Formation of a sulfide melt during Alpine metamorphism of the Lengenbach polymetallic sulfide mineralization, Binntal, Switzerland

Abstract

Massive and well-crystallized Sulfides of Fe, Pb, As, Tl, Ag, Cu and Zn occur in the Lengenbach mineral deposit (Valais, Swiss Alps), located in a low- to medium-grade metamorphosed Triassic dolomite. Inclusions arranged along healed fractures in quartz consist of complex intergrowths of Tl-rich sartorite+tennantite+orpiment and are interpreted as relics of a sulfide melt generated during metamorphism and trapped in a similar way to associated aqueous fluid inclusions. Microprobe analyses of melt inclusions homogenized at 500°C demonstrate a strong enrichment of Tl in the melt phase, consistent with the observed late-stage Tl-enrichment in the deposit. The presence of a sulfide melt is compatible with Alpine metamorphism of the Lengenbach deposit under relatively closed conditions. The unique sulfide mineral assemblage of the Lengenbach deposit can be ascribed to fractional crystallization in a slowly cooling melt-aqueous fluid system during Alpine uplift.