First zunyite-bearing lithocap in Greece: The case of Konos Hill Mo-Re-Cu-Au porphyry system.

Zunyite is a rare F- and Cl- bearing mineral related to advanced argillic alteration zones of porphyry/epithermal style mineralization and is considered as a pathfinder mineral towards high-grade Au ores. We report here the first occurrence of zunyite along with alunite, quartz, APS minerals, diaspore, pyrophyllite and kaolinite in the metallogenic province of Western Thrace. 
 The Konos Hill prospect in Western Thrace comprises a telescoped porphyry Mo-Re-Cu-Au system, overprinted by high-sulfidation mineralization. In low topographic levels, porphyry-style mineralization is exposed and comprises pyrite-chalcopyrite-bornite-molybdenite-rheniite-bearing quartz-stockwork. Host rocks are subvolcanic bodies of granodioritic composition that have suffered pervasive sericitic alteration. High-sulfidation epithermal-style alteration occupies the higher topographic levels and has caused significant overprinting of the porphyry-style mineralization and alteration. It consists of silicified zones related to N-S and E-W trending faults, which grade outwards to advanced argillic alteration assemblages. These assemblages are characterized by abundant alunite and quartz, with minor presence of diaspore, APS minerals, kaolinite, pyrophyllite and zunyite. 
Zunyite forms euhedral crystals that reach in size up to 300μm. They sometimes include minor quartz and are associated with alunite, APS minerals and pyrophyllite. EPMA data revealed variations in the F and Cl content of zunyite, that range between 3.62-6.54 wt.% and 2.65-3.15 wt.% respectively. Alunite supergroup minerals display a wide compositional range and are represented by members of the alunite, beudanite and plumbogummite subgroups. Alunite and natroalunite constitute the most common advanced-argillic alteration minerals and are found in both quartz+zunyite and quartz+diaspore+pyrophyllite assemblages. Available mineral-chemical data favor the existence of compositions that cover a complete solid-solution series between Na- and K-rich varieties. Common mode occurrences comprise euhedral, tabular-shaped and rarely pseudocubic crystals. APS minerals are usually found as pseudocubic crystals forming the cores of tabular alunites. Analyzed compositions comprise woodhouseite (Sr-, Ce- and Sr-Ce- rich members were found). Diaspore forms aggregates of euhedral, coarse-grain crystals scattered in strongly silicified rock. Finally, pyrophyllite when present, forms acicular aggregates in the matrix along with diaspore and quartz. 
Available data suggest that the formation of the studied advanced argillic alteration assemblages is hypogene and due to ascending magmatic fluids released by the subvolcanic bodies. Mineralogical variances in the different assemblages may reflect distinct degrees of hydrothermal alteration. Co-existence of zunyite, APS minerals and pyrophyllite could be used to set constraints on the physicochemical conditions of formation of the assemblage, as the volatile-rich nature of the minerals reflects a narrow range of pH and temperature in hydrothermal systems.


