A New Porphyry Mo Mineralization at Aisymi-Leptokarya, South-Eastern Rhodope, North-East Greece: Geological and Mineralogical Constraints

A new porphyry Mo prospect has been discovered in the Aisymi-Leptokarya area, along the southern margin of the Byala Reka–Kechros metamorphic dome, south-eastern (SE) Rhodope metallogenic zone. The study area is dominated by an Oligocene felsic dike complex, which hosts the porphyry Mo mineralization and intrudes into upper Eocene sandstones-marls and the Leptokarya monzodiorite pluton. The Aisymi-Leptokarya felsic dike complex displays a rhyodacitic to dacitic composition with postcollisional affinities. The porphyry Mo mineralization occurs in the form of porphyry-style quartz stockworks in the felsic dike complex associated with potassic alteration characterized by hydrothermal K-feldspar. The ore minerals consist mainly of pyrite, molybdenite, kesterite, bismuthinite and galena within both the stockwork and the rock matrix. Bulk ore analyses indicate enrichment in Mo (up to 215 ppm), Se (up to 29 ppm), Bi (up to 8 ppm) and Sn (up to 14 ppm) in the porphyry quartz veins. Late-stage, north-east (NE-) and north-west (NW-)trending milky quartz intermediate-sulfidation epithermal veins with base metals, crosscut previous vein generations and are characterized by Ag, Sn and Te anomalies. The Aisymi-Leptokarya porphyry Mo prospect is set in a back-arc geotectonic regime and shares similarities to other post-subduction porphyry molybdenum deposits elsewhere.


Introduction
Porphyry-type deposits are the largest source of the world's molybdenum. Currently, China, which hosts many porphyry Mo deposits is the most significant Mo producer [1]. Proven reserves are >25 Mt Mo metal (e.g., [2]) and comprise more than twice the rest world's supplies [3].
Porphyry Mo deposits have been classified into three types [4]: (1) The Endako-type (or high-fluorine), which is related to subduction environments and hosted in calc-alkaline magmatic rocks [5]. Figure 1. Simplified geological map of the south-eastern Rhodope (modified after [11] and [18]). Black square marks the study area. Figure 2. Geological map of the study area (this study and [11]).

Figure 1.
Simplified geological map of the south-eastern Rhodope (modified after [11,18]). Black square marks the study area. Simplified geological map of the south-eastern Rhodope (modified after [11] and [18]). Black square marks the study area. Figure 2. Geological map of the study area (this study and [11]). Figure 2. Geological map of the study area (this study and [11]). The felsic dike complex consists of a major NW trending intrusive body, up to hundreds of meters wide and several km long, which crosscuts all the other rocks in the area ( Figure 2). Alfieris et al. [26] described acid dikes (e.g., quartz ± biotite porphyries) from the south-eastern part of the study area, with disseminated pyrite and base metal veins mineralization.
The Leptokarya pluton, forms part of a NE-trending intrusive belt that includes the Kassiteres, Kirki, Leptokarya and Tris Vrisses intrusions ( Figure 1) and consists of quartz monzogabbros/monzodiorites predominantly along with tonalites and granodiorite [24,25,27,28]. Theses intrusive rocks are characterized by a medium-grained, granular to porphyritic texture consisting of plagioclase and variable amounts of pyroxene, amphibole, biotite and Fe-Ti oxides, with quartz and K-feldspar occurring as interstitial minerals. Radiometric data (Rb-Sr in biotite) yielded The felsic dike complex consists of a major NW trending intrusive body, up to hundreds of meters wide and several km long, which crosscuts all the other rocks in the area ( Figure 2). Alfieris et al. [26] described acid dikes (e.g., quartz ± biotite porphyries) from the south-eastern part of the study area, with disseminated pyrite and base metal veins mineralization.
The Leptokarya pluton, forms part of a NE-trending intrusive belt that includes the Kassiteres, Kirki, Leptokarya and Tris Vrisses intrusions ( Figure 1) and consists of quartz monzogabbros/monzodiorites predominantly along with tonalites and granodiorite [24,25,27,28]. Theses intrusive rocks are characterized by a medium-grained, granular to porphyritic texture consisting of plagioclase and variable amounts of pyroxene, amphibole, biotite and Fe-Ti oxides, with quartz and K-feldspar occurring as interstitial minerals. Radiometric data (Rb-Sr in biotite) yielded ages of 31.4 ± 0.7 to 31.9 ± 0.5 Ma [24]. Recently, Perkins et al. [28] provided a high precision age of 32.235 ± 0.026 Ma, based on Pb-U geochronology, for the Leptokarya pluton.
Finally, the sedimentary sequence in the area comprises marls and sandstones and is characterized by different degrees and styles of hydrothermal alteration.

