Rare and Critical Metals in Pyrite, Chalcopyrite, Magnetite, and Titanite from the Vathi Porphyry Cu ‐ Au±Mo Deposit, Northern Greece

: The Vathi porphyry Cu ‐ Au±Mo deposit is located in the Kilkis ore district, northern Greece. Hydrothermally altered and mineralized samples of latite and quartz monzonite are enriched with numerous rare and critical metals. The present study focuses on the bulk geochemistry and the mineral chemistry of pyrite, chalcopyrite, magnetite, and titanite. Pyrite and chalcopyrite are the most abun ‐ dant ore minerals at Vathi and are related to potassic, propylitic, and sericitic hydrothermal alterations (A ‐ and D ‐ veins), as well as to the late ‐ stage epithermal overprint (E ‐ veins). Magnetite and titanite are found mainly in M ‐ type veins and as disseminations in the potassic ‐ calcic alteration of quartz monzo ‐ nite. Disseminated magnetite is also present in the potassic alteration in latite, which is overprinted by sericitic alteration. Scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry (LA ‐ ICP ‐ MS) analyses of pyrite and chalcopyrite reveal the presence of pyrrhotite, ga ‐ lena, and Bi ‐ telluride inclusions in pyrite and enrichments of Ag, Co, Sb, Se, and Ti. Chalcopyrite hosts bornite, sphalerite, galena, and Bi ‐ sulfosalt inclusions and is enriched with Ag, In, and Ti. Inclusions of wittichenite, tetradymite, and cuprobismutite reflect enrichments of Te and Bi in the mineralizing fluids. Native gold is related to A ‐ and D ‐ type veins and is found as nano ‐ inclusions in pyrite. Titanite inclusions characterize magnetite, whereas titanite is a major host of Ce, Gd, La, Nd, Sm, Th, and W.


Introduction
The group of rare metals includes Ag, Au, Be, Bi, Cd, Ce, Co, Ga, Ge, Hg, In, Li, Nd, Rb, Re, Sc, Se, Sr, Ta, Te, Th, U, V, W, Y, the REE-lanthanides, and the platinum group elements (PGE) [1][2][3][4]. These metals are characterized by low concentrations in various rock types and deposits, resulting in scarce occurrences of economically valuable concentrations [1][2][3][4]. The term "rare" metals is usually coupled with the terms "critical" or "strategic" metals, and the elements included in both groups largely overlap [2,3]. Rarity is a prominent geological factor while criticality reflecting industrial and market values and is defined by the importance of a specific metal to the high-technological industrial sector and by demand and supply [4]. Thus, the list of critical metals is subject to periodic assessment and review Figure 1. The geological setting of the Cenozoic magmatic rocks and related porphyry-type mineralization of the Vertiskos Unit (modified after [35,38,42,53,55,56]).

Regional Geology
The Vertiskos Unit is an NW-trending polytectonic and polymetamorphosed geotectonic and basement zone, stretching from the Greek-North Macedonian border to the Chalkidiki peninsula [57][58][59][60][61] (Figure 1). It consists of gneisses, schists, and amphibolites intercalated with marble. Metagabbros-metadiabases and A-type granites of the Paleozoic to Triassic ages are also present [58,59,62]. The Cenozoic igneous evolution is characterized by four magmatic pulses spanning from the Late Paleocene to the Pliocene and includes I-type plutonic, sub-volcanic, and volcanic rocks, associated with calc-alkaline to high-K-calcalkaline and shoshonitic affinities [56,62] (Figure 1). The second and the third magmatic pulses (Oligocene-Miocene) are related to mineralized intrusions, located mainly in the Kilkis and the NE Chalkidiki ore districts [37,53,56] (Figure 1).
The Vertiskos Unit is located in the hanging wall of the Eocene-Oligocene Kerdylion first-order detachment fault, while another first-order structure of the region is the Miocene Strymon detachment fault exposed mainly in SW Bulgaria [57,60]. These two detachment faults are associated with an extensional regime, lasting from the Late Cretaceous to the Miocene, and which resulted in the collapse of the Vertiskos Unit towards the SSW [55][56][57][58][59][60]. In the Vertiskos Unit, the extension is documented by ENE-WSW-and eastwardtrending shears and related to E-W-, NW-SE-, and NE-SW-trending normal to oblique faults and N-S-trending strike-slip faults [56,58]. The Cenozoic magmatic events and the formation of the Oligocene-Miocene mineralized intrusions were structurally controlled by these faults [53,56].
