Fertility Indicators for Porphyry-Cu-Au+Pd ± Pt Deposits: Evidence from Skouries, Chalkidiki Peninsula, Greece, and Comparison with Worldwide Mineralizations

: The research interest for many authors has been focused on the origin, recovery, and exploration of critical metals, including platinum-group elements (PGEs), with the aim of ﬁnding new potential sources. Many giant porphyry Cu deposits are well known around the Paciﬁc Rim, in the Balkan–Carpathian system, Himalayas, China, and Malaysia. However, only certain porphyry Cu-Au deposits are characterized by the presence of signiﬁcant Pd and Pt contents (up to 20 ppm). This contribution provides new analytical data on porphyry-Cu-Au ± Pd ± Pt deposits from the Chalkidiki Peninsula and an overview of the existing geochemical characteristics of selected porphyry-Cu deposits worldwide in order to deﬁne signiﬁcant differences between PGE-fertile and PGE-poor porphyry-Cu intrusions. The larger Mg, Cr, Ni, Co, and Re contents and smaller LILE elements (Ba and Sr) in fertile porphyry-Cu-Au-(PGE) reﬂect the larger contribution from the mantle to the parent magmas. In contrast, the smaller Mg, Cr, Ni, Co, and Re contents and larger Ba and Sr in PGE-poor porphyry-Cu-Mo deposits from the Chalkidiki Peninsula (Vathi, Pontokerasia, and Gerakario) and Russia–Mongolia suggest the presence of parent magmas with a more crustal contribution. Although there is an overlap in the plots of those elements, probably due to the evolution of the ore-forming system, consideration of the maximum contents of Mg, Cr, Ni, and Co is proposed. Magnetite which separated from the mineralized Skouries porphyry of Greece showed small negative Eu anomalies (Eu/Eu* ≥ 0.55), reﬂecting a relatively high oxidation state during the cooling of the ore-forming system. The relatively high, up to 6 ppm (Pd+Pt), and low Cr content towards the transition from the porphyry to epithermal environment, coupled with the occurrence of Pd, Te, and Se minerals (merenskyite, clausthalite), and tetrahedrite–tennantite in fertile porphyry Cu deposits (Elatsite deposit, Bulgaria), reﬂect a highly fractionated ore-forming system. Thus, in addition to the crustal and mantle recycling, metasomatism, high oxidation state, and abundant magmatic water, other factors required for the origin of fertile porphyry-Cu deposits are the critical degree of mantle melting to release Pt and Pd in the ore-forming ﬂuids and the degree of fractionation, as reﬂected in the mineral chemistry and geochemical data.