Materials and Methods
A total of 33 rock and mineralized samples were collected from the Aisymi-Leptokarya prospect for petrographic, mineralogical and geochemical studies. From these rock samples, 10 thin sections, 16 polished-thin sections and 51 polished sections were made and underwent detailed mineralogical investigation using a Leica DM2500P optical microscopy mounted with a Nikon camera. All samples were pulverized using a tungsten carbide mill at −70 µm grain size and were scanned by X-ray diffraction, using a Bruker (Siemens, Munich, Germany) D-5005 X-ray diffractometer between 3 • and 65 • 2θ with 1 • /min velocity using the Ka radiation of a Cu X-ray tube operated at a voltage of 40 kV and a current of 40 mA, in conjunction with the DIFFRACplus software. Results were evaluated using EVA 10.0 software at the Department of Mineralogy and Petrology, National and Kapodistrian University of Athens. Bulk chemical analyses of 12 samples (prepared according to the PUL85 analytical protocol of ACME) were performed using an XRF (X-Ray Fluorescence) instrument using the method LF700 (2015) for major elements and LF100 (2015) for trace elements at the ACME Laboratories, Canada. Trace elements for all samples were measured using ICP-MS (Inductively Coupled Plasma Mass Spectrometry) method AQ200 (2015) by ACME, Canada. Electron Probe Micro Analyses (EPMA) analyses of sulphides and sulfosalts in ten polished sections were carried out using a Superprobe JEOL JXA 8200 electron-microprobe (Eugen F. Stumpfl laboratory, Leoben University, Leoben, Austria). The operating conditions were: accelerating voltage 20 kV, beam current 10 nA, beam size ≈ 1 µm. Counting times were 20 s on the peak and 10 s on the right and left backgrounds for major elements. The counting times were increased up to 60 and 30 s for peak and background, respectively, to analyse the trace elements. The minerals were analysed using pure metals as the reference material for Ge, In, W and Re, synthetic Bi 2 Te 3 , GaAs, AgBiSe 2 , cassiterite and electrum for Bi, Te, Ga, Se, As, Sn and Ag. Natural pyrite, chalcopyrite, sphalerite, molybdenite, stibnite and galena were used for Fe, Cu, Zn, Mo, Sb, Pb and S. In the following, the analytical lines and the diffracting crystals are listed for each analysed element: S = Kα, PETJ; Ag, Sb, Mo = Lα, PETJ; Bi, Re = Mα, PETJ; Fe, Cu, Zn = Kα, LIFH; As, Se, Ge = Lα, TAP; Te, In, Sn = Lα, PETH; Pb = Mα PETH; W = Lα, LIF; Ga = Kα, LIF.
All maps were made using ArcGIS 10.4 by ESRI.

