposits were later lower amphibolite metamorphosed. An important sign of ore metamorphism is the formation of structures of crystallization of sulfide melt of the low-melting-point metals.
In the Oleninskoe ores we see fine intergrowths of Ag, Cd, Pb, As, Sb, Te minerals galena, argentotetrahedrite, pyrargyrite, pyrrhotite, ullmannite, stutzite, etc. In the Pellapahk Cu-Mo deposit, multiphase fine aggregates consist of Bi, Pb, Sb, and As minerals. Other signs of metamorphism of the ore in the Pellapahk deposit are recrystallization of chalcopyrite with re-deposition along cleavage in kyanite, and recrystallization of molybdenite along cleavage in muscovite. Late metamorphism of the ore in the Oleninskoe deposit is indicated by the absence of chlorite and the scarcity of carbonate in altered amphibolite.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.