Introduction
Zunyite [Al13Si5O20(OH,F)18Cl] is a rare F-and Cl-bearing, aluminosilicate that was originally described from and named after the Zuni Mine, Anvil Mountain, CO, USA [1]. Its complicates structure has long been the subject of controversy (e.g., [2]), as it is one of the few minerals that are known to contain three different volatile components, namely H2O, F and Cl [3]. Zunyite has been recognized as a rare mineral in advanced argillic alteration assemblages, which commonly develop in shallow levels, above porphyry Cu-Au deposits (e.g., Lepanto Far Southeast, Philippines [4]) and form from acidic fluids that arise from the condensation of volatiles over the porphyry intrusives [5]. In many cases, advanced argillic alteration zones, or "lithocaps", as were named by Sillitoe (1989) [6], are a favorable environment for exploration, as they may host significant high-sulfidation ores and adjacent porphyry-style mineralization [7,8]. Commonly, advanced argillic alteration lithocaps comprise various amounts of quartz, andalusite, pyrophyllite, topaz, kaolinite-dickite, diaspore, corundum, zunyite, alunite supergroup minerals, and dumortierite [9][10][11]. Lithocaps are usually zoned: deep-level assemblages comprise quartz and pyrophyllite, whereas in shallower levels, quartz and alunite predominate and reflect the cooling and increasing acidity of the hydrothermal fluids [12].
Advanced-argillic alteration lithocaps have been described from a number of porphyry/epithermal deposits and prospects in Greece [13,14]. The most well-known examples are Kassiteres-Sapes [15,16], Mavrokoryfi [17] and Melitena [15,18] prospects in Thrace district, northern Greece, as well as the Fakos and Stypsi prospects, in Limnos and Lesvos Islands, respectively [19][20][21]. Until now, no zunyite has been found in any of these lithocaps and its only occurrence in Greece was reported from advanced argillic-altered rhyolite in Kos Island [22]. We report here the first occurrence of zunyite related to a lithocap over a porphyry system in Greece.

Materials and Methods
Twenty rock samples were collected from the advanced argillic-altered rocks of the Konos Hill area for petrographic, mineralogical, and mineral-chemical studies. From these samples, sixteen thin sections underwent detailed petrographical investigation using optical microscopy. Powders from ten representative samples were processed by X-ray diffraction, using a Brοοker (Siemens) 5005 Xray diffractometer, in conjunction with the DIFFRACplus software, at the Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens. Results were evaluated using the EVA 10.0 software. Chemical compositions of the minerals were determined using electron probe micro-analysis (EPMA) with a JEOL 8530F instrument at the Institute of Mineralogy, University of Münster, Germany. Analytical conditions were: 15 kV accelerating voltage, 5 nA beam current and counting times of 10 s for peak and 5 s for the background signal. Natural (for Na, Mg, Al, Si, Mn, Fe, Sr, Cl, Ba, K, Ca, P and S) and synthetic (for F, Ti, Cr, La, Ce, Nd and Pb) mineral standards were used for calibration prior to quantitative analyses. The phi-rho-z correction was applied to all data and error on major oxides is within 1-2%.