Petrology and Geochemistry of the Aisymi-Leptokarya Magmatic Rocks
The Aisymi-Leptokarya felsic dike complex is texturally inhomogeneous and consists of at least two variants, both of which are hydrothermally altered: (a) a strongly silicified and sericitized equigranular to porphyritic variety, dominated by subhedral to anhedral quartz, K-feldspar (replaced by kaolinite) and sericite (pseudomorph after primary biotite) (Figure 4a). Spherulitic aggregates of quartz and K-feldspar are commonly present (Figure 4a), together with graphic textures of quartz and orthoclase; (b) a porphyritic variety, which includes quartz and K-feldspar phenocrysts embedded within a microcrystalline matrix of quartz and possible feldspars (Figure 4b). This variety resembles the acid dike rocks described by Alfieris et al. [26] and mostly occurs along the south-eastern margin of the Aisymi-Leptokarya felsic dike complex.
The Leptokarya pluton is a medium-grained, equigranular to porphyritic quartz monzodiorite, that contains plagioclase and variable amounts of pyroxene, amphibole, biotite and Fe-Ti oxides. Quartz and K-feldspar occur as interstitial minerals. In places, the monzodiorite is characterized by different styles of hydrothermal alteration (e.g., potassic alteration, Figure 4c). et al. [26]), with some samples plotting in the trachyandesitic field, possibly due to hydrothermal alteration ( Figure 5a). Accordingly, these rocks can be regarded as microgranite porphyries. In the Zr/TiO2 versus Nb/Y diagram samples from the Leptokarya pluton plot in or near the andesite field. Accordingly, the Leptokarya pluton is a monzodiorite. In the Rb versus Y + Nb diagram [30,31], the magmatic rocks are mostly characterized by post-collisional affinities (Figure 5b). Compositional variations in the felsic dike samples, could be attributed to hydrothermal alteration, which could result in erroneously low values of Rb for these rocks.  Chemical compositions of the Aisymi-Leptokarya magmatic rocks are summarized in Table 1. Due to strong hydrothermal alteration, discrimination diagrams for granitoids are not suitable for the studied samples. Using the immobile element Zr/TiO 2 versus Nb/Y diagram [29] the felsic dike samples show a rhyodacitic to dacitic composition (in accordance with previous studies by Alfieris et al. [26]), with some samples plotting in the trachyandesitic field, possibly due to hydrothermal alteration ( Figure 5a). Accordingly, these rocks can be regarded as microgranite porphyries. In the Zr/TiO 2 versus Nb/Y diagram samples from the Leptokarya pluton plot in or near the andesite field. Accordingly, the Leptokarya pluton is a monzodiorite. In the Rb versus Y + Nb diagram [30,31], the magmatic rocks are mostly characterized by post-collisional affinities ( Figure 5b). Compositional variations in the felsic dike samples, could be attributed to hydrothermal alteration, which could result in erroneously low values of Rb for these rocks.  The microgranite porphyry and the Leptokarya monzodiorite display different chondritenormalized rare earth element (REE) (Figure 5c) and mantle-normalized trace element patterns (Figure 5d). The microgranite samples have subparallel chondrite-normalized REE patterns and show strong light REE enrichment (La/Sm = 3.14-6.37) but almost unfractionated heavy REEs (Tb/Y = 0.74-1.12). They also display a negative Eu anomaly (Eu/√(Sm + Gd) = 0.37-0.58). The Eu anomalies are chiefly controlled by feldspars, particularly in felsic magmas, thus the removal of feldspar from felsic melt by crystal fractionation or partial melting of the rock will give rise to a negative Eu anomaly. The monzodiorite samples have subparallel patterns with the microgranite porphyry but are more enriched in all REEs.
In Figure 5d, the samples of microgranite display enrichment of LILE relative to HSFE, negative anomalies of Nb, Ta, P, Ti and positive anomalies of K, Zr and Pb, typical for convergent boundaries magmatism, which is consistent with the findings in previous studies for igneous rocks in the Evros area by Christofides et al. [34]. Compositional variations in the microgranite samples within for same element, can be attributed to different degrees of hydrothermal alteration (e.g., Ba, Rb). The The microgranite porphyry and the Leptokarya monzodiorite display different chondrite-normalized rare earth element (REE) (Figure 5c) and mantle-normalized trace element patterns (Figure 5d). The microgranite samples have subparallel chondrite-normalized REE patterns and show strong light REE enrichment (La/Sm = 3.14-6.37) but almost unfractionated heavy REEs (Tb/Y = 0.74-1.12). They also display a negative Eu anomaly (Eu/ √ (Sm + Gd) = 0.37-0.58). The Eu anomalies are chiefly controlled by feldspars, particularly in felsic magmas, thus the removal of feldspar from felsic melt by crystal fractionation or partial melting of the rock will give rise to a negative Eu anomaly. The monzodiorite samples have subparallel patterns with the microgranite porphyry but are more enriched in all REEs.
In Figure 5d, the samples of microgranite display enrichment of LILE relative to HSFE, negative anomalies of Nb, Ta, P, Ti and positive anomalies of K, Zr and Pb, typical for convergent boundaries magmatism, which is consistent with the findings in previous studies for igneous rocks in the Evros area by Christofides et al. [34]. Compositional variations in the microgranite samples within for same element, can be attributed to different degrees of hydrothermal alteration (e.g., Ba, Rb). The monzodiorite, displays quite the opposite geochemical features, as it appears enriched in HSFE and relatively depleted in LILE, always compared to the microgranite samples.

Mineralization and Alteration
Mineralization comprises: (a) porphyry-type quartz stockworks (A-and D-type veins according to Seedorf et al. [35] in the microgranite porphyry and partly in the monzodiorite; (b) sulphide disseminations hosted in both magmatic rocks; and (c) base metal veins (E-type veins), which crosscut the previous styles of mineralization.  Table 2). In places, D-type veins are associated with phyllic alteration (Figure 4b,f; Table 2) and overprint earlier style of mineralization and alteration. Localized potassic-sodic alteration characterized by hydrothermal biotite and albite (Figure 4c) is also present within the Leptokarya monzodiorite.
Porphyry-style mineralization consists mainly of pyrite, molybdenite, magnetite, kesterite, bismuthinite and galena within the A-type veins and as disseminations in the rock matrix. Pyrite is present in anhedral crystals in all samples, whereas molybdenite occurs as isolated flakes in both the quartz veins and the microgranite (Figure 6a). Kesterite, (Cu 2 (Zn,Fe)SnS 4 ), a relatively rare member of the stannite group, is also present in porphyry-style veins. It is included in pyrite either as isolated grains, or associated with bismuthinite (Figure 6b,c). Bismuthinite forms small blebs in pyrite (Figure 6d). D-type veins host pyrite mineralization associated with sericitic alteration in both the microgranite and the monzodiorite. In the elevated part of the prospect, oxidation of ore minerals, most probably argentiferous galena, resulted in the formation of supergene acanthite (Figure 6e).

