Mineralogical, petrographic and geochemical study of hydrothermal alteration zones and related porphyry-epithermal mineralization at Sapes-Kirki area, NE Greece
Authors: Μαυρογόνατος Κωνσταντίνος 
Published: September 2021

Detailed field work (mapping and sampling) at the broader Sapes-Kirki area took place focusing on the porphyry/epithermal mineralization centers of Konos and Pagoni Rachi. Beyond these already known occurrences, new mineralized centers were also discovered (e.g., Papadokoryphi). The study area is occupied by volcanosedimetary rocks, set in E-trending tectonic basins, that were created by extensional tectonism due to a slab roll-back since Eocene times. Further to the North of the study area, metamorphic rocks of the Rhodope Massif predominate, while to the South, metamorphic rocks of the Circum-Rhodope belt occur (e.g., Makri Unit etc.). The studied mineralization is hosted in sub-volcanic to intrusive bodies of Oligocene age, which are characterized by intermediate to acidic composition and high-K calc-alkaline geochemical affinities. Three distinctive intrusive phases were identified in the study area. The granodiorite porphyry consists of biotite-hornblende-augite phenocrysts along with a few resorbed quartz phenocrysts, set in a matrix of fine-grained feldspar and quartz. It hosts most of the mineralized zones and usually suffers from severe hydrothermal alteration. The next intrusive phase, which mostly crops out in the northern part of the study area, is a monzodiorite with a characteristic equigranular texture composed by varying amounts of amphibole, pyroxene, biotite, and feldspar. Finally, the latest intrusive phase in the area comprises small bodies of microgranite porphyry, set along a N0-20°W-trending direction. These bodies commonly exhibit a severe phyllic alteration overprint. Geochemical analyses (of fresh or least-altered) granodiorite porphyry and monzodiorite show a pronounced calc-alkaline signature with post collisional affinities. The porphyry-style mineralization at Konos area is located along a tectonic line NΒ10°W and occurs as a very dense stockwork of quartz veins. At the deeper parts of the system, relict sodic (albite- and chlorite-dominated) alteration zones occur, mostly overprinted by sericite. Quartz veins (mostly A-, and B-type veins) appear banded at places, comprising fine alternations of black and milky quartz. They host pyrite, chalcopyrite, Re-rich molybdenite and rheniite, along with minor quantities of magnetite, enargite, colusite, sphalerite, galena, and tetrahedrite/tennantite. Many quartz veins have been re-opened at a later stage (D-event), and pyrite was deposited forming a center line, in association with pervasive sericitic alteration. At places, gypsum veins are also associated with sericitic alteration and host Se-bearing galena, pyrite, sphalerite, and tetrahedrite/tennantite. Porphyry-style mineralization at Konos is characterized by anomalies in Mo and Re. It is severely overprinted by a high-sulfidation epithermal system (lithocap), which comprises silicification zones and widespread alunite-rich alteration assemblages, also containing kaolinite, dickite, diaspore, pyrophyllite and zunyite. The porphyry-style mineralization at Papadokoryphi area was discovered during the present study. It occurs in the form of quartz veins (stockwork) and disseminations set in potassic-altered (secondary biotite) granodiorite porphyry, and to a lesser extent in monzodiorite. The veins carry magnetite, pyrite, chalcopyrite and minor molybdenite, while bulk ore geochemical analyses revealed high concentrations of Mo (e.g., 150 ppm). The Pagoni Rachi porphyry-style mineralization develops in the form of an impressive stockwork, composed of various generations of veins. Potassic, sodic and calcic alteration assemblages predominate in the lower topographic levels of the system, while a phyllic overprint is widespread in the higher parts. The main ore minerals of the Pagoni Rachi system are pyrite, chalcopyrite, bornite, Re-rich molybdenite, rheniite, native gold. Four major porphyry vein types were identified: early magnetite veinlets (M-type) and banded A-type quartz veins are associated with sodic/calcic-potassic alteration of the granodiorite porphyry; linear, B-type quartz veins with sharp walls, associated with sodic/potassic alteration; and finally D-type, massive pyrite (+ chalcopyrite + molybdenite) veins, accompanied by intense sericitic alteration. The porphyry system is locally overprinted by epithermal-style, quartz-calcite veins and faults (E-type veins), associated with lateral argillic alteration of the host rocks. Main ore minerals of this stage are galena, sphalerite, pyrite Ag-Au alloy and tennantite/tetrahedrite (Cu-excess and Zn-rich varieties). Peripheral to the porphyry centers, propylitic alteration assemblages occur, composed by varying amounts of albite, epidote, amphibole, chlorite, and calcite. Mineral-chemical analyses of