Regional and Local Geology
The Rhodope Massif in northern Greece is characterized by Late Cretaceous-Tertiary, syn-to post-orogenic collapse, which exhumed deep-seated metamorphic sequences along major detachment faults and created a number of E-W trending tectonic basins [23,24]. These basins comprise thick, Eocene-Oligocene clastic sequences and basic-to-acidic volcanic rocks. Calc-alkaline to shoshonitic and ultra-potassic plutons and subvolcanic bodies of Oligocene to Miocene age, display supra-subduction geochemical and isotopic signatures [25] and form intrusive bodies in the above-mentioned lithologies. In the Sapes-Kassiteres district, lithologies of the Circum Rhodope Belt crop out, especially in its southern part, with metasedimentary lithologies of the Makri unit being the most widespread. Extended outcrops of the Eocene volcanosedimentary sequence discordantly overlie the metamorphic basement and occupy most of the study area, along with hydrothermallyaltered subvolcanic intrusions. Konos Hill, its most prominent topographic feature, is located approximately 20 km N-NW of Alexandroupolis and consists of a hydrothermally-altered granodiorite which intruded into the volcanosedimentary sequence ( Figure 1). Further to the E-NE part of the study area, a monzodioritic body intruded into the volcanosedimentary sequence and the granodiorite. Available geochronological data for the monzodiorite yielded cooling ages of 31.9 ± 0.5 Ma (Rb/Sr on biotite, [26]) and 32.6 ± 0.5 Ma ( 40 Ar/ 39 Ar on biotite, [27]). Recently, Perkins et al., 2018 [28] based on U-Pb zircon geochronology, provided additional geochronological data for the broader Kassiteres magmatic suite, that display a time interval between 32.05 ± 0.02 and 32.93 ± 0.02 Ma. Previous studies in this area have shown that granodiorite hosts the Konos Hill porphyry Cu-Mo-Re-Au porphyry prospect [15,29].
Advanced argillic alteration at Konos Hill is related to E-W, N-S and NNW-SSE trending faults and produced a significant overprint of the porphyry-style alteration and mineralization, which is exposed in lower topographic areas (Figure 2a). In the uppermost part, acidic leaching of the granodiorite resulted in a structurally-controlled and spatially restricted silicification zone, which comprises residual and vuggy quartz with minor alunite. Silicified zones grade outwards into advanced argillic assemblages (Figure 2b-e). Adjacent to silicification, alunite + APS minerals + quartz + zunyite ± pyrophyllite assemblages predominate, while more distal assemblages are composed of quartz + alunite + APS minerals + diaspore + kaolinite ± pyrophyllite. The Konos Hill lithocap does not host any significant high-sulfidation ores, apart from pyrite + enargite + colusite, and could be considered as barren, but nearby lithocaps host high-sulfidation, gold-enargite mineralization, which is preferably found in the western part of the study area [15,16,31,35,[37][38][39]. Advanced argillic alteration assemblages evolve through a transitional zone of quartz + alunite + pyrophyllite + sericite, into a typical sericite-rich assemblage, which is the most widespread type of hydrothermal alteration. Porphyry-style quartz stockwork veins (Figure 2f), hosted in granodiorite, comprise chalcopyrite-pyrite-molybdenite-rheniite mineralization [33] and are associated mostly with sericitic, as well as relics of sodic alteration. Sericitic alteration has also affected extended outcrops of the volcanosedimentary sequence, as well as parts of the monzodiorite intrusion, especially along fault planes. It grades further outwards into propylitic alteration, dominated by varying amounts of epidote, chlorite, and carbonates.

Diaspore
Diaspore is usually found as euhedral to subhedral grains up to 0.2 cm in length (Figures 3e and  4d). It can form aggregates in the quartz + alunite dominated matrix, or it may be scattered as isolated grains in fissures or cracks in the matrix. Electron microprobe analyses revealed almost stoichiometric compositions with traces of TiO2, BaO, Ce2O3 and Nd2O3 (up to 0.18, 0.98, 0.38 and 0.22 wt %, respectively; Table 1).

Alunite Supergroup Minerals
Alunite supergroup consists of four major subgroups, each one containing a varying number of different members. These subgroups namely are the alunite group (e.g., alunite, natroalunite), the beudantite group (e.g., woodhouseite, svanbergite), the plumbogummite group (e.g., crandalite, florencite), and the dussertite group (e.g., arsenocrandalite, segnitite) [40][41][42][43]. All members of the supergroup share in common the formula DG3(TX4)2(X')6. In D site, a tetravalent, trivalent, divalent, monovalent cation or partial vacancy can be found (e.g., Ce, La, Nd, Ca, Sr, Ba, Pb, Na, K, NH4, H3O), G site is mainly occupied by trivalent and rarely divalent cations (e.g., Al, Fe 3+ , Cu 2+ , Zn 2+ ), and T site contains a hexavalent or pentavalent cation (e.g., S, Cr6+, P, As) or minor Si 4+ . Finally, X and X' stand for O, (OH), minor F and possibly H2O [43].  Figure 5a, display a progressive substitution of PO4 3− by SO4 2− , coupled with substitution of monovalent (K, Na) by divalent (Ca, Ba, Sr) cations in the D site. Compositions that plot further below this line, display a significant variation in P, while the occupancy of D site seems quite stable. This points towards protonation of one of their trivalent anions, in order to establish neutralization. Moreover, those APS minerals that are devoid of monovalent cations (P > 1apfu), are also characterized by a 1:1 substitution in the monovalent-bearing D site by divalent cations (Figure 5b), whereas compositions that plot below this line indicate significant vacancies, due to charge balance. Alunite and natroalunite are the most common representatives of the supergroup and are found in the restricted zone of a vuggy silicification zone, in both quartz + zunyite + kaolinte ± pyrophyllite and quartz + daspore + kaolinite ± pyrophyllite assemblages, as well as in the transitional zone to the sericitic alteration. They are generally found in tabular-shaped or rhombohedral crystals, replacing feldspars or even mafic minerals of the host rock, but pseudocubic shapes were also observed. In other cases, tabular alunites form small veinlets crosscutting the silicified matrix. The majority of alunites are K-rich, with K2O values reaching up to almost 9 wt %. Na-rich alunite is also common and usually forms euhedral, tabular-shaped crystals with sizes up to 500 μn. In this case, Na2O content is higher than K2O and is up to 5.12 wt %, but there are crystals where intense zoning is present as shown by alternations of concentric K-and Na-rich areas in the crystal. Many alunites, especially the tabular-shaped natroalunites can include a core of APS mineral, commonly woodhouseite.