Mineral Chemistry
Molybdenite is characterized by varying rhenium concentration from below detection to 0.3 wt.%. Electron-microprobe data of molybdenite compositions are presented in Table 3 and all data are plotted in a binary Re versus Mo diagram (Figure 7). Minor amounts of Sn and W were also identified (up to 0.08 and 0.02 wt.% respectively). Finally, traces of Se (up to 0.04 wt.%) substitute S in the structure of the molybdenite. Late-stage, NE-and NW-trending, epithermal-style (E-type), milky quartz veins with base metal sulphides crosscut all earlier-formed vein generations and are associated with sericite-rich alteration assemblages (Figure 3e,f). Mineralization is composed of early pyrite followed by sphalerite and then by galena (Figure 3l). Fahlores (zincian tennantite) and minor chalcopyrite are also present.

Mineral Chemistry
Molybdenite is characterized by varying rhenium concentration from below detection to 0.3 wt.%. Electron-microprobe data of molybdenite compositions are presented in Table 3 and all data are plotted in a binary Re versus Mo diagram (Figure 7). Minor amounts of Sn and W were also identified (up to 0.08 and 0.02 wt.% respectively). Finally, traces of Se (up to 0.04 wt.%) substitute S in the structure of the molybdenite.
Kesterite revealed almost stoichiometric composition, with a minor substitution of Sn by In (values up to 0.70 wt.%) (Figure 8).
Galena is generally characterized by very low values of trace elements (Se up to 0.05 wt.% and Sb p to 0.02 wt.% respectively (Table 4).
Sphalerite has very low Fe content (up to 1.0 wt.%) and is generally devoid of any trace elements (Table 4).

Bulk Ore Geochemistry
The Aisymi-Leptokarya prospect has not yet been drilled or has been the subject of exploration utilizing modern techniques. Preliminary bulk ore analyses of surface samples from the porphyry-style veins revealed a significant enrichment in Mo, with values up to 215 ppm (Table 5), with a mean of 110 ppm. It must be noted that elevated Mo concentrations lacks correlation with any other element such as Cu or Au, which characterizes mineralization elsewhere in the nearby area [15]. Selenium is also enriched, up to 29 ppm, whereas Bi and Sn display relatively elevated concentrations (up to 8 and 14 ppm, respectively). Gold and Ag content is rather insignificant.
The base metal veins are characterized by enrichment in Bi and Sn, with values up to 16 and 17 ppm, respectively. Mo displays elevated concentration as well (up to 20 ppm). Au and Ag values reach up to 24 ppb and 9 ppm respectively.

