Robert Marschik, Ludwig-Maximilians University (Germany)
Tobias Bauer, Ludwig-Maximilians University (Germany)
Ana-Sophie Hensler, Ludwig-Maximilians University (Germany)
Nikos Skarpelis, National and Kapodistrian University of Athens (Greece)
Stefan Hölzl, Bayerische Staatssammlung für Paläontologie und Geologie (Germany)
Field mapping, Sr, Pb, S, and O isotope analysis, whole rock geochemical and mineralogical studies have been carried out to test previously suggested genetic models for the Pb-Zn-Ba(-Ag-Au) mineralization at Triades-Galana, western Milos, Greece. Milos island is part of the South Aegean Active Volcanic Arc, which formed on continental crust and is related to subduction of the African lithosphere under the Aegean microplate. The rocks of the island record a transition from a shallow marine volcano-sedimentary environment in the upper Miocene-lower Pleistocene to a subaerial volcanic environment that was established after the emergence probably in the middle to late Pleistocene. HP/LT metamorphic basement rocks are exposed locally, though they probably underlie a major part of the island. The ore and most alteration zones in the Triades-Galana district occur along NE-SW or N-S brittle structures, which are related to the still active extension in the Aegean realm. The metallic mineralization comprises mainly Ag-free galena, Fe-poor Cd-bearing sphalerite, barite, and subordinately tetrahedrite-tennantite, chalcopyrite, enargite, and is associated with pervasive silicification, sericitization, and illitization. Native sulfur is locally present. Elevated concentrations of Au, As, Cd, Li, and W have been detected in the Pb-Zn-Ba ore and/or intensely altered rocks. Lead isotope ratios of galena and sphalerite are similar to those of South Aegean Arc volcanic and Aegean Miocene plutonic rocks, and compatible with Pb derived from magmatic and/or crustal sources. The Sr isotope signature of sphalerite and barite is consistent with a significant seawater component in the hydrothermal system, and interaction of the ore-forming fluids with basement rocks. Oxygen isotope values of barite have also been determined. A calculated oxygen isotope composition of a fluid in equilibrium with the barite assuming a temperature of 300°C, as suggested for the mineralization by previous works, would have δ18O(VPDB) values between +5.7 and +9.8 . These values are similar to that of Milos volcanic rocks, and consistent with involvement of magmatic-hydrothermal fluids, or interaction and equilibration of non-magmatic fluids with the volcanic rocks. Sulfides show a narrow range of sulfur isotope ratios from +1 to +3.6 , which is similar to that of magmatic sulfur from subduction-related magmatism. Sulfate sulfur (barite) has δ34S(VCDT) from +22.8 to +24.4 , which is close to, though somewhat higher than that of seawater sulfur. Disproportionation of magmatic SO2 into H2S and H2SO4 and precipitation of sulfides and sulfate at around 300°C could account for the observed Δ34S composition. The new analytical data permits genetic models, which predict mixing of seawater ±meteoric water with magmatic fluid/vapor. The geological context, the metallic and alteration mineralogy, and these anaylyical data are compatible with an epithermal intermediate-high sulfidation mineralization.