The mineralization is hosted by latite and is genetically associated with a quartz monzonite (18 ± 0.5 Ma and 17 ± 1 Ma; U-Pb zircon ages) [52,53], which intruded the crystalline basement rocks and the latite ( Figure 2). Both magmatic rocks are silica-saturated and exhibit calc-alkaline to slightly alkaline chemical affinities [52,53]. A phreatic breccia crosscuts the latite and is genetically related to the quartz monzonite intrusion, whereas a latite cataclasite located in the northern part of Ragian Hill 1 is associated with a late epithermal event [53] (Figure 2). Old exploration drilling projects at Vathi revealed the development of a ~100 m-deep oxidation zone, and a narrow secondary enrichment zone [53].
Quartz monzonite outcrops are rare but are characterized by a weak potassic alteration, whereas the latite was affected by a local propylitic alteration. An intense sericitic alteration overprinted these two alteration styles [51][52][53]. Locally, a subsequent epithermal overprint is observed in the latite. Mineralization forms disseminations, aggregates, and veins (M, A, and D types), while the metallic assemblage related to the epithermal overprint is accommodated in E-type veins [53].
The porphyry-style mineralization was developed in a shallow (<2.6 km in depth) environment from hydrothermal fluids with moderate to high salinities at a minimum temperature of 311 °C. The late epithermal overprint was associated with less saline fluids (1.4-2.9 wt.% NaCl equiv.) and lower temperatures ranging from 205 to 259 °C [53]. Surface samples contain as much as 9297 ppm Cu, 341 ppm Mo, 7 ppm Au, and 263 ppm U. In addition, an enrichment of La (up to 613 ppm), Ce (up to 894 ppm), and Nd (up to 211 ppm) is attributed to the formation of phreatic breccia [53].
Potassic-calcic alteration occurs in the quartz monzonite and is characterized by disseminations and M-type veins. These veins are up to 1 cm wide, continuous, sharp-edged, and branched ( Figure 3a). They exhibit a massive texture and consist mainly of magnetite and titanite (Figure 4a). This hydrothermal alteration is characterized by quartz, biotite, titanite, actinolite, rutile, and minor ilmenite and chlorite (Table 1).

Materials and Methods
Ten surface samples from the Vathi deposit were collected, with a focus on hydrothermal alteration styles and mineralization stages. In addition, a set of eight older surface samples and 16 drill core samples were evaluated for further mineralogical and geochemical study. The core samples are from two legacy exploration drilling projects that targeted the uranium and the porphyry-style mineralization conducted during the 1970s and 1980s [53]. Eight polished sections studied here were from drill core Η4 (43 to 166 m interval), and seven were from drill core Η5 (53 and 73 m interval) ( Figure 2). Description and characterization of the mineralogy and the alteration styles of the available drill cores can be found in Stergiou et al. [53].
Bulk geochemical analyses of eight samples were conducted for trace elements, including REE, by inductively coupled plasma atomic emission spectroscopy (ICP-AES) at MSALABS, Langley, Canada. Additionally, 19 previously published geochemical analyses were considered in this study (see Supplementary Table S1; [53]).
The samples were studied with an optical microscope and a JEOL JSM-840A scanning electron microscope (SEM) equipped with an OXFORD INCA 300 energy-dispersive system (EDS) (Oxford Instruments Ltd., Abingdon, UK) at the Faculty of Sciences, Aristotle University of Thessaloniki. The operating conditions were a 20 kV accelerating voltage and 0.4 mA probe current, 80 s analysis time, and a beam diameter of ≈1 μm, in the backscattering electron (BSE) mode. Internal empirical corrections were performed to compensate for peak overlap. SEM-EDS micro-analyses were conducted for the determination of iron and titanium contents used as internal standards in LA-ICP-MS analyses for pyrite, chalcopyrite, magnetite, and titanite (see Supplementary Table S2).