Kaolinite-Pyrophyllite
Kaolinite and pyrophyllite are present as minor constituents in the studied alteration types. They usually form small, acicular aggregates that accompany quartz-alunite-APS-diaspore and quartzalunite-zunyite-APS assemblages. Their presence, beyond microscopic examination was verified by X-ray diffraction patterns (Table 3).
Advanced argillic lithocaps form in higher topographic levels, from magmatic gases, contemporaneously with the formation of porphyry-style alteration and mineralization at depth [4]. Field and mineralogical data from the Konos Hill area, strongly support the idea of hypogene formation of the advanced argillic alteration which is in agreement with the findings of Voudouris (2014) [16], who described similar assemblages from the nearby occurrences of advanced argillic altered rocks in the broader Sapes-Kassiteres district. Mineralogical differences that were identified in the studied occurrence, may be due to different degrees of hydrothermal alteration. The occurrence of both advanced argillic and transitional to sericitic alteration zones in the Konos Hill area is similar to other analogous porphyry/epithermal transition environments (e.g., Lepanto-Far Souheast, Philippines, [4]; Asarel porphyry Cu deposit, Bulgaria, [50]). The presence of zunyite in a lithocap that overlies a porphyry deposit, is described here for the first time and constitutes the second occurrence of this mineral in Greece, beyond the advanced argillic altered rhyolite of Kos Island [22]. Its occurrence suggests the availability of F and Cl in the hydrothermal fluid, which resembles the results of Voudouris (2018), who described a topaz-bearing advanced argillic assemblage from the nearby Koryphes Hill area. Estimations of the temperature of formation of zunyite formation in the Konos Hill area is below 450 °C, based on the fact that at this temperature, zunyite is replaced by topaz [51]. A low-temperature limit could be set by the coexistence of zunyite with pyrophyllite, which according to Reyes (1990Reyes ( , 1991 [52,53], does not form below 200 °C . Thus, zunyite probably formed between 200 and 450 °C . A more restrictive T range of 280-350 °C for its formation is based on the similarity of zunyite-bearing assemblages at Konos Hill with those observed in the Hugo Dummet porphyry Cu-Au deposit [46] which formed at this range of temperatures. Further constraints can be made on the fact that the studied assemblages contain alunites that include APS minerals, which according to Hedenquist et al. (1998) [4], formed in a high-temperature environment, at the margins of a magmatic intrusion, compared to APS-free alunites. Finally, the fact that advanced argillic zones in the area of Konos Hill display a more-or-less E-W trending symmetry, that follows the major tectonic line of granodiorite emplacement, suggests that the low pH hydrothermal fluids from which advanced argillic alteration assemblages were derived, were channeled through fault planes.
Author Contributions: C.M. collected the studied samples. C.M. assisted by S.K. and J.B. acquired the mineralchemical data and evaluated them along with P.V., P.G.S., V.M., R.M. and C.K. C.M. wrote the manuscript. This paper is part of the first author's PhD thesis.