Introduction
In recent decades, the exploitation of primary raw materials has rapidly increased worldwide due to the continuous growth of the global population and technological innovations [1,2].The research interest for many authors has been focused on the origin and exploration of new sources for critical and strategic raw materials, including platinumgroup elements (Os, Ir, Ru, Rh, Pt, Pd) or PGE.Among the PGE, Pt and Pd are the most economically relevant due to their use in electronics, jewelry, medical applications, and in the catalytic converter of vehicles to reduce the pollutants of exhaust gases [3][4][5][6].With few exceptions that consist of placer deposits, economic PGE deposits occur in mineralized The progressively collisional subduction of Tethys has led to mineralized ophiolitic slabs thrust over onto the Paleozoic continental margin of the Serbo-Macedonian massif (SMM) basement rocks and generated intra-continental syn-orogenic faults, which facilitated extensive Tertiary magmatic plutonic and sub-volcanic rocks, including porphyry Cu-Au intrusions [26,27].The Skouries Vathi, Pontokerasia, and Gerakario porphyry-Cu intrusions belong to the Vertiskos unit of the SMM, extending to Serbia and Bulgaria (Figure 1; [28][29][30][31]).
The Vertiskos unit, at the central part of the SMM, is composed of an alternation of gneisses and schists, hosting mafic-ultramafic bodies (ophiolites) of Jurassic age, known as the Gomati-Therma-Volvi (GTV) complex, which has been metamorphosed to highgrade amphibolite facies [28,[32][33][34][35]. Calc-alkaline magmatism started in the Early Miocene  is characterized by the intrusion of several subvolcanic rocks that host porphyrystyle mineralization, including Vathi, Pontokerasia, Gerakario, and Skouries [36,37].Based on whole-rock SiO 2 and alkali, the Skouries intrusion can be classified as a high K calcalkaline type [11,18,34].Also, although the contents of trace elements are scattered over a wide range, the high K/Rb and low Rb/Sr ratios from the Skouries porphyry stock are similar to PGE-fertile porphyry at the Allard stock, La Plata Mountains Colorado [8, 9,11].Three major fault groups characterize the regional structures, which controlled the emplacement of the intrusions in the crystalline basement of the Vertiskos Unit.Based on crosscutting and overprinting relationships, at least four stages of mineralization have been described in the porphyry Cu-Au deposit of Skouries (Figure 1b): (1) the initial quartz monzonite porphyritic phase and (2) the main stage of porphyritic syenite, associated with the mineralization of defined reserves approximately 205 Mt at 0.5% Cu, and 0.53 ppm Au, (3) the porphyritic mela-syenite dykes, and (4) the late stage, which crosscuts all earlier phases [18,29,32,33,38,39].According to these authors, magnetite-chalcopyrite (reaching up to 10 vol.%, average 6 vol.%) and bornite-chalcopyrite, linked to a pervasive potassic and propylitic alteration, crop outs in the central parts of the deposit, while chalcopyritepyrite dominates around the periphery of the deposit.Molybdenite is rare and occurs in pyrite-sericite-carbonate-bearing veinlets at the marginal parts of the deposit, with only a minor quantity of chalcopyrite.
The presence of isolated fragments of dark-green mafic fragments in drill core samples has been noted (Figure 1d) [13,19].In addition, drill-hole samples from a depth of more than 300 m contain relatively large fragments of metamorphosed basic rocks, intruded by porphyry intrusions that affect the sulfide mineralization (Figure 2).Three major fault groups characterize the regional structures, which controlled the emplacement of the intrusions in the crystalline basement of the Vertiskos Unit.Based on crosscutting and overprinting relationships, at least four stages of mineralization have been described in the porphyry Cu-Au deposit of Skouries (Figure 1b): (1) the initial quartz monzonite porphyritic phase and (2) the main stage of porphyritic syenite, associated with the mineralization of defined reserves approximately 205 Mt at 0.5% Cu, and 0.53 ppm Au, (3) the porphyritic mela-syenite dykes, and (4) the late stage, which crosscuts all earlier phases [18,29,32,33,38,39].According to these authors, magnetite-chalcopyrite (reaching up to 10 vol.%, average 6 vol.%) and bornite-chalcopyrite, linked to a pervasive potassic and propylitic alteration, crop outs in the central parts of the deposit, while chalcopyrite-pyrite dominates around the periphery of the deposit.Molybdenite is rare and occurs in pyrite-sericite-carbonate-bearing veinlets at the marginal parts of the deposit, with only a minor quantity of chalcopyrite.
The presence of isolated fragments of dark-green mafic fragments in drill core samples has been noted (Figure 1d) [13,19].In addition, drill-hole samples from a depth of more than 300 m contain relatively large fragments of metamorphosed basic rocks, intruded by porphyry intrusions that affect the sulfide mineralization (Figure 2).A calc-alkaline syenite and a quartz granodiorite intrusion host the Vathi, Gerakario, and Pontokerasia porphyry deposits in SMZ are of Miocene age (18 Ma) too, with more than 258 Mt of ore at 0.40 wt.% Cu and 0.9 g/t Au [36].

Mineralogical Characteristics
New data presented here are combined with those available from previous detailed descriptions on the porphyry deposits hosted in the Vertiskos Formation [11,13,18,19,[36][37][38][39].Magnetite-chalcopyrite ± bornite, linked to pervasive potassic and propylitic alteration, is exposed in the central parts of the Skouries deposit, while pyrite, that is dominant around the periphery of the deposit, is characterized by a significant (Ni±Co) content, for example, those from the SOP76 drill-hole (Figure 1c) (Table 1).The hypogene mineralization at the Vathi occurs in the quartz monzonite, while the potassic alteration is associated with vein-type ore assemblage: pyrite + chalcopyrite + bornite + molybdenite + magnetite, the propylitic alteration is related to pyrite + chalcopyrite, and the sericitic alteration is A calc-alkaline syenite and a quartz granodiorite intrusion host the Vathi, Gerakario, and Pontokerasia porphyry deposits in SMZ are of Miocene age (18 Ma) too, with more than 258 Mt of ore at 0.40 wt.% Cu and 0.9 g/t Au [36].

