The deposits Prabornaz and Servette (Aosta valley, NW Italy) are located within the Zermatt-Saas-zone, which is a part of the ophiolithic Piemont zone. Prabornaz is the biggest known manganese deposit of the western Alps. The enrichment of manganese occurred by the sea floor spreading of the penninic ocean, which started in the middle Jurassic and proceeded until the early Cretaceous. From this point the deposit shared the whole high pressure metamorphism of the alpine orogeny. During the metamorphism a plurality of different manganese minerals were formed (piemontite, spessartine, braunite, macfallite, calderite, greenovite, manganoan calcite). The focus of this thesis is the mineralogy and petrology of the metamorphic rock, which have been studied with the aid of optical microscopy, Raman spectroscopy, scanning electron microscopy and microprobe. Fluid inclusions in quartz, garnet and epidote have been studied with microthermometry and Raman spectroscopy. The manganese rich minerals are analyzed with chemical methods and their forming circumstances are defined by density and composition of fluid phases in their inclusions. The fluid phase in manganese rich lithologies is H2O-rich with minor amounts of CO2 and N2, and a salinity at an average of 6.17 equivalent mass% NaCl. The density is about 0.9138 g/cm3. In contrast, the adjacent HP-metamorphic manganese-poor rocks contain highly saline fluids, with up to 19.95 equivalent mass% NaCl.