magmatic and hydrothermal biotite (form the Pagoni Rachi prospect, as well as from Papadokoryphi and the nearby Koryphes prospect, for comparison) revealed significant differences: Hydrothermal (secondary) biotite is Mg-rich (phlogopite), it displays varying amounts of Ti and high concentrations of halogens (up to 5.21 wt% F and up to 0.36 wt% Cl). In contrast, magmatic (primary) biotite is typically iron-rich) and commonly carries a high-Ti contnet. LA-ICP-MS analyses, conducted for the first time in hydrothermal biotite from porphyry systems in Greece revealed a relative enrichment in elements like Cs, La, Th etc. Especially the Pagoni Rachi hydrothermal biotite, appears to be slightly enriched in REE’s, compared to secondary biotite form the Papadokoryphi and Koryphes porphyry-style prospects. Significant F values (up to 0.8 apfu) were measured in secondary calcic amphiboles while mineral-chemical analyses of epidote from various alteration zones (e.g., calcic/potassic, propylitic) yielded typical clinozoisite compositions, with no systematic differences. This fact was also remarked for chlorites from different alteration zones, except for some variation of the Fe content in chlorites from the calcic/potassic assemblages. Native gold was not remarked in the Konos Hill porphyry system, however, at the Pagoni Rachi prospect, it is present in various forms and mineralization stages: Early M-type and A-type veins carry electrum (aver. 78% at. Αu), D-type veins host native gold (aver 89% at. Au), and finally epithermal-style veins carry Ag-Au alloy (aver. 30% at. Au). The rare sulfide rheniite was identified in both porphyry prospects (Konos Hill and Pagoni Rachi), along with Re-rich molybdenite. (up to 2.20 and 3.15 wt% Re, at Konos Hill and Pagoni Rachi, respectively). At Pagoni Rachi, the Re content correlates with gold, and displays an important enrichment in the D-type veins, compared to the other vein types. Chalcopyrite is always present with stoichiometric compositions. Enargite, sphalerite, galena, bismuthinite, colusite and tetrahedrite/tennantite, occur mostly in the epithermal-style veins as well as disseminations, overprinting/postdating the porphyry-style minerals. Bulk ore geochemical analyses showed an enrichment of the porphyry-style ores in a series of precious and critical metals like Au, Ag, Bi, Te, Se, Mo, Re, In, Sn and V, with prominent correlations between Mo-Re and Au-Se.The Konos Hill porphyry system is characterized by very low gold grades (up to 0.12 g/t), however, the gold grade of the Pagoni Rachi prospect is very high (0.36 g/t), partly due to the very high gold content of its D-type veins (up to 6.3 g/t Au). In contrast to the porphyry-style ores, the epithermal-style mineralization at the study area is characterized by enrichment in elements like Ag, Ga, In, Sn, Te, Bi, Se και V. In an attempt to identify any spatial/genetic correlations between the various ore stages and alteration assemblages, that could be used as exploration tools, part of this dissertation focused on the advanced argillic alteration zone that crops out at the western part of the study area. The Konos Hill porphyry system is telescoped by an advanced argillic lithocap that is located at the higher topographic levels of the area and hosts high sulfidation epithermal mineralization. Vuggy and massive silica zones at the central part of the lithocap trend N-S, NNW-SSE and E-W and grade outwards in assemblages rich in advanced argillic alteration minerals. Towards the lower topographic levels, the alteration is dominated by sericite. Detailed mineralogical and petrographic investigation, revealed the presence of the rare mineral zunyite (described here for the first time in a porphyry-related lithocap in Greece), along with alunite supergroup minerals (from which florencite-Ce is described for the first time in Greece), kaolinite, dickite, diaspore and pyrophyllite. Mineral chemical analyses of zunyite revealed varying F and Cl contents while the members of the alunite supergroup (also including APS minerals) are characterized by a great chemical variation, covering the range of all three subgroups (i.e. alunite, beudantite and plumbogummite). Bulk ore geochemical analyses from the HS-style mineralization at Konos Hill revealed a relative enrichment in Se, Mo, and Bi, elements that are known to reflect a magmatic contribution to the system, thus suggesting a genetic link between the epithermal-style event and the porphyry-style mineralization. Trace element geochemistry of magnetite was tested as a potential exploration tool at the Pagoni Rachi porphyry prospect. Magnetite is a common mineralogical constituent at Pagoni Rachi and occurs in two distinct modes: (i) as accessory magmatic/primary mineral in fresh to slightly propylitised granodiorite porphyry, and, (ii) as an important alteration mineral formed during the early stages of porphyry-style mineralization (such type of magnetite was not remarked at