Mineralogical Characteristics
New data presented here are combined with those available from previous detailed descriptions on the porphyry deposits hosted in the Vertiskos Formation [11,13,18,19,[36][37][38][39].Magnetite-chalcopyrite ± bornite, linked to pervasive potassic and propylitic alteration, is exposed in the central parts of the Skouries deposit, while pyrite, that is dominant around the periphery of the deposit, is characterized by a significant (Ni±Co) content, for example, those from the SOP76 drill-hole (Figure 1c) (Table 1).The hypogene mineralization at the Vathi occurs in the quartz monzonite, while the potassic alteration is associated with vein-type ore assemblage: pyrite + chalcopyrite + bornite + molybdenite + magnetite, the propylitic alteration is related to pyrite + chalcopyrite, and the sericitic alteration is associated with the assemblage pyrite + chalcopyrite + native gold ± tetradymite.The assemblage sphalerite + galena + arsenopyrite + pyrrhotite + pyrite ± stibnite ± tennantite is related to a subsequent epithermal overprinting event [36].Disseminated magnetite as part of the quartz-bornite-chalcopyrite assemblages in the potassic alteration zones, proximal to the centers, is a characteristic feature in the Skouries and Elatsite deposits [16][17][18][19][20]. Merenskyite (the most common PGE-mineral) occurs commonly as inclusions or on the edge of hydrothermal chalcopyrite [18,19,37].More attention was paid here on the texture and mineral chemistry of magnetite (Figure 3; Table 3) in order to define significant differences compared to PGE-poor porphyry-Cu intrusions.Disseminated magnetite as part of the quartz-bornite-chalcopyrite assemblages in the potassic alteration zones, proximal to the centers, is a characteristic feature in the Skouries and Elatsite deposits [16][17][18][19][20]. Merenskyite (the most common PGE-mineral) occurs commonly as inclusions or on the edge of hydrothermal chalcopyrite [18,19,37].More attention was paid here on the texture and mineral chemistry of magnetite (Figure 3; Table 3) in order to define significant differences compared to PGE-poor porphyry-Cu intrusions.Magnetite in the Skouries porphyry Cu deposit results in the occurrence of intergrowths with Ti-magnetite, ilmenite, zircon, very fine Cu minerals (bornite and chalcopyrite), thorite, U-bearing thorite, rare earth element (REE) minerals (mostly monazite) and Cl,(OH)-apatite (Figure 3; [19].Zircon often shows zoning, with Fe, Th, Hf, and S in the core in contrast to the rim (Table 3).Uranium-rich thorite is associated with Ti-magnetite hosted by quartz (Table 1).
Separates of disseminated magnetite, derived from large porphyry-Cu samples, were analyzed for major and trace elements.The Th contents ranging from 28 to 110 ppm (mean 65), U ranging from 5.3 to 31 (mean 15 ppm), and Zr ranging from 119 to 700 ppm (mean 323) in magnetite separates are much larger than in bulk analyses and confirm their association with magnetite (Figure 3c,d).In addition, a characteristic feature of the Magnetite in the Skouries porphyry Cu deposit results in the occurrence of intergrowths with Ti-magnetite, ilmenite, zircon, very fine Cu minerals (bornite and chalcopyrite), thorite, U-bearing thorite, rare earth element (REE) minerals (mostly monazite) and Cl,(OH)-apatite (Figure 3; [19].Zircon often shows zoning, with Fe, Th, Hf, and S in the core in contrast to the rim (Table 3).Uranium-rich thorite is associated with Ti-magnetite hosted by quartz (Table 1).
Separates of disseminated magnetite, derived from large porphyry-Cu samples, were analyzed for major and trace elements.The Th contents ranging from 28 to 110 ppm (mean 65), U ranging from 5.3 to 31 (mean 15 ppm), and Zr ranging from 119 to 700 ppm (mean 323) in magnetite separates are much larger than in bulk analyses and confirm their association with magnetite (Figure 3c,d).In addition, a characteristic feature of the magnetite is the relatively high Cr (average 0.8 wt.% Cr 2 O 3 (Table 1), reaching up to 2.3 wt.% Cr 2 O 3 ) in SEM/EDS analyses, and in magnetite separates, the high Cr, Ni, and Co contents reach values up to 1060 Cr, 640 Ni, and 69 Co (all in ppm, Table 2).The average Cr content in magnetite from the Vathi porphyry [37] and Cr (16 ppm) from the Pagoni Rachi Cu-Mo-Re-Au porphyry prospect, northern Greece (Thrace), was 110 ppm and 16 ppm, respectively [41], meaning that they are much lower compared to that in magnetite from the Skouries deposit (Tables 2 and 3).The rare earth element (REE) content of magnetite separates is relatively low, and the chondrite-normalized REE patterns (Figure 4a) are similar to those for porphyry (Figure 4b) in terms of highly fractionated LREE [42].Mean ratios for Ce/Ce* = 0.96 and Eu/Eu* = 0.95 were calculated according to [43] (Table 3 The rare earth element (REE) content of magnetite separates is relatively low, and the chondrite-normalized REE patterns (Figure 4a) are similar to those for porphyry (Figure 4b) in terms of highly fractionated LREE [42].Mean ratios for Ce/Ce* = 0.96 and Eu/Eu* = 0.95 were calculated according to [43] (Table 3).

