Experimental Constraints on Baryte–Sulfide Ore in the Miocene Seawater-Dominated Mykonos Vein System, Cyclades

Abstract

This study presents experimental and geochemical modeling results that validate a fluid-mixing model for baryte and sulfide mineralization in vein-type hydrothermal systems, with reference to the Mykonos granodiorite, Cyclades. Synthetic Ba-rich hydrothermal fluids, representing those released during retrograde alteration of granitoids, were mixed with SO₄²⁻-bearing solutions, simulating Miocene seawater under controlled conditions (200–300 °C, <100 bars). Baryte precipitated rapidly upon mixing, accompanied by the co-precipitation of sulfides, such as sphalerite, chalcopyrite, galena, and minor native silver. The experiments reproduced key mineral assemblages observed in the Mykonos vein system, emphasizing the importance of a second fluid boiling at 250 °C, and redox shifts as triggers for ore formation. Complementary geochemical simulations (Solveq) constrained the stability fields of Ba–sulfate and base-metal sulfides, highlighting the critical influence of pH (5.0–6.2) and SO₄²⁻/H₂S ratios on mineral precipitation. The integration of experimental and simulation approach supports a robust model for baryte–sulfide deposition in shallow, extensional settings, where fault-controlled fluid flow promotes episodic mixing and boiling of magmatic and seawater-derived ore fluids.

1. Introduction

Baryte (BaSO₄) is a mineral of critical industrial and geochemical significance, primarily used as a weighting agent in drilling fluids within the oil and gas industry [1]. Globally, baryte mineralization occurs in a variety of geological settings, including Sedimentary-Exhalative (SEDEX) systems (e.g., Rammelsberg, Germany), Kuroko-type volcanogenic massive sulfide deposits (e.g., Hakurei, Japan), Mississippi Valley-Type (MVT) and Irish-type carbonate-hosted deposits (e.g., Walton, Canada, Alston Moor, UK), and hydrothermal veins (e.g., Wolkenhügel, Germany) [1,2,3,4,5,6]. In Cyclades, Greece, baryte is found in both epithermal and vein-type systems, such as the Mn-Ba-rich deposits of Milos [7] and the high-grade baryte-sulfide veins within the Mykonos granitoid intrusion in the Cyclades, where estimated reserves exceed 6 Mt [8].

While multiple formation mechanisms have been proposed for baryte deposits, including direct hydrothermal precipitation, replacement of carbonates in metasomatic fronts, and biogenic or bacterially mediated growth in marine settings, the processes that govern large-scale baryte mineralization remain under investigation. Hydrothermal baryte in carbonate rocks often results from the interaction of Ba²⁺-rich fluids with SO₄²⁻-bearing seawater under temperatures between 100 and 300 °C, producing zoned metasomatic columns characterized by baryte, pyrite, and associated silicates. Experimental studies have reproduced these assemblages by mixing Na₂SO₄ and BaCl₂ under controlled pH and temperature, revealing diffusion-controlled zoning and temperature-dependent phase stability. In parallel, laboratory experiments simulating baryte precipitation in seawater have shown that marine bacteria can mediate BaSO₄ nucleation details in [1,2,3,4,5,6,7].

This study aims to experimentally investigate the physicochemical conditions that control baryte and sulfide ores in vein-type hydrothermal systems and compare them to numerical simulations. Mixing experiments that replicate Miocene seawater and Ba-rich magmatic/hydrothermal fluids were conducted to simulate fluid evolution in shallow-crustal, fault-controlled settings analogous to the Mykonos vein system. The interaction between magmatic fluids and host granitoids during cooling, combined with extensional tectonics, shaped ideal physicochemical conditions for baryte and sulfide co-precipitation. The roles of temperature, pressure, and redox conditions in baryte precipitation and its co-deposition with sulfates; sulfides such as pyrite, galena, sphalerite, and chalcopyrite; as well as native silver are exploited. These experiments are designed to validate a precipitating model for baryte–sulfide vein-type ores in order to improve our understanding of ore-forming mechanisms involving mixing, phase separation, and cooling in extensional tectonic settings characterized by magmatic to seawater fluid interaction.