Following electron microscopy study, seven polished sections from the drill core samples and three from surface samples were selected to be analyzed by LA-ICP-MS. Trace element concentrations in pyrite, chalcopyrite, magnetite, and titanite were measured using a PerkinElmer ELAN DRC-e ICP mass spectrometer combined with a New Wave UP193-FX excimer laser ablation system at the Geological Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria. The ablation was conducted in He medium. In order to maximize sensitivity, the ICP-MS was optimized daily with respect to the oxide production rate of ThO/Th (0.5%). Operating conditions of the laser system include: 5 Hz repletion rate; 20 to 50 μm spot size; and energy density on analyzed minerals and standards of 5.0 J/cm 2 (at 35 and 50 μm spot) and 4.5 J/cm 2 (at 25 and 20 μm spot). The nebulizer gas flow rate was 0.8 L/min, while auxiliary and make-up gas flows rates were 0.92 L/min. The analysis time was 100 s (background: 40 s, laser-on the sample: 60 s). The acquisition dwell time was set to 0.02 s for 74 Ge, 82 Se, 115 In, 125 Te, and 202 Hg, to 0.03 s for 107 Ag, to 0.04 s for 197 Au, and to 0.01 s for all other elements. The monitored isotopes include: 27 Al, 31 P, 34 S, 49 Ti, 51 V, 53 Cr, 55 Mn, 57 Fe, 59 Co, 60 Ni, 65 Cu, 66 232 Th, and 238 U. The targeted areas in the polished sections were predefined to try and avoid mineral inclusions. Repeated external standardization was conducted by analyzing NIST SRM 610 and USGS GSD-1G glass standards, and the USGS Mass 1 sulfide standard.
Data reduction was performed by using the Fe contents of pyrite, chalcopyrite, and magnetite, as well as Ti contents of titanite measured by SEM-EDS as internal standards and the SILLS software [64]. During data reduction, peak-shaped fluctuations of the intensity signal of some isotopes were investigated in order to exclude the influence of other minerals on the chemical composition of the studied minerals. Several elements occur as nano-scale mineral inclusions in pyrite and chalcopyrite and as nanoparticles in magnetite, and in the following, they are described separately. For the full list of the LA-ICP-MS data, including elements ascribed to nano-scale mineral includions, see Supplementary  Table S3. Additionally, the average detection limits for the analyzed elements are given for each studied mineral in Supplementary Table S3.
For the statistical analysis of the bulk geochemistry and the LA-ICP-MS analytical results, univariate analysis (minimum, maximum, standard deviation, and average) and bivariate analysis (Pearsonʹs correlations) were obtained. The SPSS statistics software (IBM) was used for the statistical analyses. Statistical graphs and time-resolved analytical profiles of LA-ICP-MS analyses were plotted using Grapher software (Golden Software LLC).

Bulk Geochemistry of Rare and Critical Metals in Metallic Mineralization
Bulk geochemical analysis of mineralized samples was conducted in order to investigate their endowment in rare and critical metals. Eight surface samples were analyzed, five of which were from latite, one was from propylitic alteration (Vath 42), and four were from sericitic alteration (Vath 34, Vath 38, Vath 40, Vath 45). Among the three samples from quartz monzonite, Vath 41 is relatively fresh, while Vath 43 is from a D-type vein in sericitic alteration, and Vath 44 is from an M-type vein in potassic-calcic alteration. Table  2 includes the rare metal concentrations of these samples, and the average values for the same elements from previously published bulk geochemical analyses [53]. The complete analytical datasets for both groups are given in Supplementary Table S1.
Pearson product-moment correlation coefficients and associated p-values were calculated for the bulk geochemical analyses conducted in this study along with previously published geochemical analyses (Table 2; Supplementary Tables S1 and S4). The statistical analysis included all the elements listed in Table 1.

Pyrite and Chalcopyrite Mode of Occurrence
Pyrite at Vathi is ubiquitous, especially in potassic and sericitic alterations. It is found as disseminations and aggregates, and in veins (Table 1). In the potassic alteration of the quartz monzonite, pyrite occurs in A-type veins crosscutting the host rock ( Figure 4b). It is massive, filling the inner parts of the veins, and is intergrown with chalcopyrite and bornite. Native gold and galena are found in the interstices of pyrite (Figure 6a). In the hydrothermal halo around the A-type veins, pyrite disseminations and aggregates vary in size from 100 to 500 μm. In the potassic alteration of the latite, pyrite (Py1) forms subhedral to euhedral disseminations and rounded aggregates (<1 mm in size; Figure 6b). It occurs along with magnetite, chalcopyrite, and traces of bornite, molybdenite, and pyrrhotite ( Figure 6b).