Platinum-Group Minerals (PGMs) in the Porphyry-Cu-Au+Pd±Pt Deposits
Consistently with the geochemical data, the presence of discrete platinum-group minerals (PGMs) of Pd and, to a lesser extent, of Pt have been described in several porphyry-Cu-Au+Pd±Pt deposits, including the Skouries.In the Afton and Mt Milligan deposits of British Columbia, pyrite contains small amount of Pd [18,22,45,46].In the Cusulfides associated with potassic alteration of the Skouries deposit, intergrowths of merenskyite [(Pd,Pt)(Te,Bi)2], hessite (Ag2Te), electrum, and Cu minerals (bornite and chalcopyrite) have been described [39].More recently, the PGM identified in PGEenriched porphyry deposits consist exclusively of Pd minerals such as merenskyite that

Platinum and Palladium Distribution
Any systematic variation between the Pd and Pt contents with the drill-hole depth and the location of the drill-hole is not obvious.However, it seems likely that the maximum (Pd+Pt) contents were measured in the central part of the ore body (Figure 1c), for example, in the drill-hole labeled as SG6, whereas they are much lower in marginal drill-holes.In general, a common feature of the Skouries deposit and other (Pd, Pt, Au)-fertile porphyry-Cu deposits is the occurrence of the quartz-bornite-chalcopyrite assemblages in the potas-sic alteration zones, proximal to centers, during the primary hypogene mineralization event [10][11][12][13][14][15][16][17][18][19]51].A relatively high PGE content in the Skouries porphyry-Cu deposits (up to 2.4 ppm Pd in chalcopyrite concentrates) measured in vein-type highly mineralized portions from drill-hole samples, covering deeper parts of the whole mineralized porphyry of Skouries (Figure 3; Table 4), is consistent with the analyzed composite drill-hole sample (~15 kg), showing 75 ppb Pd at 0.5 wt.% Cu or 3300 ppb Pd (measured contents of Pd are normalized to 100% chalcopyrite or 33 wt.%Cu) [53].
Although an overlapping may occur in the plots of the Pd and Pt contents, versus the (Ba+Sr) and (Cr+Ni) contents, the smaller (Ba+Sr) content and the maximum contents of Mg, Cr, Ni, and Co seem to be characteristic of the precious metal fertile intrusions (Table 5; Figure 5).(Pt, Pd, and Au) relative to their primitive mantle values [66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81].The presence of mafic (gabbroic) rocks fined-grained "basaltoid" dykes are known in the environment of Elatsite porphyry mineralization [17,[67][68][69][70]. Also, detailed field studies in the Valerianov-Beltau-Kurama magmatic arc, Uzbekistan, revealed the occurrence of gabbroic intrusions at depth and along the periphery of the ore field, and have established their spatio-temporal relationship with fertile porphyries [71,72].In addition, the formation of melts more favorable for generating porphyry Au-Cu deposits has been attributed to several processes, such as slab rollback caused asthenospheric mantle upwelling, the melting of metabasaltic amphibolites that underplated subducted continental crust, and shoshonitic lower Miocene magmatic rocks on Skouries (Chalkidiki) [73][74][75][76][77]. Ophiolites and associated ore deposits (mainly chromitites) in the SMM (Figure 1) [82][83][84] may have been affected during their multistage evolution by several processes in the mantle wedge above a subduction zone and crust, such as partial melting, fractional crystallization, interactions between mantle and lithospheric slab, and metal recycling [85][86][87][88].During the progressively collisional subduction of Tethys [26], an interaction between a varying degree of mantle-derived magma with the crust and the genesis of magma rich in water and sulfur with a significant mafic contribution to the magma source may have played an important role in the fertility of the Skouries porphyry-Cu deposit.Therefore, we can argue that an influx of a PGE-rich mafic magma related with the complex processes occurring in the supra-subduction zone was probably responsible for the original enrichment of Pd and Pt in certain porphyry-Cu deposits, as illustrated in Figure 6.

