Alpine metamorphism of calcareous metasediments in the Western Hohe Tauern,tyrol: mineral equilibria in COHS fluids

During Tertiary regional metamorphism in the Western Hohe Tauern, reaching maximum P, T conditions around 6 kb, 550° C in calcareous metasediments, reaction of pyrite to pyrrhotite is suggested by regional distribution and textural relations. In rocks without graphite pyrite is common at all metamorphic grades. In graphite bearing rocks, however, the dominant Fe-sulfide is pyrite at lower grade and pyrrhotite at higher grade. Furthermore, in graphite bearing high grade rocks, pyrite is restricted to assemblages with Mg-rich silicates. Several factors control pyrite-pyrrhotite relations. Increase of temperature is most effective by increase of pyrrhotite vs. pyrite stability field, shift of silicate-sulfide reactions toward the stability field of pyrrhotite, creation of sulfur free fluids from devolatilization reactions, and increase in the proportions of sulfur bearing fluid species. Presence of graphite also favours progress of pyrite to pyrrhotite reaction, as shown by different -stabilities and changes in the amount of minerals and fluid during metamorphic heating of graphite bearing and graphite free assemblages. An opposite effect is shown by assemblages with low Fe-contents in Fe-Mg silicates, due to the enlarged stability field of such minerals with increasing Mg (and F) content. Another inhibition of pyrite to pyrrhotite reaction is suggested to be due to relatively high sulfur contents of H₂O rich infiltrating fluids.