2. Regional and Baryte Deposit Geology

Mykonos Island, located within the Attico-Cycladic Massif, consists of three Alpine nappes: the Basal Unit, the Cycladic Blueschist Unit, and the Upper Cyclades Unit (Figure 1A). The Basal Unit is a Late Triassic to Early Cretaceous succession of dolomite, phyllite, and quartzite [9], overlain by the Cycladic Blueschist Unit, a Mesozoic continental margin assemblage consisting of a pre-Alpine basement and volcanic–sedimentary cover [10,11]. The Cycladic Blueschist Unit is, in turn, overlain by the Upper Cycladic Unit, which includes Late Cretaceous ophiolites [12]. Miocene back-arc extension led to the exhumation of the Mykonos Metamorphic Core Complex through the activity of the North Cycladic Detachment System (NCDS), a major low-angle extensional fault with top-to-the-north or -northeast kinematics, active between ~14 and 9 Ma [13,14]. Intrusion of the ~13.5 Ma Mykonos pluton [15] into the Metamorphic Core Complex occurred syn-tectonically during NCDS activity. The pluton is a multiphase, ENE-trending laccolith consisting of calc-alkaline, I-type granitoids, namely granodiorite, monzogranite, tonalite, quartz monzonite, and minor gabbro [16,17]. Its root zone is exposed on Delos Island, while the apical cupola, preserved in eastern Mykonos, is characterized by porphyritic monzogranite with orthoclase megacrysts. Dating, thermal, and structural data [18] indicate that post-intrusion cooling occurred between ~12 and 11 Ma, modified by normal Late Miocene faulting and the formation of baryte tension gashes (~11 to 10 Ma [19]). As first noted by [20], a system of baryte-bearing dikes intruded syn-tectonically along NW-trending sinistral oblique-thrust faults [21,22]. Associated with these NW faults are less-developed ENE-trending dextral strike-slip faults. Fault kinematics and stress analysis point to two main deformation phases: an initial middle Miocene transpressional regime, followed by a late Miocene transtensional phase [21]. The baryte gashes exploited structural discontinuities, such as foliation, cleavage, and detachment faults, facilitating hydrothermal fluid circulation in Mykonos granitoid.

The Cape Evros vein system (Figure 1B) consists of over fifteen NW-NNW-striking, subvertical baryte veins, some exceeding 3 m in width and extending several kilometers in strike length [8]. Hosted primarily in the Mykonos monzogranite, but also crosscutting the Cycladic Blueschist Unit and Upper Cyclades Unit greenschists and silica breccias, these veins contain multiple generations of mineralization and are often associated with brecciation and stockwork textures. Baryte is dominant across all paragenetic stages, accompanied by clear quartz, pyrite, sphalerite, chalcopyrite, and late-stage argentiferous galena. Mineralogical zoning from margin to core (Zones A–E) records systematic variations in texture and fluid composition—from fine-grained, sulfide-rich baryte at the margins to coarse, comb-textured baryte and Mn-oxides at the core. Zone A comprises fine-grained baryte ± muscovite with fine pyrite, sphalerite, chalcopyrite, and fragments of altered monzogranite. Zone B is characterized by medium-grained baryte and clear quartz ± chlorite, epidote, and muscovite, accompanied by fine chalcopyrite and colloform pyrite. Zone C contains medium-grained baryte with goethite, marcasite, and chalcopyrite. Zone D consists of medium- to coarse-grained baryte, goethite and coarse galena ± calcite, witherite, and muscovite. Zone E is dominated by coarse, comb-textured baryte with psilomelane, pyrolusite, hematite, ferrihydrite, and goethite. These compositional and textural differences warrant a thorough description to capture the progressive changes in fluid chemistry, redox state, and depositional conditions that controlled ore formation. Ore and SEM microscopy reveal over 40 ore and gangue minerals, including sulfosalts (geocronite, jordanite), native Ag-Au, and Pb-Sb-As-Te phases, formed through three hypogene stages and a subsequent supergene oxidation stage [8,19].

Hydrothermal alteration halos surrounding major veins at Cape Evros include the following: (i) an inner silica-rich breccia zone with baryte, quartz, hyalophane, and pyrite; (ii) a chlorite–muscovite–albite zone with epidote and minor feldspars; and (iii) an outer zone characterized by montmorillonite, goethite, pyrophyllite, and Fe-oxides. Geothermometric data indicate that mineralization occurred under shallow crustal, near-hydrostatic conditions at pressures of ~100 bars and temperatures between 275 °C and 325 °C. These fluids were sourced from oxidized, Ba-rich magmatic ore solutions derived from the alteration of the Mykonos granitoids, with additional contributions from Miocene seawater [8,19,20].