Sources of Precious Metals in Porphyry-Cu Deposits
Potential source regions of lithospheric re-fertilization include subduction-modified lower crust, sub-continental lithospheric mantle, or lower crust formed by under plated arc mafic rocks [7,43,[59][60][61][62][63][64][65].Melting the mantle at oxidizing conditions destabilizes and removes sulfide phases, while the coexisting melt may be enriched in precious metals (Pt, Pd, and Au) relative to their primitive mantle values [66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81].The presence of mafic (gabbroic) rocks fined-grained "basaltoid" dykes are known in the environment of Elatsite porphyry mineralization [17,[67][68][69][70]. Also, detailed field studies in the Valerianov-Beltau-Kurama magmatic arc, Uzbekistan, revealed the occurrence of gabbroic intrusions at depth and along the periphery of the ore field, and have established their spatio-temporal relationship with fertile porphyries [71,72].In addition, the formation of melts more favorable for generating porphyry Au-Cu deposits has been attributed to several processes, such as slab rollback caused asthenospheric mantle upwelling, the melting of metabasaltic amphibolites that underplated subducted continental crust, and shoshonitic lower Miocene magmatic rocks on Skouries (Chalkidiki) [73][74][75][76][77]. Ophiolites and associated ore deposits (mainly chromitites) in the SMM (Figure 1) [82][83][84] may have been affected during their multistage evolution by several processes in the mantle wedge above a subduction zone and crust, such as partial melting, fractional crystallization, interactions between mantle and lithospheric slab, and metal recycling [85][86][87][88].During the progressively collisional subduction of Tethys [26], an interaction between a varying degree of mantle-derived magma with the crust and the genesis of magma rich in water and sulfur with a significant mafic contribution to the magma source may have played an important role in the fertility of the Skouries porphyry-Cu deposit.Therefore, we can argue that an influx of a PGE-rich mafic magma related with the complex processes occurring in the suprasubduction zone was probably responsible for the original enrichment of Pd and Pt in certain porphyry-Cu deposits, as illustrated in Figure 6.The larger Mg, Cr, Ni, Co, Re, and 187 Os contents and smaller LILE elements (Ba and Sr) in fertile porphyry-Cu-Au-(PGE) reflect the larger contribution made by the mantle to the parent magmas [67][68][69][70].In contrast, the smaller Mg, Cr, Ni, Co, and Re contents and larger Ba and Sr in PGE-poor porphyry-Cu-Mo deposits from the Chalkidiki Peninsula (Vathi, Pontokerasia and Gerakario) and Russia suggest the presence of parent magmas with a more crustal contribution [76,77].Although the overlapping in relevant plots (Figure 5) is not precluded, it is obvious that in selected plots (Figure 5), two populations can be distinguished, the PGE-enriched and PGE-poor porphyry-Cu deposits.In addition, there is not any distinction between the PGE-rich whole rock analyses and those of PGE-rich flotation concentrates.

Transport of PGE in Hydrothermal Systems
Merenskyite (the most common PGE-mineral) occurs commonly as inclusions or on the edge of hydrothermal chalcopyrite [18,19,37], suggesting that their crystallization was related with the presence of hydrothermal fluids [63].The current state of knowledge of the solubility of PGE has been reviewed and applied in hydrothermal systems [63][64][65].Recent experimental data have shown that the solubility of Au and Pd both have positive relationships with f O 2 , temperature, acidity, and total chloride concentration (Cltotal), while Pt is efficiently mobilized at magmatic temperatures in relatively oxidized, slightly acidic, highsalinity brines [65].According to these authors, calculated fluid/melt partition coefficients for Au and Pd in low-density magmatic vapors suggest that Pd may experience decoupling from Au in porphyry Au-Cu (±Pd, Pt) systems due to the restricted compatibility of Pd in the fluid phase (requiring strongly acidic and substantially high f O 2 conditions).Also, the main mechanisms that promote the deposition of Pt from high-temperature aqueous brines are a reduction in f O 2 , temperature, acidity, and the total chloride concentration.Therefore, we suggest that local and randomly distributed enrichment in Pt and Pd in some porphyry Au-Cu (±Pd, Pt) deposits may have been caused by the circulation of complex hydrothermal fluids.

