Gypsum as a precursor to pyrrhotite in metamorphic rocks

During low-grade regional metamorphism, pyrrhotite can form from gypsum by the reaction: CaSO₄·2H₂O+Fe _sol. ²⁺ +2C_org.FeS+Ca _sol. ²⁺ +2H₂O+2CO₂. This reaction takes place in the anchizone, below 350°C and might be initiated by the thermal dehydration of gypsum (200°C) and aided by the generation of gaseous hydrocarbons. Evidence for the reaction is the occurrence in dolomitic layers in the Ballachulish Slate, East Larroch quarry, Argyll of lath and swallow-tail shaped quartz+dolomite+pyrrhotite pseudomorphs after gypsum. Quartz+pyrrhotite bodies in slate represent replaced and deformed (mainly flattened) crystals, concretions and possible veinlets of gypsum. Pyrite porphyroblast growth, after the peak of metamorphism and under relatively high fS₂ conditions, failed to destroy some early pyrrhotite because it is encapsulated in quartz. Pyrite-silicate reactions and hydrothermal exhalations have been suggested previously to account for pyrrhotite-enriched horizons in regionally metamorphosed rocks. Replacement of gypsum by pyrrhotite is an additional explanation for pyrrhotite-enriched horizons, especially in dolomitic and graphitic lithologies.