3. Setting the Experiments

The Mykonos vein system that hosts the baryte–sulfide ores is defined by shallow emplacement depths, moderate hydrostatic pressures (~100 bars), and mesothermal temperatures ranging from ~275 °C to 330 °C. These conditions, constrained by mineral equilibria, fluid inclusion microthermometry, and structural context, reflect a low-pressure, high-permeability regime. The mineralization is spatially associated with zones of intense brecciation, alteration, and pervasive silicification within the apical zone of the Mykonos pluton. These features strongly suggest focused hydrothermal flow and repeated fracturing, creating transient supersaturated conditions conducive to baryte precipitation. Salinity and redox conditions were further defined through detailed fluid inclusion microthermometry and Raman spectroscopy, revealing a chemically complex fluid system dominated by H₂O-NaCl with minor CH₄, H₂S, SO₂, and trace CO₂ [8]. Fluid salinities ranged from ~2 wt.% to ~15 wt.% NaCl eq. The co-occurrence of vapor-rich (V-L) and liquid-rich (L-V) inclusions, along with colloform and jigsaw textures in quartz, provided robust evidence for boiling, a critical mechanism for local baryte supersaturation [8]. In addition, the chemical speciation, pH, and redox conditions of the ore fluid were critical for reproducing baryte precipitation experimentally [8].

The literature on submarine hydrothermal systems and mid-crustal baryte deposits (e.g., [23]) has emphasized that fluid boiling and degassing strongly affect sulfide, sulfate, and barium solubility, validating our inclusion of immiscibility and degassing mechanisms in the experimental approach. The geochemical and isotopic data from baryte and associated sulfides further constrained fluid composition and source. According to [8,23], the δ³⁴S values for baryte (i.e., +22 to +26 per mil) and sulfides (i.e., +4 to +25 per mil) indicate a dual sulfur source, meaning magmatic and crustal, while δ¹⁸O and δD values from baryte (i.e., +0.6 to +5.1, and −34 per mil) and quartz (i.e., +6.4 to +9.5 and −80 to −75 per mil) suggest fluid mixing between magmatic–hydrothermal and seawater-derived fluids. Similarly, initial ⁸⁷Sr/⁸⁶Sr ratios between 0.712 and 0.715 in baryte and quartz indicate interactions in the altered Mykonos monzogranite [8]. By replicating the natural P-T-X conditions, i.e., pressures of ~100 bars, temperatures between 275 °C and 330 °C, salinities between ~2 wt.% and ~15 wt.% NaCl eq., and fluid mixing between oxidized and reduced components, we ensured that the experimental environment faithfully represented the hydrothermal system responsible for the Mykonos baryte veins. The documented role of feldspar alteration in liberating Ba, supported by EMPA data and comparable systems [24,25,26,27], reinforced our use of Ba-rich silicate phases as the barium source in the experiments. Prior studies (e.g., [8,26]) emphasized the importance of labile Ba release during organic and mineral breakdown in seawater for baryte precipitation, which complements our findings on Ba mobilization from alkali feldspars during phyllic alterations of the Mykonos monzogranite. Moreover, the observed boiling in the field guided the staged introduction of gas phases and temperature gradients in the lab, facilitating baryte nucleation under realistic saturation pathways. These constraints allowed us to not only reproduce baryte textures and mineral associations observed in Mykonos but also to assess the physicochemical thresholds critical for BaSO₄ precipitation in natural hydrothermal environments. The δ³⁴S_SO4²⁻ and δ³⁴S_H2S values further constrained the fluid sources and sulfur speciation, allowing us to mimic the dual magmatic and crustal sulfur inputs [8]. Together, these field-based, petrographic, and geochemical observations directly informed the design of our baryte precipitation experiments, which were effectively reproduced in the laboratory setting.