Genetic Significance of the Oxidized Nature of Parent Magmas
The incorporation into the melt of precious metal-bearing Fe-Ni-Cu-sulfides, hosted in mantle-derived rocks, may be an important step in the genesis of those deposits, because the magmatic sulfide melts can act as intermediate Cu and precious metals hosts during the evolution of the magmatic system [89][90][91].It has been established that the oxidized nature of parent magma is connected with the ability to produce a magmatic-hydrothermal system with ideal chemistry that facilitates the capacity for transporting sufficient precious metals [73,89].Specifically, the oxidized nature of the alkaline arc magmas inhibits PGE to precipitate as sulfides, allowing the Pd and Pt to remain in the magmas, and thus they can be transported by magmatic-hydrothermal fluids and precipitated in the porphyry environment [59].Since hydrothermal magnetite associated with chalcopyrite and bornite in porphyry systems is favored at high temperatures and f O 2 , and low f S 2 [91], the occurrence of abundant (average 6 vol.%) magnetite (reaching up to 10 vol.%) in the Skouries deposit, that is linked to pervasive potassic and propylitic alteration type [19], is considered to be a characteristic feature of the oxidation state of Pd-bearing porphyry Cu deposits.In contrast, "reduced" porphyry Cu-Au deposits, lacking primary magnetite and sulfate minerals (anhydrite), contain abundant pyrrhotite and are relatively Cu-poor, but they are Au-rich [59,92,93].
Dilles et al. [94] investigated porphyry Cu (±Mo±Au) and epithermal Au-Ag deposits formed by magmatic-hydrothermal fluids in Phanerozoic convergent margin settings and reported SHRIMP-RG ion microprobe analyses of Hf, Ti, and REE abundances in zircon.These authors compared the compositions of zircons generally in fertile and barren granitic plutons, and observed an Eu/Eu* ratio higher than 0.4 only in zircons associated with fertile granites, thus concluding that zircon composition is potentially a valuable tool for mineral exploration [94].Similar analytical data on the zircons from the Skouries mineralized porphyry are not available.Tiny zircon crystals occurring as inclusions and/or at the peripheral parts of magnetite are common in the Skouries deposit (Figure 3c,d), suggesting a genetic link between zircon and magnetite.The bulk rock analysis on magnetite separates from the Skouries deposit indicated values for the Eu/Eu* ratio higher than 0.35, ranging from 0.55 to 1.7 (Table 4).Although the presence of magnetite is considered to be a characteristic feature of the oxidation state of porphyry Cu deposits [19,59,92], the above-reported Eu/Eu* ratios in the magnetite separates are consistent with a relatively high oxidation state during the cooling of the ore-forming system, which permitted the transportation of Pd and Pt in the magmatic-hydrothermal system.