In the sericitized quartz monzonite, pyrite (Py2) forms subhedral to euhedral disseminations and aggregates varying in size from 50 to 400 μm, and D-type veins (Figures 4c-e and 6c). Pyrite is intergrown with chalcopyrite, galena, native gold, and tetradymite. Galena (< 500 μm in size) is found in interstices or as replacements in disseminated pyrite, while chalcopyrite, native gold (Figures 4d and 6c), and tetradymite are found in micro-cracks and interstices in pyrite filling D-type veins. The sericitic alteration of latite hosts disseminations and aggregates of pyrite as well as massive pyrite in D-type veins (Figure 4f,g). Disseminated pyrite (Py3) is intergrown with chalcopyrite and locally hosts inclusions (<100 μm in size) of galena and pyrrhotite (Figure 6d; [53]). Pentlandite (<10 μm in size) was found in pyrrhotite. In Dtype veins, massive pyrite is the dominant sulfide, while minor chalcopyrite inclusions are also found.
In the E-type veins related to the epithermal overprint, pyrite is a minor constituent in assemblage 1, and a major constituent in assemblage 2 (Table 1). In assemblage 1, euhedral pyrite (<80 μm in size) is found as inclusions in galena and sphalerite. Locally, it hosts tetrahedrite inclusions (<10 μm in size; Figure 6e). In assemblage 2, massive pyrite (Py4) is intergrown with pyrrhotite filling the inner parts of the veins (Figure 4h). Euhedral pyrite is found in the periphery of the veins and hosts insclusions of galena (Figure 6f). Chalcopyrite is the second most abundant sulfide in the Vathi deposit (Table 1). In the potassic alteration of the quartz monzonite, chalcopyrite is found in A-type veins along with pyrite, native gold, and minor bornite and galena. In the hydrothermal halo around the A-type veins, chalcopyrite forms disseminations up to 200 μm in size ( Figures  4b and 7a). In the potassic alteration of the latite, chalcopyrite (Cpy1; <1 mm in size) overgrew magnetite and coexists with pyrite, bornite, molybdenite, and pyrrhotite (Figures 6b  and 7b).
In the sericitically altered quartz monzonite, chalcopyrite appears as disseminations (<50 μm in size) and as aggregates along with pyrite ( Figure 4e). Galena and tetradymite are found as inclusions (<100 μm in size) in chalcopyrite (Figure 7c). In the D-type veins, chalcopyrite occurs in fractures along pyrite (approximately < 50 μm in width; Figures 4e and 7d).
In the sericitic alteration of the latite, chalcopyrite (Cpy2) locally exhibits a more extensive distribution than pyrite and forms large aggregates reaching 1 cm in size. Chalcopyrite hosts inclusions of molybdenite, galena, sphalerite, bornite, pyrrhotite, and pentlandite (< 100 μm in size), as well as native gold, tennantite, tetrahedrite, wittichenite, fletcherite, and cuprobismutite, which do not exceed 50 μm in size (Figure 7e,f; [53]). In D-type veins, crosscutting the sericitic alteration of latite, chalcopyrite is a minor constituent. It is found in pyrite as inclusions reaching up to 100 μm in size.

Magnetite and Titanite Mode of Occurrence
Magnetite occurs in the potassic-calcic alteration of the quartz monzonite and in potassically altered latite (Table 1). In the potassic-calcic alteration, magnetite (Mag1) occurs as disseminations (<400 μm in size) and in M-type veins (<1 cm in width). It is intergrown with titanite and ilmenite (<100 μm in size; Figure 8a,b). In the M-type veins, massive magnetite is the dominant metallic mineral and includes titanite and ilmenite (Figures 4a  and 8c). In the potassic alteration in latite, magnetite (Mag2) occurs as disseminations and aggregates (<500 μm in size; Figures 6b and 7b). It is overgrown by chalcopyrite and hosts minor inclusions of pyrrhotite (<40 μm in size).
Titanite (Ttn1) is found in the potassic-calcic alteration of the quartz monzonite as disseminations and in M-type veins (Figure 4a). Disseminated titanite (<100 μm in size) appears mainly as subhedral crystals, as intergrowths with ilmenite, and as overgrowths on magnetite. In M-type veins, titanite (<400 μm in size) is included in massive magnetite (Figure 8b).