4. Experimental Design and Modeling Rationale

The sample MY10 (zone A, Y/Ho ratio of 100 [8], Figure 1B) was crushed and separated into −16 and −200 mesh fractions. It contained baryte, Fe-oxides (mostly goethite), sphalerite, pyrite, and galena, with an average chemical composition of SiO₂ 0.5 wt.%, Fe₂O₃ 1.8 wt.%, BaO 63.7 wt.%, SO₃ 32.7 wt.%, and Cu 395 ppm, Pb 337 ppm, Zn 44 ppm, and Ag 1.3 ppm. These minerals typically occur as intergrown aggregates, with sulfides commonly cemented within a baryte–Fe-oxide matrix and particle sizes ≥ 74 μm. The MY10 sample was roasted to enhance metal mobility (particularly for Pb, Zn, Cu, and Ag) and subsequently leached. Leaching was carried out using acidified NaCl and H₂SO₄ solutions (1 to 4 M NaCl, pH 3.5 to 6.2, adjusted with HCl) in 250 mL Erlenmeyer flasks for 12 h. The resulting leachate was then mixed with 1M Na₂SO₄ and 1 M H₂SO₄ solutions in a proportion of 1:1. Heating/cooling was maintained between 200 °C and 300 °C for 6 to 12 h, consistent with the post-boiling evolution of a hydrothermal fluid that had experienced a major boiling event at ~330 °C. The solid-to-liquid ratio and agitation time were optimized to maximize Ba²⁺ extraction, with negligible improvement beyond 6 h of stirring. Iron was selectively removed from the leachate, by adding 10 mL of chlorine dioxide 0.5 M, to isolate and recover valuable base/precious metals (Zn, Cu, Pb, and Ag), and the Ba-rich solution was then filtered to remove residual solids.

Baryte/sulfides/sulfates were precipitated by introducing pre-filtered Na₂SO₄ solution into the Ba-rich leachate under controlled conditions. Simulated scenarios included the following: (i) simple mixing of two fluid reservoirs, (ii) mixing accompanied by a second boiling phase at 250 °C caused by a sudden pressure release, and (iii) passive cooling post-mixing using a 10 °C step. Redox buffers such as sodium dithionite (reducing) and hydrogen peroxide (oxidizing) were used to reproduce the evolving fO₂ and fS₂ conditions observed in natural fluid inclusions. The precipitation setup employed a gravity-feed system (2–5 mL/h) to mimic slow natural mixing rates and to avoid the formation of fine-grained precipitates caused by turbulence. Stirring was avoided during precipitation to promote larger, well-formed baryte crystals, in line with prior experimental results ([26,27]). To ensure purity ≥95%, all glassware was cleaned using chromic–sulfuric acid and steam-rinsed or treated with 0.1 M EDTA solution. Solutions were aged and filtered before use to ensure chemical homogeneity. At the end of each experimental run, the hot suspension was filtered to avoid secondary crystallization during cooling. The solid products were dried and then were analyzed via SEM microscopy, whole-rock geochemistry, and XRD diffraction (Supplementary Material, Figures S1–S4 and Tables S1 and S2).

X-ray diffractometry was carried out at the Department of Geology, University of Patras, using a Bruker D8 Advance diffractometer (Brucker AXS GmbH, Karlsruhe, Germany) with Ni-filtered Cu(Kα) radiation at 40 kV/40 mA, scanning the 2θ interval of 2–90° with a 0.015° step size. Mineral phases, including magnetite, pyrite, sphalerite, chalcopyrite, and galena, were identified with DIFFRACplus EVA 12^® and quantified semi-quantitatively using the Rietveld method in TOPAS3. Coherent scattering domain (CSD) sizes of pyrite and sphalerite were further determined with the Bertaut–Warren–Averbach (BWA) technique in WINFIT, while grain sizes for analysis ranged between 5 and 50 μm. Mineral microanalyses were conducted using a JEOL JSM-6300 SEM (Jeol Ltd., Tokyo, Japan) equipped with energy-dispersive (EDS) and wavelength-dispersive (WDS) spectrometers, along with INCA software, at the Laboratory of Electron Microscopy and Microanalysis, University of Patras, Greece. The operating conditions included an accelerating voltage of 25 kV, a beam current of 3.3 nA, and a beam diameter of 4 μm. The total counting time was 60 s, with a dead time of 40%. Standards used for gangue and ore minerals included natural marialite (Cl), tourmaline (B, F), orthoclase (K), diopside (Ca, Si), ilmenite (Ti), rhodonite (Mn), fayalite (Fe), jadeite (Na), forsterite (Mg), corundum (Al), baryte (Ba), chlorite, epidote, plagioclase, and muscovite. Additional standards comprised natural chalcopyrite, tetrahedrite, tennantite, stibnite, pyrite, sphalerite, galena, hausmannite (Mn²⁺), manganite (Mn³⁺), pyrolusite (Mn⁴⁺), as well as synthetic CoNiAs, SnO₂, and CdTe. Native metals, Ag, Au, Te, and Se, were also used. Detection limits were in the range of approximately 0.01%, and the accuracy was better than 5%. Geochemical analyses of the products were performed by ActLabs Ancaster, Ontario, Canada. Major and trace elements compositions were measured using ICP-OES and ICP-MS. The detection limits were 0.001 wt.% for MnO and TiO₂ and 0.01 wt.% for the other major elements. For the trace elements, they were as follows: Au (2 ppb); Lu and Ge (4 ppb); Ir, Pr, Eu, and Tm (5 ppb); La, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb, Ta, Tl, Th, U, Se, and Te (0.1 ppm); Hf and In (0.2 ppm); Cd (0.3 ppm); Bi (0.4 ppm); Br, Ag, Cs, and Sb (0.5 ppm); Sc, Be, Co, Ga, Nb, Ni, Cr, Mo, W, Sn, and Hg (1 ppm); Sr, Y, and Rb (2 ppm); Ba (3 ppm); Zr (4 ppm); Pb, As, and V (5 ppm); and Cu (10 ppm) and Zn (30 ppm).

