Critical Factors Controlling Pd and Pt Potential in Porphyry Cu–au Deposits: Evidence from the Balkan Peninsula

Porphyry Cu–Au–Pd±Pt deposits are significant Au resources, but their Pd and Pt potential is still unknown. Elevated Pd, Pt (hundreds of ppb) and Au contents are associated with typical stockwork magnetite-bornite-chalcopyrite assemblages, at the central parts of certain porphyry deposits. Unexpected high grade Cu–(Pd+Pt) (up to 6 ppm) mineralization with high Pd/Pt ratios at the Elatsite porphyry deposit, which is found in a spatial association with the Chelopech epithermal deposit (Bulgaria) and the Skouries porphyry deposit, may have formed during late stages of an evolved hydrothermal system. Estimated Pd, Pt and Au potential for porphyry deposits is consistent with literature model calculations demonstrating the capacity of aqueous vapor and brine to scavenge sufficient quantities of Pt and Pd, and could contribute to the global platinum-group element (PGE) production. Critical requirements controlling potential of porphyry deposits may be from the metals contained in magma (metasomatized asthenospheric mantle wedge as indicated by significant Cr, Co, Ni and Re contents). The Cr content may be an indicator for the mantle input.


Introduction
Many important porphyry Cu-Au, Cu-Mo, Mo-W deposits are located around the Pacific rim, in Mediterranean and Carpathian regions of Europe, and in the Alpine-Himalayan system, extending from western Europe through Iran and the Himalaya to China and Malaysia (Figure 1).Porphyry Cu deposits typically have been formed along subduction-related convergent plate margins associated with island arcs and continental arcs or in extensional back-arc or post-collisional rift settings [1][2][3][4][5][6][7][8][9][10][11].
Alkaline or K-rich calc-alkaline porphyry deposits worldwide represent significant gold resources, owing to their large sizes.Moreover, during the last decades, elevated contents of palladium (Pd) and platinum (Pt) have been noted.These precious metals belong to the platinum-group elements (Os, Ir, Ru, Rh, Pd and Pt or platinum-group elements (PGEs)), which are the most valuable elements, of strategic importance, due to their growing use in advanced technologies (medicine, electronics) and automobile catalyst converters.Although they are traditionally associated with mafic-ultramafic complexes, significant Pd and Pt contents were described in certain alkaline porphyry deposits, such as the Cordillera of British Columbia (Copper Mountain, Galore Creek), Allard Stock, La Plana Mountains and Copper King Mine in USA [12,13], Skouries porphyry deposit, Greece [14,15], Elatsite, Bulgaria [16][17][18][19], Santo Tomas II in the Philippines [20,21] and elsewhere (Figure 1).
The research interest for many authors has been focused on the discovery of new PGE sources.Present study has focused on the characteristics of the Skouries (Greece) and Elatsite (Bulgaria) porphyry deposits.New and previously published geochemical data are combined with the current state of knowledge on the PGE solubility and partitioning of PGE which will lead to a better understanding of the controlling factors of the PGE mineralization and PGE potential in porphyry-Cu systems.

Analytical Methods
Polished sections were examined by reflected light combined with backscattered electron (BSE) images of the sections, obtained on a JEOL JSM 5600 (Tokyo, Japan) scanning electron microscope, at the University of Athens, Department of Geology and Geoenvironment.
Major and trace elements on mineralized porphyry samples were determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis, at the Acme Laboratories Ltd., Vancouver, BC, Canada.Detection limits are 0.1 ppm for Ag, Cu, Pb, Ni, and Mo, 0.2 ppm for Co and 1 ppm for Zn and Cr.Platinum-group elements (PGE) were determined by ICP-MS analysis after pre-concentration using the nickel fire assay technique from large (30 g) samples, at the Acme Laboratories Ltd, Canada.This method allows for complete dissolution of samples.Detection limits are 10 ppb for Pt, 2 ppb for Pd and Au.
Oxygen isotopic and hydrogen compositions presented herein were determined on quartz veinlets containing Cu-Fe-minerals, magnetite and precious metal-minerals, at Geochron Laboratories.The hydrogen (δD) and oxygen (δ 18 O) data were normalized to the SMOW (Standard Mean Ocean Water) standard [22].

