Sulfide melts in ore deposits from low-grade metamorphic settings: Insights from fluid and Tl-rich sulfosalt microinclusions from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy)

Sulfide melting is increasingly recognized as an efficient ore remobilization process in several ore bodies deformed and metamorphosed under amphibolite to granulite facies conditions. Actually, sulfide melts may also occur at lower metamorphic conditions, provided the abundance of the so-called Low-Melting point Chalcophile Elements (LMCE). The Monte Arsiccio ore body (Apuan Alps, northern Tuscany, Italy) is strongly enriched in several LMCE, i.e., Ag, As, Hg, Pb, Sb, and Tl, and experienced greenschist facies metamorphism (T = 350–450 °C; P = 0.3–0.4 GPa) during the Alpine orogeny. Trails of polyphase sulfosalt inclusions (up to five phases), coexisting with fluid inclusions, were observed within quartz and baryte crystals found in late-stage veins. Sulfosalt assemblages are formed by LMCE-rich phases, the main being ferdowsiite, Ag8As3Sb5S16, chabournéite, Tl2Pb(Sb,As)10S17, and intermediate members of the arsiccioite/routhierite isotypic pair, (Ag1−xCux)Hg2TlAs2S6, along with minor realgar. Sulfosalt inclusions range between 200 nm and more than 100 µm in size, whereas larger inclusions, likely formed through the coalescence of smaller melt droplets, are up to 500 µm. Sulfosalt inclusions are associated with primary two-phase liquid-rich fluid inclusions (Type 1a), containing CO2 and N2, as revealed by microthermometric investigations and micro-Raman spectroscopy. Secondary two-phase liquid-rich fluid inclusions (Type 1b) were also identified. Microthermometric measurements carried out on Type 1a fluid inclusions gave homogenization temperatures (Th) in the range 265–296 °C, whereas Type 1b fluid inclusions showed lower Th, between 222 and 246 °C. As microthermometric heating experiments on sulfosalt inclusions revealed homogenization temperatures around 270 °C, it is very likely that sulfosalt inclusions were in the liquid state during their entrapment, in agreement with many Tl-bearing systems. These results are fully consistent with the occurrence of LMCE-rich sulfide melt during the formation of the sulfosalt assemblages observed at the Monte Arsiccio mine. In addition, the polyphase nature of most of the observed sulfosalt inclusions may be related to the unquenchable nature of sulfide melts. The formation of LMCE-rich sulfosalt assemblages could be related to the release of these elements from pyrite during its metamorphic recrystallization. Consequently, pyrite ores may produce sulfide melts, with potential implications in the change of speciation of LMCE, from trace to major elements.