Geochemical modeling for the Mykonos baryte/sulfide/sulfate vein system was performed using XRD (Brucker AXS GmbH, Karlsruhe, Germany ) and bulk geochemical analyses in combination with chemical equilibrium calculations via the Solveq software [28]. The initial composition of the baryte-enriched ore fluid was reconstructed from fluid inclusion microthermometry and Raman spectroscopy [8]. These parameters were used as input for Solveq to model the fluid evolution under shallow and NaCl-saturated conditions. Ore fluid modification was tested, incorporating the physicochemical constraints observed in our laboratory experiments for continuous mixing, boiling at 250 °C, and simple cooling (350–200 °C, with a 5 °C step). The experimental results, especially the enhanced baryte and co-precipitated sulfide formation during boiling and mixing, were used to refine model parameters for Ba²⁺, SO₄²⁻, and base-metal activities. Chemical stability fields of sulfides, sulfates, and associated silicate phases were further evaluated using SUPCRT92 [29], allowing for phase stability relationships to be compared directly with experimental precipitation products (Supplementary Material, Figure S5). The close agreement between predicted and observed mineral assemblages helped to validate our experimental interpretation.

5. Results

5.1. Continuous Mixing with Seawater

Between 220 °C and 280 °C, continuous mixing of Mykonos magmatic fluids (~35%) with ~4 wt.% NaCl seawater (~65% simulating Miocene seawater [8,30]), drives mineral precipitation under evolving redox conditions (Figure 2). Seawater dilution reduces the solubility of metal–sulfate complexes, facilitating supersaturation. Baryte shows an increased concentration of up to ~250 °C, after which the Ba content declines due to its precipitation, indicating a peak stability just below this threshold. Goethite (FeO(OH)) precipitates between ~260 °C and ~230 °C, and its mass precipitating decreases at higher temperatures, suggesting Fe²⁺ remobilization. No pyrite was precipitated. Anglesite (PbSO₄), goslarite (ZnSO₄·7H₂O), and chalcanthite (CuSO₄·5H₂O) tend to precipitate between 200 °C and 210 °C under low Pb, Cu, and Zn contents. Quartz shows no temperature dependence.

Sphalerite displays a decreasing Zn content with temperature increase, stabilizing ~275 °C, and precipitates at ~250 °C, implying high Pb²⁺, Zn²⁺, Cu²⁺, and SO₄²⁻ activities before declining. Chalcopyrite reaches maximum Cu content at ~220 °C, then diminishes. Native silver precipitates below ~240 °C, followed by a slight decline, implying higher metal solubility and redox-sensitive conditions. Collectively, these patterns confirm that seawater mixing at ~4 wt.% NaCl promotes baryte, goethite, and sphalerite precipitation, while redox evolution facilitates co-deposition of native silver.

5.2. Mixing with Second Boiling

The observed mineral assemblage between 220 °C and 300 °C corresponds to mixing, cooling, and chemical evolution of the ore fluid after the main boiling event at 330 °C, which experienced a second boiling event at ~250 °C. The boiling events caused a rapid drop in pressure (~200 to ~100 bars) and escape of vapor. This physical separation significantly altered the fluid composition, salinity, and redox conditions. In this post-boiling regime, Figure 3 shows increased precipitation of sulfates such as goslarite and chalcanthite, along with baryte. The presence of anglesite and a marked increase in native silver deposition at ~280 °C point to redox changes driven by phase separation during mixing with oxidized seawater. This promotes supersaturation and mineral precipitation. After the first boiling event, the increase in sulfate activity (α_SO4²⁻) enhances deposition of baryte and silver, related to the drop in baryte solubility.