Trace Element Composition of Pyrite and Chalcopyrite
Trace elements in pyrite (Py1, Py2, Py3, Py4) and chalcopyrite (Cpy1, Cpy2, Cpy3) were measured by means of LA-ICP-MS. The trace element composition of pyrite and chalcopyrite exhibits notable variations between the different mineralization stages (Tables 3 and 4; Figure 9). In addition, ablation patterns reveal the occurrence of nano-scale mineral inclusions in pyrite. These inclusions are excluded from Tables 3 and 4. The complete analytical dataset for pyrite and chalcopyrite is given in Supplementary Table S3.

Trace Element Compositions of Magnetite and Titanite
Magnetite (Mag1, Mag2) exhibits variations between the analyzed samples, while titanite Ttn1 is characterized by distinctive geochemical patterns. Several rare metals were partitioned between coexisting magnetite and titanite, with titanite being a major host of REE (Table 5; Supplementary Table S3).

Statistical Analysis of Trace Element Concentrations
In order to further investigate the chemistry of pyrite, chalcopyrite, magnetite, and titanite, and the geochemical association of the detected and measured rare metals, Pearson product-moment correlation coefficients and associated p-values were calculated for the LA-ICP-MS analyses. Heat maps of the Pearson correlation coefficient matrix for pyrite, chalcopyrite, magnetite, and titanite are given in Supplementary Table S4. Based on data reduction and ablation patterns, trace element concentrations related to nano-scale mineral inclusions, or nanoparticles in the case of magnetite, were excluded before the statistical analyses.
Pearson product-moment correlation coefficients for pyrite are described by significant positive correlations (p < 0.01; values ≥ 0.883) defined for the elemental pairs Ag-Sb, Co-In, Co-V, Se-V, and Se-W (Supplementary Table S4 Table S4). Strong positive correlation coefficients (p < 0.01; values ≥ 0.960) occur for the elemental pairs Ce-La, Gd-Nd, Gd-Sm, Nd-Sm, Ge-Sb, Nd-V, and Sm-V in titanite (Supplementary Table S4), while smaller positive correlations (p < 0.05; values ≥ 0.884) occur between Ce and Ge, Nd, Th, and V, between Ge and La, Nd, Th, and V, and for the pairs Ga-Th, Gd-V, Nd-Th, Sm-Th, and Th-U (Supplementary Table S4). Significant negative correlations (p < 0.01; values ≥ −0.996) occur for the pairs Ge-W and Sb-W.
At the Vathi deposit, the hydrothermal alteration, the porphyry and epithermal-related mineralization stages, the bulk geochemistry, and the mineral chemistry reveal specific mineralogical and geochemical relations to various rare metals (e.g., Ag, Au, Bi, Co, Se, Te, and W; Table 2). The REE distribution and relative REE enrichment at Vathi are ascribed to monazite and titanite, and possibly to other rock-forming and alteration minerals (e.g., feldspar [53]). The relatively fresh and unoxidized quartz monzonite (Vath 41) includes the highest concentrations of Ce, Gd, La, Nd, and Sm, which are significantly correlated with each other (Figure 5; Supplementary Table S4). The comparison between the new and the previously published bulk geochemical analysis at Vathi emphasizes the effect of supergene processes on the enrichment of the rare metals (Table 2; Supplementary  Table S1; [53]).
The metallic and alteration assemblages related to potassic-calcic alteration and the complete absence of sulfide minerals suggest the highly oxidized nature of the initial fluids, as well as the high solubility of Fe and FeCl2 [30] (Table 1). This type of alteration exhibits the lowest enrichment of rare metals (Table 2; Figure 5). Tungsten, Ce, Th, Nd, and La are the most enriched rare metals at concentrations <75 ppm and are significantly correlated with each other (Supplementary Table S4). In Greece, M-type veins have been previously described from the Pagoni-Rachi, the Maronia, and the Stypsi porphyry deposits [33,49,66]. The M-type veins at Vathi are different in terms of the contained metallic and alteration assemblages and are not associated with the introduction of any hypogene mineralization, as is the case at Pagoni-Rachi [33,49,66].