the Konos Hill porphyry prospect). Counting on the ability of magnetite to incorporate a plethora of trace elements, LA-ICP-MS analyses were used to identify any geochemical differences in magmatic and hydrothermal magnetite for the first time from a porphyry system in Greece. Detailed textural and geochemical investigation revealed that the two types of magnetite display significant differences. Primary (magmatic) magnetite usually occurs as disseminated euhedral grains in the matrix of fresh granodiorite porphyry. In contrast, the secondary (hydrothermal) magnetite forms fine-grained anhedral crystals that occur either as disseminations or, most commonly, as tiny veinlets crosscutting sodic/calcic-potassic altered granodiorite porphyry, and is associated with ore minerals like pyrite, chalcopyrite, molybdenite and electrum. Trace element analyses showed that hydrothermal magnetite is enriched in elements like V, Pb, W, Mo, Ta, Zn, Cu, Nb, in contrast to its magmatic counterpart, which carries mostly Ti, and minor Cr, Ni, and Sn. Especially the decrease of the Ti content in the hydrothermal magnetite is a common feature in many porphyry deposits and marks the transition from the magmatic to the hydrothermal environment. An uncommon feature for porphyry-related magnetite is its relatively high Mo, Pb and Zn content. This fact emerges as a unique feature of hydrothermal magnetite from the Pagoni Rachi prospect and can be further evaluated as a potential exploration tool, since magnetite-rich zones spatially coincide with that the high-Au part of the system. An analogous trace element geochemical study focused on another very common mineral at both Pagoni Rachi and Konos Hill systems. Pyrite, which is present throughout the depositional history of the systems, displays significant textural and geochemical differences. From a morphological standpoint, in the porphyry-style stages, pyrite forms fine, anhedral grains, occasionally with a spongy texture or rich in small inclusions of pyrrhotite and chalcopyrite. In contrast, epithermal-style pyrite forms coarse-grained, euhedral crystals. Geochemical analyses of different pyrite generations through LA-ICP-MS, revealed different enrichment patterns between the porphyry and the epithermal pyrite. Porphyry-style pyrite is generally depleted in trace elements, except for Co, Se, Cu, and minor Zn. In contrast, epithermal-style pyrite is enriched in As, Bi, Pb, Ni και Se. Gold is present in the form of non-stoichiometric substituting element and not as submicroscopic inclusions. Its values reach up to 4ppm in pyrite from D-type veins at Pagoni Rachi. The increase of As content in pyrite from the D event towards the E event comes in agreement with remarks form other porphyry mineralization elsewhere, however at Pagoni Rachi, a distinctive feature is that the As content increases from the early M towards the D event as well, in accordance with the gold content of pyrite Overall, the identified chemical differentiations come in agreement with analogous remarks from porphyry/epithermal deposits elsewhere, fact the highlights the significance of pyrite chemistry as an exploration tool in the ore systems of Thrace. In addition, the enrichment of pyrite in Se seems to be higher than in any other porphyry deposit elsewhere, thus emerging as a unique geochemical feature that points towards high-Au zones at the studied systems (especially since geochemical analyses have already shown a close correlation between the two elements). Moreover, pyrite chemistry stands for (an at least partial) contribution of the acidic (microgranite porphyry bodies) magmatism to the mineralization, through the addition of elements like Sn, Ta, etc., thus recording a multi-stage evolution of the ore-forming systems. To summarize, the detailed geochemical and mineralogical research on porphyry and epithermal style mineralization in the study area highlights its promising precious and critical metal potential. The studied porphyry mineralization is characterized by an extreme enrichment in Re, which along with an enrichment in Mo and Au, seem to be their most important commodities. In contrast to other porphyry-style ore systems in Greece (e.g., Skouries, Chalkidiki peninsula), the studied systems are relatively Cu-poor (Cu sulfides are not the most common metallic minerals) and the dominance of pyrite is their most important characteristic. The abundance of pyrite and the presence of banded quartz veins have been recently described as important features of Au-only porphyry systems (e.g., Kişladağ, Turkey, Biely Vrch, Slovakia). Given the fact that (especially) the Pagoni Rachi prospect shares these similarities with other porphyry-Au systems elsewhere, as well as its relatively high gold grade (0,36 g/t based solely on surface data), it becomes obvious that the studied systems should become targets for drilling projects, as they may significantly increase the precious metal endowment of N. Greece.