Furthermore, the boiling event at ~250 °C marked a critical threshold where fluid immiscibility caused, for a second time, phase separation, degassing, and rapid physicochemical changes. Boiling created overpressures and fluid expulsion, reducing solubility for sulfide/sulfate species and initiating widespread precipitation. Figure 3 indicates that shortly after boiling, baryte, goethite, anglesite, and goslarite begin to precipitate more intensely with chalcanthite. Between 220 °C and 280 °C, the mass of baryte deposited increases gradually, reaching a peak at ~250 °C, indicating that Ba²⁺ and SO₄²⁻ activities decreased in solution. This coincides with the drop in baryte solubility during boiling, consistent with the ore fluid becoming supersaturated. Anglesite, goslarite, chalcanthite, galena, sphalerite, and chalcopyrite show clear peaks between 230 °C and 220 °C, implying efficient Pd, Zn, and Cu precipitation just after the second boiling conditions. Sulfide/sulfate precipitation is most likely driven by temperature drop, redox changes, enhanced mixing with seawater, continued sulfate availability, and ongoing water–rock interaction at declining temperatures. Galena also shows a peak at ~280 °C. Native silver precipitates most effectively at ~250 °C. The precipitation of goethite follows the opposite pathway, pointing to oxidizing conditions prevailing after boiling, possibly from seawater influx supplying dissolved oxygen. No pyrite was precipitated.

5.3. Simple Cooling

Figure 4 illustrates how the reaction rate, which was used as a proxy for supersaturation conditions, and fluid interaction modes during simple cooling without mixing and boiling, influence mineral precipitation. At higher reaction rates, representing rapid post-boiling supersaturation, only the quartz mass increases significantly, likely due to its low solubility within this reaction rate range. Under simple cooling conditions, overall precipitation is more subdued. Goethite, anglesite, goslarite, galena, sphalerite, chalcopyrite, and native Ag show relatively constant precipitation masses across different reaction rates, suggesting that their formation is less dependent on kinetics and more controlled by fluid composition. No pyrite was precipitated. Chalcanthite exhibits a modest increase at low reaction rates.

Baryte mass increases slightly with decreasing temperature, peaking at a reaction rate of ~4, in the absence of boiling or intense mixing. This highlights that simple cooling alone does not create the degree of disequilibrium necessary for substantial baryte deposition. The observed pattern implies that simple cooling generates chemical gradients insufficient to trigger selective precipitation, particularly of metals such as Zn, Cu, and Ag.

6. Discussion

6.1. Actual Conditions of Baryte Precipitation

Baryte precipitation in the Mykonos vein system was controlled by a complex interplay of fluid mixing, pressure–temperature evolution, redox changes, and chemical saturation. Calculated saturation indices for baryte (SI_BaSO4 ≈ 15.3) confirm that the ore fluids remained consistently supersaturated between 250 °C and 320 °C [8,19]. Supersaturation intensified after the main boiling event at ~330 °C, during which quartz solubility increased sharply while baryte solubility dropped. These changes suppressed silica precipitation and enhanced baryte formation. The observed increase in logαBa²⁺ values (between −10.1 and −7.4 [8]) as temperature decreased suggests a continued influx of Ba-rich magmatic fluid, providing a sustained barium supply to the vein system.

Fluid inclusion and stable isotope data support a two-step mineralization model involving both fluid mixing and phase separation. Evidence for boiling includes vapor–liquid inclusions and broad salinity variations (~2 and ~15 wt.% NaCl eq. [8]). The timing of mineralization is also consistent with this model. The Mykonos monzogranite was emplaced at ~13.5 Ma and exhumed by ~10 Ma during Tortonian marine transgression [19], creating a brittle regime that favored brecciation and hydrothermal breccia formation [8,19]. Rb-Sr dating of baryte (~8.5 Ma) places mineralization during or shortly after final pluton cooling [8,19].

Altered monzogranite shows fluid-mediated leaching patterns, including negative Ce and positive Tb anomalies. In contrast, baryte and associated sulfides exhibit REE profiles typical of seawater-derived sulfates, while quartz retains a magmatic signature [8]. Seawater-normalized ratios, such as La/Lu(N) ≥ 1, Ce/Sm(N) ≤ 4, and Y/Ho ≈ 100, confirm the seawater influx. These geochemical fingerprints indicate that baryte precipitation was driven by the mixing of Ba-rich magmatic fluids with SO₄²⁻-bearing seawater, and boiling drives baryte supersaturation [8]. Precipitation occurred under optimal conditions (~300 °C, ~100 bars, ~4 wt.% NaCl), closely matching the thresholds defined by [26,27].