Skouries Deposit (Greece)
The Skouries porphyry Cu-Au deposit, located at the Chalkidiki Peninsula, northern Greece, is hosted in the Vertiskos Formation of the Serbo-Macedonian massif (SMM) of Miocene age (18 Ma), which is younger than the intrusions of the Serbo-Macedonian massif (Figure 2).
The defined reserves in the porphyry Cu-Au deposit of Skouries are approximately 205 Mt at 0.5% Cu, and 0.53 ppm Au [23].At least four monzonite porphyries have been described [24][25][26][27].Two mineral assemblages of mineralization, occurring as veinlets/disseminations, can be distinguished: (a) magnetite-(reaching up to 10 vol %, average 6 vol %) bornite-chalcopyrite, linked to pervasive potassic and propylitic alteration, in the central parts of the deposit, and (b) chalcopyrite-pyrite, which dominates around the periphery of the deposit.Molybdenite occurs in small amounts, mainly at the marginal parts of the deposit that contains a minor quantity of chalcopyrite commonly in late pyrite-sericite-carbonate bearing veinlets [23][24][25][26][27]. Chalcopyrite, and to a lesser extent bornite are the main Cu-minerals.The most salient texture feature in the Skouries porphyry deposit is the intergrowth between merenskyite (Pd,Pt,Bi)Te 2 , hessite (Ag 2 Te), electrum and Cu-minerals (bornite and chalcopyrite) (Figure 3).Such an association of the palladium telluride, merenskyite, as the main PGE mineral in porphyry Cu-Au-Pd-Pt deposits, is considered to be of genetic significance and an important factor for the recovery of Pd and Pt as by-products.Relatively high Pd content in the major vein-type mineralization of Skouries has been documented by the analysis of a composite drill hole sample (~15 kg) showing 76 ppb Pd and 5000 ppm Cu [14,15].Furthermore, mineralized material and highly mineralized portions (up to 2.5 wt % Cu) from deeper parts of the deposit (potassic, propylitic alteration zones) from drill holes covering the whole mineralized porphyry of Skouries (Figure 4) were analyzed for precious metals and trace elements (Table 1) in order to define their spatial distribution and probable interelement relationships.
A wide heterogeneity in the spatial variation of Cu, precious metal and trace element contents throughout the Skouries deposit is evident.A special attention was given to the chromium and nickel distribution, because they may provide evidence for the magma source and evolution of the mineralized system, due to their association with mafic-ultramafic rocks (parent magmas) and their compatible behavior [28,29].There is a good positive correlation between Cr and Ni (R = 0.89, Table 1) at the Skouries porphyry.In addition, to constrain the origin of fluids trapped in quartz veins (which is unaltered in all alteration assemblages) stable isotope analyses of oxygen and hydrogen for quartz veins from various drill holes and depths performed.All samples were collected from depths of 60-525 m, covering the range of the recorded chromium contents (Table 1; Figure 4).These data were used to calculate the isotopic composition of fluids in equilibrium with quartz, using as crystallization temperature minimum 350 °C and maximum 440 °C, based on the intergrowth of Cu-Fe sulphides and precious metal tellurides [30].Isotopic trends of fluids, co-existing with quartz from various depths and drill holes in the Skouries porphyry Cu deposit are characterized by relatively high (δ 18 O = 4.33‰-9.45‰)equilibrium fluid compositions (Table 2), which are comparable to those given in previous publications [26,31].Furthermore, the analysis of D/H isotopes showed low δD (−110‰ to −73‰) values (Table 1).
Although any systematic variation of the isotopic data with depth is uncertain, it seems likely that the samples (group A) showing higher Cr contents (average = 290 ppm) correspond to lower calculated δ 18 O values (average = 5.43) and those (group B) with lower Cr contents (average = 8.2 ppm Cr) correspond to higher δ 18 O = 6.7‰ (Table 1; Figure 5).In addition, same samples with higher (Pd+Pt) contents (average = 274 ppb) and Pd/Pt ratios (average = 5.9) correspond to higher δ 18 O values and lower Cr contents as well (Table 2).     1 and 2.