Genetic Implications
This study demonstrates a new porphyry-style prospect in the Kassiteres-Leptokarya magmatic suite, located in the Aisymi-Leptokarya area. The studied mineralization is hosted in an Oligocene felsic dike complex, which intrudes into upper Eocene volcanoclastic sediments and the Leptokarya pluton. The felsic dike complex displays a rhyodacitic to dacitic composition with post-collisional geochemical signature. Mineralization is composed by porphyry-style quartz stockwork in the felsic dike complex and is associated with potassic alteration, dominated by K-feldspar, which grades outwards into propylitic assemblages. Early alteration and mineralization are overprinted by D-type veins and late-stage, base metal veins associated with quartz-sericite±carbonate alteration assemblages. Ore minerals within the porphyry stockworks consist mainly of pyrite, molybdenite, kesterite, bismuthinite and galena. Late base-metal veins are dominated by sphalerite, galena, chalcopyrite and tennantite.
The absence of drilling data and the altered nature of the analysed samples, make it difficult to classify the studied mineralization. Preliminary bulk ore analyses showed that the Aisymi-Leptokarya porphyry deposit is characterized by an enrichment in Mo, which lacks correlation with any other commodities (e.g., Cu, Au), thus it should be possibly considered as a porphyry-Mo prospect.
The magmatic rocks hosting the Aisymi-Leptokarya porphyry Mo deposit were emplaced in a back-arc geotectonic regime according to Menant et al. [36].
A post-subduction tectonic environment of porphyry formation has been recognized for porphyry deposits worldwide [37] and has implications for some porphyry molybdenum deposits such as the 27 Ma old Quartz Hill, Alaska (USA) [38], the 33 Ma old Mačkatica (Serbia) [39], the 18 Ma old Pinarbaşi (Turkey) [40], the 4 Ma old Malala (Indonesia) [41] and the Shapinggou and Tianmugou, China, which were formed in a post collisional tectonic setting [42].
These post-subduction porphyry Mo deposits, as well as the studied Aisymi-Leptokarya porphyry Mo deposit, share common features with the arc-related, or Endako-type and the collision to post-collision-related, or Dabie Shan, porphyry Mo deposits, as for example the low Nb and Rb contents in the causative intrusion [43]. Despite its post-subduction geochemical affinities, it cannot be classified as Climax-type porphyry Mo deposit, since the hosting intrusives display no features of anorogenic magmatism.
The Re anomalies in the analysed molybdenite are in accordance with the observations of Voudouris et al. [15], who reported moderate to extremely-high Re content in molybdenites from a number of porphyry-type deposits in Greece.
The enrichment of Bi, Pb and Zn in the polymetallic veins in the studied prospect is also a common feature of many porphyry Mo deposits worldwide [38]. Furthermore, these deposits are known to be enriched in elements like Sn, Bi, Zn, Pb and Ag [38,44]. This is consistent with the presence of kesterite, bismuthinite, sphalerite and galena in the studied prospect.
Aplitic textures in the microgranite porphyry, which include micrographic and spherulitic quartz-feldspar intergrowths, are common in many porphyry Mo deposits [38,45]. According to Taylor et al. [38] the formation of aplitic texture is regarded as a quenching feature because of rapid ascent and loss of volatiles from the magma due to rapid crystallization of the viscous melt. Candela [45] considered textural variants like those present at Aisymi-Leptokarya microgranite (i.e., porphyritic, aplitic, micrographic and dendritic, acicular and/or skeletal crystals) to be indicative of undercooling, volatile exsolution and ore formation.
Recently, Perkins et al. [28], extending previous studies of Del Moro et al. [24], classified the magmatic rocks of the Maronia-Leptokarya corridor, into two, temporally and geochemically distinct, magmatic complexes, the Kassiteres-Leptokarya magmatic suite and the Maronia plutonic complex respectively. Both complexes comprise magmatic rocks with calc-alkaline to shoshonitic compositions and post-collisional affinities. Porphyry-style Cu-Mo-Au-Re mineralization, has been recognized in both magmatic complexes and has been attributed to three mineralizing events [15,16]. These events are related to granodiorite-and microdiorite porphyries in the Kassiteres-Leptokarya complex (e.g., the Pagoni Rachi and Konos Hill porphyry-epithermal deposits) and to granite porphyry in the Maronia complex.
The igneous rock hosting the studied mineralization is a microgranite porphyry, similar in composition to that hosting the Maronia porphyry Cu-Mo deposit. However, the studied porphyry event lacks any enrichment in Au, Cu and Re.

Conclusions
The newly-described porphyry Mo prospect in the Aisymi-Leptokarya area, is hosted within a hydrothermally-altered microgranite porphyry and to a lesser extent in the Leptokarya monzodiorite, emplaced in a back-arc geotectonic regime. Hydrothermal alteration includes potassic rimmed by propylitic, overprinted by silicic/sericitic assemblages. Mineralization is found in the form of quartz stockwork veins as well as disseminations in the host rocks. It comprises pyrite, molybdenite, kesterite, bismuthinite and galena. Base metal veins overprint and occur peripherally to the porphyry style mineralization and are associated with sericitic alteration. Bulk ore analyses from porphyry-style mineralized samples yielded very high Mo values. Se, Bi, Sn are also enriched, while the base metal veins are characterized by enrichment in Bi, Sn and Ag.
The studied mineralization shares similarities to other porphyry Mo deposits emplaced in post-subduction settings elsewhere.
Author Contributions: E.G. assisted by P.V. and C.M mapped the study area and collected the samples. E.G. and F.Z. obtained the mineralogical and geochemical data which were evaluated by P.S., V.M., C.H. and D.A. Finally, E.G., P.V. and C.M. wrote the manuscript. This paper is part of first author's PhD project.
Funding: This research was partially funded by the University of British Columbia, Mineral Deposit Research Unit under the Western Tethyan Metallogeny Project.