6.2. Experimental Insights into the Mykonos Ore System

The experimental results strongly validate the observations from the Mykonos baryte vein system, particularly regarding the mechanisms controlling baryte/sulfate/sulfide precipitation. Across the 220 °C to 280 °C range, mixing of Ba-rich magmatic fluids with ~4 wt.% NaCl seawater reproduced the mineral assemblages observed in the field, including baryte, goslarite, chalcanthite, and native silver. These results demonstrate that fluid mixing, not simple cooling alone, was the dominant control on baryte and associated sulfide/sulfate deposition [8,19,23]. When mixing was accompanied by boiling, i.e., at ~250 °C, the hydrothermal system underwent redox shifts and salinity changes that sharply reduced baryte solubility and enhanced precipitation. This mirrors fluid inclusion and isotopic evidence from Mykonos vein system, where boiling-induced brecciation and seawater infiltration created the key chemical gradients needed for deposition. Importantly, the experiments show that high reaction rates, simulating fast fluid influx or episodic boiling, led to rapid crystal growth of baryte and sulfides/sulfates. Native silver precipitation at ≤250 °C in the experiments supports a redox-sensitive control tied to seawater input, matching the δ³⁴S and ⁸⁷Sr/⁸⁶Sr isotopic signatures of natural baryte-sulfide/sulfate assemblages. As the system cooled post-boiling, continued fluid mixing between 220 °C and 250 °C remained favorable for dense baryte growth, supported by stable Ba²⁺ and SO₄²⁻ activity.

In contrast, experiments simulating simple conductive cooling without mixing resulted in minimal mineral precipitation. The quantities of chalcanthite, sphalerite, native silver, and chalcopyrite remained very low, with baryte precipitation barely detectable. This clearly demonstrates that cooling alone was insufficient to create the chemical disequilibrium necessary for ore deposition. These findings reinforce the field-based conclusion that mineralization in Mykonos was not a passive result of cooling but a product of dynamic processes, such as boiling, mixing, and redox cycling. The experiments also show that the rate of fluid–rock interaction was critical, i.e., slow reaction kinetics produced lower yields, while fast rates promoted the deposition of Cu-, Zn-, and Pb-sulfides/sulfates.

Overall, the experimental series successfully recreated the physicochemical conditions of the Mykonos vein system and demonstrated how baryte and metal sulfates formed in response to fluid mixing and episodic and boiling. The strong overlap between experimental results and field data confirms the following depositional model: Mineralization occurred in a structurally controlled, shallow-crustal setting where fluid chemistry evolved rapidly due to magmatic input, seawater influx, and pressure changes across the flow conduits. These results not only validate the inferred controls on baryte precipitation but also offer a transferable framework for understanding similar hydrothermal systems in the Cyclades [23] and other extensional metallotectonic settings.

6.3. Thermodynamic Modeling

The geochemical modeling with Solveq provides important constraints on the solubility behavior of ore-forming ligands under the physicochemical conditions prevailing at ~250 °C during mixing and the second boiling event. At near-neutral pH values, Ba²⁺, Pb²⁺, Zn²⁺, and Ag⁺ all display strongly decreasing solubility, consistent with their tendency to precipitate as baryte and sulfides during fluid evolution. The solubility plots (Supplementary Material, Figure S5) highlight the strong dependence of baryte and sulfide solubility on both pH and sulfate activity, with baryte and galena stability fields expanding as sulfate availability increases. Similarly, Pb²⁺ and Zn²⁺ exhibit pronounced solubility drops under elevated sulfate activities, consistent with the natural paragenesis of galena and sphalerite in the Mykonos veins. Notably, Ag⁺ follows the same trend, explaining the co-precipitation of native silver observed in both the natural samples and experimental runs at ~250 °C, where the second boiling episode occurred. This solubility variability corresponds to the modeled stability of baryte and associated sulfates at higher sulfate activities, reinforcing the role of seawater-derived SO₄²⁻ influx in promoting mineral deposition. In contrast, metals such as Cu²⁺ and Fe²⁺ remain more soluble across this pH window (i.e., 5.0–6.2), suggesting that their precipitation is more sensitive to redox and temperature fluctuations rather than pH alone.