Fissoka Group
About 5 km east of Skouries is located the Fissoka porphyry (Figure 1), along the same structural trend, including several ore bodies within an area of about 1000 m × 600 m, and it is considered a prospect that may represent a separate deposit from that at Skouries.Diorite porphyry ages, 23.0 ± 1.2 (K-Ar on sericite) and 24.5 ± 1.2 Ma (K-Ar whole rock), at Fissoka suggest that the system is slightly older than Skouries [11].Aeromagnetic data for the area suggests that the Skouries-Tsikara and the Fissoka areas are not connected in the shallow subsurface, and that only the supergene zone has been drilled to date.Several prospects are characterized by strong alteration, fissures with iron oxides and gossan, with anomalous copper in rock and soil samples [11].Limited mineralogical data on the eastern margin of the Fissoka porphyry Cu (OP-65 prospect) indicated disseminated mineralization consisting of pyrite with inclusion of gold, chalcopyrite, galena, sphalerite and arsenopyrite within the alteration zone, lower Pd and Cu content, Pd/Pt ratio and higher Au, Te, As, Pb and Zn content than at Skouries [15] suggest the prospect for further exploration.

Elatsite
The Elatsite-Chelopech ore field can be divided into three sections: (a) the northern sector, the Elatsite porphyry Cu deposit is related to subvolcanic dikes and larger dike-like bodies intruding the basement rocks; (b) the central sector comprises the Chelopech volcano and associated subvolcanic intrusives, accompanied by the Chelopech Au-Cu massive high sulphidation deposit; and (c) the southern sector, a small intrusive body and several dikes intrude Precambrian metamorphic rocks along a system of radial-concentric faults [32][33][34][35][36][37].
A salient feature is the unusually high precious metal content, recorded in high Cu-grade ores (magnetite-chalcopyrite-bornite) from the level 1310 South, which are similar to those analyzed by previous authors [18,19]; Table 3.In addition, the elevated Ag (up to 220 ppm), Bi (up to 290 ppm), Se (up to 800 ppm) contents, and the common occurrence of Au-Ag-Te-Se minerals in the magnetite-bornite-chalcopyrite ore assemblage [18,19] may be of genetic significance.
Plots of precious metals versus Cu (Figure 7) indicate that they define a broad positive trend between Cu and precious metal contents, and the Pd/Pt ratio, in contrast to the negative correlation between Pd and Cr contents.Given that there is a wide variation in the Cu-grade and the associated precious-metal ( [18,19]; Table 3), the precious metal contents in the ore samples were normalized to 1% Cu, in order to obtain comparable data (Figure 7e).However, any clear correlation between Pd/Pt vs. Pd/Cu (Figure 7c) and Pd/Pt vs. (Pd+Pt) (Figure 7e), after re-calculation is uncertain.1 and 2, and [18,19,38,39].

Discussion
Although more analytical data are required for the evaluation of the precious metal potential in porphyry deposits, examples from the Balkan Peninsula indicate that they may contribute significantly to the global PGE production.Using 206 Mt reserves [23] and average metal contents (0.5 wt % Cu, 75 ppb Pd and 17 ppb Pt [15], then the potential of the Skouries deposit is approximately 15 t Pd and 3.5 t Pt.In addition, using reserves and average Pd and Pt contents for the Elatsite deposit [33], the potential is about 13 t Pd and 3.47 t Pt.Such a precious metal potential is consistent with experimental data showing the ideal nature of porphyry-stage fluids for PGE scavenging, transport and deposition [40][41][42][43][44][45][46].
The metal potential in porphyry Cu+Au+Pd±Pt deposits may be related to the following factors:

Precious/Base Metal Endowment in Parent Magma
The enrichment in Cr, Co±Ni, recorded in the Skouries, Elatsite, Medet, Assarel and Tsar Assen porphyry deposits of the Balkan, having significant Pd and Pt contents, in contrast to the porphyry deposits of Russia and Mongolia with lower Pd-Pt content (less than 25 ppm Cr [39] supports the origin of their parent magma from an enriched mantle source [47][48][49].Favorable tectonic settings include subduction of very young lithosphere or very slow or oblique convergence, flat subduction, and the cessation of subduction ( [7,33,37,[49][50][51]; Figure 8).The decreasing Pd, Pt contents, as well as in Re, from a few thousand ppm in porphyry Cu-Au deposits (Balkan Peninsula) to hundreds or tens of ppm in porphyry Cu-Mo type (Russia and Mongolia), and furthermore to less than tens of ppm Re in porphyry Mo-W (Lavrion, Greece) deposit [52][53][54] may be related to differences in their geotectonic environment.
The Re-Os system has been applied as both a geochronometer in molybdenite and as a tracer of the source of metals by direct determination of the source of Os in the ore minerals [55][56][57][58][59][60][61][62][63].Elevated initial 187 Os/ 188 Os ratios (0.5-2.5) that are substantially more radiogenic than mantle indicate a contribution by a crust source (recycled in a metasomatized mantle, lower/upper continental crust) by inference other metals of porphyry Cu deposits and have been used to discern the influence of different reservoir [57,62].On the basis of the initial 187 Os/ 188 Os ratios, a strong correspondence between copper tonnage and initial Os isotopic ratios (the larger deposits showed lower initial Os ratios than the smaller) has been established in Chilean porphyry deposits and American Southwest province (Bagdad, AZ, USA), suggesting a greater mantle component [63,64].Also, elevated Os isotope compositions recorded in deposits of varying ages, have been interpreted to reflect recycling of discrete intracrustal domains with high 187 Os/ 188 Os ratios reflecting the process of crustal hybridization and homogenization [62].Rhenium-osmium (Re-Os) ages and Re content data for molybdenites from the porphyry deposits in the Apuseni-Banat-Timok-Srednogorie (ABTS) magmatic metallogenic belt have shown high to extremely high Re and constrain the geochemical-metallogenic evolution of the belt in space and time [10].The highest Re (thousands of ppm) and 187 Os (thousands of ppb) values were recorded in the Elatsite Cu-Au-Mo-(PGE) porphyry deposit, reflecting probably higher contribution from mantle of post-collisional mantle-derived magmas [49][50][51].
Therefore, Os isotope compositions provide new constraints on amounts of intra-crust recycling in young subduction-zone environments that reflect the magmatic history of the arc, as previously suggested by Sr-, Nd-and Hf and Pb isotope data [35,65,66].In addition, the initial 187 Os/ 188 Os values for sulphides from the Lavrion mine, suggest that ore-forming components were derived from mixed sources, one of which was a radiogenic crustal source, and the other, intrusive rocks that are less radiogenic [67].

Physico/Chemical Conditions
Arc magmas with high potential to produce hydrothermal systems with ideal chemistry for transporting precious metals (origin of Au and Cu deposits) are characterized by fO 2 more than two log units above FMQ, where fO 2 is oxygen fugacity and FMQ is the fayalite-magnetite-quartz oxygen buffer [68,69].It is consistent with the abundance of magnetite in all porphyry Cu-Au-Pd deposits of the Balkan Peninsula [14][15][16][17][18][19].In contrast, "reduced" porphyry Cu-Au deposits, lacking primary hematite, magnetite, and sulphate minerals (anhydrite), contain abundant pyrrhotite, and are relatively Cu-poor, but Au-rich deposits [6].
Several experimental studies have shown that the low pH and high fO 2 nature of porphyry-stage fluids are ideal for PGE transport via chlorides, at relatively low-temperature [40][41][42][43][44].Moreover, it has been demonstrated that sufficient quantities of Pt and Pd may be scavenged from the silicate magma by vapor and brine over the magmatic system [45,46].

