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

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.