The potassic alteration that was overprinted by sericitic alteration is associated with Atype veins in the quartz monzonite and with disseminations in the latite at Vathi (Table 1; Figures 4b, 6a and 7a). The formation of the A-type veins was a restricted event in terms of size and occurrence. According to Stergiou et al. [53], the formation of the A-type veins is related to boiling hydrothermal fluids at temperatures > 390 °C (646 bars; <2.6 km in depth). The A-type veins are related to a restricted introduction of gold in the Vathi magmatic-hydrothermal system (Table 1; Figure 6a).
The sericitic alteration is characterized by D-type veins and disseminations in both the quartz monzonite and the latite (Table 1; Figure 3d,e). The sericitic alteration and the associated metallic and alteration assemblages formed at temperatures ranging between 311 and 392 °C, from a moderately saline (8.4-11.2 wt.% NaCl equiv.) fluid [53]. Tetradymite and native gold are spatially related to the mineralization of the quartz monzonite (Figures 6c and  7c,d). Bismuth tellurides, Bi-sulfosalts, and Au-Ag-tellurides have been previously reported from calc-alkaline to alkaline porphyry deposits in northern Greece [67]. Tetradymite associated with sericitic alteration was recognized from the Koryfes porphyry deposit, whereas in the Papadokoryfi and Pagoni-Rachi porphyry deposits, tetradymite is found in the porphyryepithermal transition environment [38]. The occurrence of pyrrhotite, pentlandite, wittichenite, fletcherite, and cuprobismutite found in the latite at Vathi is uncommon or is rarely described for the sericitic alteration of the porphyry deposits found elsewhere in Greece [38]. At Vathi, pyrrhotite and pentlandite are found as inclusions along with chalcopyrite in pyrite disseminations and aggregates, while fletcherite, wittichenite, and cuprobismutite occur as inclusions in chalcopyrite aggregates (Figures 6d and 7e). These textural characteristics suggest a Ni, Co, Bi, and Ag enrichment and shifts in fO2/fS2 conditions for the mineralizing fluids of this stage. Shifts in sulfur fugacity caused by the interaction between the late-mineralizing fluids and carbonaceous wall rocks have been previously suggested for Vathi [53].
Bulk geochemical analysis showed that rare metals are enriched in the sericitic alteration (Table 2; Figure 5). Gold (0.8 ppm), Te (1.5 ppm), and Co (450 ppm) exhibit their highest enrichment in this alteration, while Se (2.8 ppm), Bi (3.4 ppm), and Ag (0.9 ppm) are also enriched ( Table 2). In addition, Pearson's correlation coefficients suggested significant correlations for the pairs Ag-Au, Au-Te, and Au-Se (Supplementary Table S4). These results verify the association among tetradymite and native gold. According to Voudouris et al. [38], the porphyryand epithermal-style gold mineralization in Greece is classified as follows for deposits where: (a) native gold and Au-Ag-tellurides are associated with Bi-sulfosalts and oxidized-type Bisulfotellurides; (b) native gold is related to Bi-sulfosalts; and (c) native gold and Au-Ag-tellurides are only present. Using this classification scheme, Vathi belongs to the first category.

Mineral Chemistry of Pyrite and Chalcopyrite and Nano-Scale Inclusions
Pyrite and chalcopyrite can be associated with significant amounts or rare metals incorporated as nano-scale mineral inclusions or as nanoparticles [13,14,[18][19][20][21][22]24]. These inclusions can be formed due to exsolution from the pyrite or chalcopyrite matrix, as well as through precipitation from the hydrothermal fluid into the growing mineral [14,24]. In pyrite, nano-scale mineral inclusions are mostly related to electrum, galena, tellurides, and cinnabar, whereas in chalcopyrite, sphalerite and micro-inclusions related to Zn-, As-, Pb-, and Au-bearing phases are reported [14,24]. The interpretation of the scanning electron microscopy and LA-ICP-MS analyses for pyrite and chalcopyrite at Vathi suggests that specific minerals occur as nano-inclusions.