Fractionation in the Mineralized System
Differences in the Pd/Pt ratios, ranging from 0.71 (up to 1.1) in the porphyry deposits of Russia and Mongolia, to 4.3 (up to 16) in Skouries, 10 (up to 130) in Elatsite and 13 (up to 57) in British Columbia, may reflect differences in their evolution systems.In addition, the extremely low Cr contents (<1 ppm) in high Cu-Pd-Pt-grade ores from Elatsite deposit (Table 3), and the increasing (Pd+Pt) content and Pd/Pt rations with decreasing Cr content in the Skouries deposit (Table 2; Figure 4) are consistent with the compatible behaviour of Cr.There is a decoupling between precious metals and Cu in the plots Pd/Pt vs. (Pd+Pt) and Pd/Pt vs. Cu, after re-calculation of Pd and Pt values at the typical Cu-grade (~1 wt % Cu), due probably to the much higher values of the partition coefficient for Pd and Pt compared to that for Cu [48,70].Nevertheless, the increasing (Pd+Pt) content with the increase of the Pd/Pt ratio (Figure 7), due to the more incompatible behavior of Pd than that of Pt [40][41][42][43][44][45][46] suggest an enrichment in Pt, Pd, Au, and Cu with fractionation.Thus, the magmatic component, as it is supporting by the δ 18 O values in the liquid fluids carrying metals (Table 2), seems to be a driving force for the precious metal potential in evolved porphyry Cu deposits.
In addition, the elevated Au, Ag, Te, Bi, Se contents and the abundance of corresponding minerals, like tetrahedrite and tennantite, at the Elatsite deposit resemble geochemical and mineralogical characteristics of epithermal deposits, and have been interpreted as the latest stage of the porphyry system [18,33,34,38,49,51].Hydrothermal veins in volcanic rocks, adjacent to Cu-Au porphyry mineralization, have been considered as the equivalent of transitional (post-porphyry, pre-epithermal) like in other porphyry Cu deposits [71].The enrichment in Au, PGE, As, Sb, Hg, Bi, Te and base metals, has been attributed to the migration away from the source intrusion of mineralized fluids, resulting in the deposition of metals in the veins at a depth of up to several kilometers [71].

Conclusions
The compilation of new and previously published data on porphyry Cu-Au-Pd±Pt deposits, and differences within and between porphyry deposits, lead to the following conclusions: • Critical requirements for a significant base/precious metal potential in porphyry Cu+Au+Pd±Pt deposits are considered to be the geotectonic environment, controlling the precious/base metal endowment in the parent magma, the oxidized nature of parent magmas, that facilitate the capacity for transporting sufficient Au and PGE, and the degree of evolution of the mineralized system.• The elevated contents (hundreds of ppm) in Cr, Co, Ni and Re in porphyry Cu-Au-Pd±Pt deposits of the Balkan Peninsula, in contrast to the porphyry Cu-Mo deposits of Russia and Mongolia are attributed to a direct mantle input by metasomatized asthenospheric mantle wedge.• The elevated values of the Pd/Pt ratios in porphyry deposits of the Balkan Peninsula, coupled with the extremely low Cr contents (<1 ppm) in high Cu-Pd-Pt-grade ores from Elatitse, and the negative correlation between Cr content and the Pd/Pt, δ 18 O values, support their genesis from more evolved mineralized fluids.• The Pd and Pt upgrade in massive Cu-ore at the Elatsite porphyry deposit may point to the possibility for their concentration in "transitional" porphyry deposits, during late stages of the evolution of the hydrothermal systems.• The estimated Pd, Pt and Au potential for certain porphyry-Cu deposits are consistent with the capacity of aqueous vapor and brine to scavenge sufficient quantities of Pt, and Pd, and hence porphyry Cu-Au-Pd-Pt deposits may contribute significantly to global PGE production.

Figure 4 .
Figure 4. Location of the studied drill holes, xenoliths and mineral intergrowths.(a) Location of drill holes in the Skouries porphyry [23], and (b) representative drill core sample, showing residues of assimilated dark green angular mafic fragments, and crosscutting relationships between successive quartz veins.

Figure 5 .
Figure 5. Relationship between calculated δ 18 O values in quartz and Cr content.A plot of the calculated δ 18 O values in quartz from mineralized quartz veinlets, in the range of maximum and minimum temperatures versus chromium content at the Skouries mineralized porphyry.The red squares represent average values for the groups A and B. Data from Tables1 and 2.

Table 1 .
Precious metal and selected trace element contents in the Skouries deposit.