Fertility of Porphyry-Cu Deposits
The mantle source that underwent a partial melting of existing sulfides is a critical control of the PGE contents in the resulting melts.Therefore, the predominant factors controlling the distribution of PGE are the partial melting of the mantle source, the interaction of the magmas with magmatic sulfide liquids, the partition coefficients of PGE from silicate into sulfide liquids being of the order of thousands, and the Pd solubility in silicate melts orders of magnitude higher than that of other PGE [7].In some cases, the formation of an immiscible sulfide liquid enriched in the more chalcophile PPGE (Pt, Pd, and Rh sub-group) can be achieved in certain magmas during their cooling, giving rise to the precipitation of PPGE minerals together with Ni-Cu-Fe sulfides, as interstitial in chromite ores and cumulus silicates [6,94].
However, the fertility of the porphyry Cu-Au-Pd-Pt deposits and the mechanism permitting the transfer of these elements from the mantle to the magma source for porphyryepithermal deposits still remain unclear [22,24,25].The available texture intergrowths, mineral chemistry, and geochemical data (Figure 3; Tables 2-5) are consistent with the significant role of the critical degree of mantle melting to release Pt and Pd in the oreforming fluids, the high oxidation state, high magmatic water content, and the degree of fractionation.The larger Mg, Cr, Ni, and Co contents in bulk rock analyses, in the mineral chemistry (sulfides and magnetite), and smaller Sr and Ba (LILE elements) contents in fertile porphyry-Cu deposits from the Balkan Peninsula and British Columbia, in contrast to those from the Vathi, Pontokerasia, Gerakario, Russia, and Mongolia deposits (Tables 2-5; Figure 5), indicate that the composition of the parent magma is mainly affected from a mantle component rather that lower crust.The investigation of the characteristic features of porphyry-Cu-Au-Pd±Pt intrusions, such as the Elatsite, has shown that mantle-derived methane was identified in two-phase (CO 2 -CH 4 ) high-temperature (>500 • C) fluid inclusions [68].That type of fluid inclusion may suggest that carbon, as a supercritical CO 2 fluid, could be present in the primitive Elatsite magmas, promoting the physical transport of sulfides and/or telluride melt droplets [18,94].It has been noted that a small amount of reduced gases in fluid inclusions cannot argue against the oxidized feature of the magmas during the final mineralization process of porphyry deposits [92].Although most of the hydrogen and methane should have been oxidized by ferric Fe, some of the reduced gases may be trapped in fluid inclusions [92].
Also, detailed Pb, Sr, S, O, and C isotopic data have been applied on the porpyry Cu deposits of the Chalkidiki Peninsula [34].These Tertiary intrusions are characterized by a rather diffuse Sr isotopic pattern, suggesting heterogeneous crustal rocks (meta-igneous and metasedimentary), while relatively low initial Sr values (<~0.707)indicated either a mantle component and/or a component from lower crustal rocks [34].Also, the Pb isotope composition outlined by whole rock, feldspar, and ore minerals from Skouries suggests a major contribution of country rocks from the Vertiskos Formation at depths prior to the final emplacement [34].Bonev et al. [95] have reported on the isotopic compositions of metamafic rocks exposed in the SMM, consisting of high-and low-Ti gabbroic and basaltic rocks, showing Nd-Sr-Pb isotope signatures compatible with mantle-derived MORB (Mid-Ocean Ridge Basalts) and OIB (Ocean Island Basalts) components with a small amount of crustal material involved in their melt source.These authors suggested that such isotopic features, coupled with field observations, reflect an intra-continental rift origin of those metamafic rocks protolith [95].
In addition to the contribution by mantle-derived components, the fractionation degree of the mineralized system seems to be of particular importance too.The Pd/Pt ratio, that may reflect the degree of fractionation in the ore-forming system (due to the higher solubility of Pd compared to Pt), ranges from 0.6 to 0.9 in the PGE-poor porphyry deposits of Russia and Mongolia, and 0.13 to 0.9 in the Vathi, Pontokerasia, and Gerakatio PGE-poor deposits, in contrast to the (Pd, Pt, Au)-fertile deposits in Skouries (average 4.3 up to 60), Elatsite (average 7.8), and British Columbia (average 19) (Table 5).The variation in the Pd/Pt ratio may reflect differences in their evolution systems.Tetrahedritetennantite are frequently associated with the magnetite-bornite-chalcopyrite assemblage of an early stage of mineralization in porphyry deposits, which are in a spatial association with epithermal deposits.Some examples include the Elatsite deposit associated with the Chelopech Au-Cu high-sulfidation epithermal deposit, the Assarel deposits in Bulgaria, and the Bor/Majdanpek porphyry deposits in Serbia and in British Columbia, although they are rare in the Skouries deposit [21,68,69,96,97].Thus, a higher degree of fractionation for the fertile porphyry-Cu-Au-Pd±Pt-forming system compared to porphyry Cu-Mo deposits is consistent with the often proximity and overlapping of the former with epithermaltype deposits [24,26,68,69].Similarly, the presence of zircons having low Th/U and high Yb/Gd associated with the mineralized phases ratios in porphyry-fertile Late Triassic and Early Jurassic plutons in British Columbia suggests their precipitation after fractional crystallization to some extent [24].The elevated values of the Pd/Pt ratios, the extremely low Cr contents (<1 ppm) in high Cu-Pd-Pt-grade ores, and a negative correlation between Cr content and the Pd/Pt values in porphyry deposits of the Balkan Peninsula [13] suggest a genesis from more evolved mineralized fluids in porphyry Cu-Au-Pd-Pt deposits.The significant role of the crystal fractionation process that led to the formation of immiscible sulfide enriched in PPGE (Pt, Pd and Rh sub-group), giving rise to the precipitation of PPGE minerals together with Ni-Cu-Fe sulfides, as interstitial in chromite ores and cumulus silicates, has been emphasized [6,52].

Conclusions-Implications for Mineral Exploration
The presented mineral chemistry and geochemical data on selected porphyry-Cu deposits distributed worldwide, coupled with petrological evidence, may allow to define significant differences between PGE-fertile and PGE-poor porphyry-Cu intrusions, especially in the deposits of Balkan Peninsula, Russia, and Mongolia.

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Critical factors controlling the PGE-fertility of porphyry-Cu systems may be the spatial and temporal relationship of mafic (gabbroic) rocks with porphyries (for example, in the Elatsite deposit), which is reflected in the composition of the magma, and the favorable conditions for the transfer of metals from the magma to the fluids and fertile porphyry-Cu deposits.

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Fertile porphyry Cu+Au+Pd±Pt deposits from the Balkan Peninsula are characterized by larger Mg, Cr, Ni, and Re contents and smaller Ba and Sr (LILE elements) compared to PGE-poor porphyry-Cu-Mo from Russia, Mongolia, and Vathi-Pontokerasia-Gerakario deposits.