All the pyrite stages (Py1-4) at Vathi are enriched with Ag, Co, Sb, Se, and Ti (Table  3; Figure 9a). In contrast, Hg, Te, and V are elevated in Py1, Py2, and Py3, whereas Bi and W are present in Py4 of the late-stage epithermal overprint. Indium is associated with Py1 in the potassic alteration and the epithermal overprint (Py4) (Figure 9a). It is suggested here that based on the ablation patterns, Co and Se are incorporated in pyrite as stoiciometric substitutions. Cobalt and Se are considered common stoichiometric substitutions for Fe and S, respectively [13]. In addition, restricted amounts of Ag, Bi, Hg, In, Sb, Ti, V, and W could be accommodated in the pyrite structure as solid solutions (Table 2; [13,24]). Based on the concentrations of some elements and the laser ablation patterns, several enrichments in trace elements of pyrite are attributed to nano-scale mineral inclusions (Supplementary Table S3; Figure 11). The rare metals related to inclusions comprise: Ag, Au, Bi, Cd, and In in Py1; Au, Bi, and Te in Py2; Bi, Co, Th, Ti, and V in Py3; and Ag, Bi, and U in Py4. Lead enrichment could be attributed to nano-scale inclusions of galena in Py2, Py3, and Py4. These trace elements could be associated with Bi-sulfosalts for Py1, with tetradymite for Py2, with pentlandite for Py3, and with galena for Py4 (Figure 11a-d). Furthermore, based on the concentrations of Au and Cu, nano-inclusions of native gold are accommodated in Py1 and Py2, and chalcopyrite is hosted in Py2. Chalcopyrite Cpy1 (potassic alteration; latite) and Cpy2 (sericitic alteration; latite), as well as chalcopyrite Cpy3 (E-type veins), contain enrichments of Ag, In, and Ti (Table 4; Figure 9b). Cadmium, Ga, and V are related to Cpy1, and Hg, Se, and Te were found only in Cpy2, while Bi and Co are associated with Cpy1 and Cpy2 (Figure 9b). Germanium and Sb were found in Cpy2 and Cpy3, while W was measured only in Cpy3 from the E-type veins. Based on the ablation patterns Ag, Bi, Cd, Co, Ga, Ge, Hg, In, Sb, Se, Te, Ti, V, and W are accommodated in chalcopyrite in solid solutions.
Concentrations of trace elements ascribed to nano-scale mineral inclusions in chalcopyrite include mainly Bi and Pb, while Ag, Cd, and Zn were attributed to inclusions in Cpy1 (Supplementary Table S3; Figure 12a,b). Zinc and Cd enrichments (Cpy1) are likely related to the presence of sphalerite nano-scale inclusions, while Bi and Pb (Cpy1, Cpy2), and Ag (Cpy1) are probably due the presence of Bi-sulfosalts (Figure 12a,b). Chalcopyrite Cpy3 exhibits minor Pb enrichments, probably due to galena inclusions. It is noted that the presence of Bi-sulfosalts in Cpy2 is also supported by the presence of wittichenite and cuprobismutite (Figure 7e).

Mineral Chemistry of Magnetite and Titanite and Nano-Scale Inclusions
Aluminum, Co, Cr, Ga, Mg, Mn, Ni, Ti, V, and Zn typically substitute primary cations in magnetite participating in a crystal lattice [15,16]. Additionally, Al, Ca, Mg, Na, Si, Ti, and V can be found as nanoparticles in growth or sector zones in magnetite [25]. Titanite is considered a major host of Zr, Hf, Nb, Ta, Ti, Sr, V, Sc, and REE [31]. These elements are found structurally bound in titanite [17,31,32]. The combined interpretation of the scanning electron microscopy and LA-ICP-MS analyses for magnetite and titanite at Vathi supports these conclusions.
Magnetite (Mag1) from the M-type veins related to the potassic-calcic alteration in the quartz monzonite and magnetite (Mag2) from the potassic alteration in latite are associated with rare metals including Co, Ga, Ge, Ti, and V, while W occurs in Mag2. These elements are likely incorporated as stoichiometric substitutions (Table 5; Figure  10a). Trace elements associated with nanoparticles include Ag, Bi, Ce, Cu, La, Pb, Th, and Ti for Mag1, and Bi, Ce, Sb, and U for Mag2 (Supplementary Table S3). Ablation patterns and SEM-EDS analysis support the concept that titanite is hosted as inclusions of various sizes in Mag1 (Figures 8b and 12c). The presence of Pb and S in Mag2 could be related to minor nano-scale galena inclusions.