Importantly, the modeled conditions align with the estimated fluid pH values. At 320 °C, the pH is constrained to ~5, while at 300 °C, it rises to ~6.2, conditions that are compatible with both baryte precipitation and the co-deposition of galena, sphalerite, chalcopyrite, and native silver, with estimated activities of logαPb²⁺ = −4.4, logαFe²⁺ = −5.9, and logαAg⁺ = −10.9, corresponding to Pb, Fe, and Ag concentrations between 0.25 and 25 ppb. In addition, the calculated activities of sulfate and sulfide species (logαSO₄²⁻ = −2.8 to −2.1; log α(SO₄²⁻/H₂S) = +2.6 to +5.5) indicate that fluids were relatively oxidized and strongly sulfate-dominated during precipitation. Corresponding redox conditions yield logfS₂ values from −12.6 to −13.6 and logfO₂ values from −30.9 to −31.9 (see [8]), indicating a mineralizing fluid buffered between reduced and mildly oxidizing conditions. These values reinforce the interpretation that seawater-derived SO₄²⁻ was the dominant oxidizing agent in the ore system.

The modeling outcomes are also consistent with the experimental results. Our boiling and mixing simulations showed that baryte dominates the precipitated assemblages, with sulfides (galena, sphalerite, chalcopyrite) and minor Ag phases co-precipitating under conditions of elevated sulfate activity and moderately acidic to neutral pH. The experimental formation of galena, sphalerite, and chalcopyrite mirrors the modeled solubility decrease in Pb²⁺, Zn²⁺, and Cu²⁺ under evolving pH-sulfate conditions. The presence of Ag⁺ in the modeling trends, where its solubility decreases steeply with increasing pH, explains why native silver was observed in our experiments only after boiling and rapid mixing conditions that promote supersaturation and destabilization of Ag complexes. The subdued precipitation under simple cooling alone agrees with the geochemical diagrams, which indicate that disequilibrium from rapid mixing or boiling is required to overcome the stability fields of dissolved species.

Taken together, the integrated evidence from thermodynamic modeling and experimental results demonstrates that baryte–sulfide–silver deposition in the Mykonos veins was controlled by the interplay of the pH, redox state, and sulfate availability. The observed increase in baryte and goethite at higher reaction rates in the laboratory closely mirrors the solubility trends predicted by the modeling. This suggests that sulfate-rich seawater input and progressive neutralization of magmatic fluids played a central role in triggering mineral deposition. Boiling and fluid mixing not only shifted the pH toward near-neutral values but also increased the SO₄²⁻/H₂S ratio, creating favorable conditions for the co-precipitation of baryte, base-metal sulfides, and native silver.

7. Conclusions

This study used controlled laboratory experiments to replicate the physicochemical conditions responsible for baryte precipitation in the Mykonos vein system. By simulating key mechanisms, such as fluid mixing, boiling, redox shifts, and temperature evolution, we were able to test the dominant mechanisms influencing baryte/sulfide/sulfate assemblages under conditions analogous to those found in shallow vein systems:

• Experiments confirmed that fluid mixing, particularly between Ba-rich magmatic solutions and ~4% NaCl seawater, was the dominant trigger for baryte precipitation.
• Boiling at ~250 °C significantly enhanced baryte/sulfide precipitation by lowering solubility, inducing redox shifts, promoting oxidation of Fe²⁺ and H₂S, and thereby favoring the co-precipitation of baryte and native silver.
• High reaction rates led to deposition of baryte, while simple cooling without mixing, boiling, or changes in the redox state of the mineralizing fluid yielded limited precipitation.

By reproducing natural mineralizing conditions under controlled laboratory settings, this study validates a geologically realistic model for baryte/sulfide/sulfate ore formation in extensional, low-pressure, fault-controlled hydrothermal vein systems. The proposed fluid–mixing–boiling model for the Mykonos vein hydrothermal system suggests that baryte and sulfide/sulfate co-deposition was triggered by seawater influx, sulfate enrichment, and episodic boiling in a shallow, extensional tectonic setting. The Mykonos case study offers a transferable framework for interpreting baryte and base-metal sulfide/sulfate ores in hybrid magmatic–marine systems, especially those hosted in intensively altered granitoids. Future research should refine the kinetic controls on baryte/sulfide nucleation, particularly under variable salinity, pH, and redox conditions, and further investigate how tectonic architecture governs fluid flow, reaction fronts, and mineral zoning.