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The common presence of high PGE mineralization [thousands ppb (Pd+Pt)] and low Cr content in the transition from the porphyry to epithermal environment, coupled with the occurrence of Pd-, Te-, and Se-bearing minerals (merenskyite, clausthalite) and tetrahedrite-tennantite in fertile porphyry Cu deposits (Elatsite deposit), reflect a highly fractionated ore-forming system.

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The occurrence of abundant Cr-bearing magnetite, abundant amphibole and biotite (abundant magmatic water), high Mg, Cr, Ni, and Re contents, and low Ba and Sr in bulk rock analyses is considered to be encouraging evidence for finding PGE-rich Cu+Au+Pd±Pt deposits in the future.

Figure 1 .
Figure 1.(a): Simplified geology of the Serbo−Macedonian Massif the studied part is marked by the red frame (modified after [30,31]); (b): schematic cross−section showing crosscutting successive monzonite porphyry intrusions; (c): the location of drill holes in the Skouries porphyry deposit [29]; (d): representative drill core sample, showing the presence of dark-green angular mafic fragments to a sharp contact with the host porphyry, and crosscutting relationships between successive quartz veins.Abbreviation: Cp = chalcopyrite, Py = pyrite, K-SPAR = K-feldspar.

Figure 1 .
Figure 1.(a): Simplified geology of the Serbo−Macedonian Massif the studied part is marked by the red frame (modified after [30,31]); (b): schematic cross−section showing crosscutting successive monzonite porphyry intrusions; (c): the location of drill holes in the Skouries porphyry deposit [29]; (d): representative drill core sample, showing the presence of dark-green angular mafic fragments to a sharp contact with the host porphyry, and crosscutting relationships between successive quartz veins.Abbreviation: Cp = chalcopyrite, Py = pyrite, K-SPAR = K-feldspar.

Figure 2 .
Figure 2. Photographs of dark-green biotite-amphibole metamorphic country rocks, which are dominant in drill cores as xenoliths in the Skouries porphyry.(a,b): Sulfide mineralization in those rocks intruded by porphyry intrusions are common.(c): layered amphibolite.Bar scale (a-c) = 1 cm.

Figure 2 .
Figure 2. Photographs of dark-green biotite-amphibole metamorphic country rocks, which are dominant in drill cores as xenoliths in the Skouries porphyry.(a,b): Sulfide mineralization in those rocks intruded by porphyry intrusions are common.(c): layered amphibolite.Bar scale (a-c) = 1 cm.

Figure 6 .
Figure 6.Schematic section of a subduction zone and continental arc, showing the geotectonic setting for porphyry-Cu-Au-Pd±Pt systems.Partial melting of metasomatized mantle generates mafic melts, which may contribute metals, including PGE and sulfur, potentially incorporated into the overlying felsic magma chamber.Hydrous, metal-bearing extracted from the subducting slab rising into the mantle wedge may cause metasomatism [78-81].The larger Mg, Cr, Ni, Co, Re, and 187 Os contents and smaller LILE elements (Ba and Sr) in fertile porphyry-Cu-Au-(PGE) reflect the larger contribution made by the mantle to the parent magmas [67-70].In contrast, the smaller Mg, Cr, Ni, Co, and Re contents and larger Ba and Sr in PGE-poor porphyry-Cu-Mo deposits from the Chalkidiki Peninsula

Figure 6 .
Figure 6.Schematic section of a subduction zone and continental arc, showing the geotectonic setting for porphyry-Cu-Au-Pd±Pt systems.Partial melting of metasomatized mantle generates mafic melts, which may contribute metals, including PGE and sulfur, potentially incorporated into the overlying felsic magma chamber.Hydrous, metal-bearing extracted from the subducting slab rising into the mantle wedge may cause metasomatism [78-81].

Table 1 .
Selected characteristics of the porphyry-Cu deposits from the Chalkidiki Peninsula.

Table 3 .
Composition of disseminated magnetite separates from the Skouries drill-holes.Present study. ).

Table 4 .
[13]r and trace element contents in the Skouries porphyry-Cu-Au-Pd-Pt samples from drill-holes of varying depths [present study] and Bulgaria[13].

Table 5 .
Mean values of selected trace elements taken into consideration to divide the PGE poor for the PGE fertile, in highly mineralized ore (Sf.C) and flotation concentrates (F.C) samples from porphyry-Cu deposits; n.d.= not detected; n.a.= not available.