Titanite (Ttn1) from the potassic-calcic alteration associated with quartz monzonite is enriched with Ce, Gd, La, Nd, and Sm and hosts several rare metals including Co, Ga, Ge, In, Sb, Se, Th, and V (Table 5). These elements constitute stoichiometric substitutions since nano-scale mineral inclusions were not detected (Figure 12d). their associated inclusions (highlighted in gray). Iron, In, Ag, and Sn, where present, exhibit similar distribution patterns to Cu, in Cpy1 and Cpy2: (a) in Cpy1, the spiky patterns of Zn and Cd suggest the presence of sphalerite as nano-scale inclusions, while the more spiky patterns for Bi, Pb, and Ag could be related to nano-scale inclusions of silver-rich Bi-sulfosalts; (b) in Cpy2, the curved patterns of Bi and Pb could be associated with Bi-sulfosalts found as nano-scale inclusions; (c) in Mag1, vanadium, Ni, and Ga follow similar distribution patterns to iron, while the spiky patterns indicate a titanite inclusion in magnetite; in the titanite inclusion, La, Ce, and thorium follow the distribution patterns of Ti; (d) in Ttn1, vanadium, Sn, La, Ce, Nd, Sm, Gd, and Th show similar distribution patterns. Mineral inclusions were not detected in titanite (Ttn1).

Conclusions
The Vathi porphyry Cu-Au±Mo deposit shows enrichements of rare and critical metals including Ag, Au, Bi, Cd, Co, Ga, Ge, Hg, In, Nb, Sb, Se, Te, Th, Ti, V, W, and REE (Ce, Gd, La, Nd, and Sm). Native gold occurs in A-type veins crosscutting the potassic alteration that is overprinted by sericitic alteration in quartz monzonite, and in D-type veins found in both the quartz monzonite and the latite. Native gold is more widespread in the sericitic alteration, where it occurs with tetradymite in the quartz monzonite, and with wittichenite and cuprobismutite in the latite.
Samples from the potassic and sericitic alterations show significant enrichments of rare metals including Ag, Au, Bi, Co, Se, Te, and W. Rare-earth element enrichment is not related to the porphyry and epithermal mineralizing events but is attributed mostly to rock-forming and supergene processes. Pyrite at Vathi is enriched with Ag, Co, Sb, Se, and Ti, while moderate and weak enrichments have been detected for Bi, Hg, Te, V, and W, and for In, respectively. Chalcopyrite contains elevated concentrations of Ag, In, and Ti and is moderately enriched with Bi, Cd, Co, Ga, Hg, Se, Te, V, and W, and weakly enriched with Nb. High concentrations of Co, Ga, Ge, Ti, and V and low concentrations of Se and W occur in magnetite. Titanite is a major host of REE including Ce, Gd, La, Nd, and Sm, as well as being a major host of Co, Ga, Ge, In, Sb, Se, Th, and W. Nano-scale mineral inclusions are hosted in pyrite and chalcopyrite, while enrichments of Ag, Bi, Ce, Cu, La, Pb, Th, and U are related to nanoparticles partitioning into magnetite. Nano-scale mineral inclusions are likely related to the presence of Bi-sulfosalts and native gold in pyrite in the potassically altered latite. In addition, chalcopyrite, tetradymite, and native gold in pyrite from the sericitic alteration in quartz monzonite, pentlandite in pyrite from the sericitic alteration in latite, and galena in pyrite in the epithermal overprint are also likely nano-inclusions. Nano-scale inclusions occur as sphalerite and Bi-sulfosalts in chalcopyrite from the potassic alteration in latite, and as Bi-sulfosalts in chalcopyrite from the sericitic alteration of the latite. Nano-scale inclusions of galena are accommodated in chalcopyrite from E-type veins. Inclusions of wittichenite, tetradymite, and cuprobismutite reflect enrichments of Te and Bi in the mineralizing fluids associated with sericitic alteration. Magnetite from the potassic-calcic alteration of the quartz monzonite hosts titanite inclusions, while minor amounts of nano-scale inclusions of galena likely occur in magnetite frοm the potassic alteration in latite. Titanite from the potassic-calcic alteration of the quartz monzonite contains no nano-inclusions.

Data Availability Statement:
The data presented in this study are available in the Supplementary Materials in Tables S1, S2, S3, and S4.