Abstract
Graphitic sulfide-rich schists from a Buchan-type metamorphic terrain contain assemblages of silicate, sulfide, oxide, and carbonate minerals metamorphosed from chlorite through sillimanite zone conditions. Sulfidic marine black shales, the protoliths of the graphitic sulfide-rich schists, contain pyrite but either only traces of pyrrhotite or no pyrrhotite at all. The appearance of abundant pyrrhotite in the graphitic sulfide-rich schists, therefore, documents the conversion of pyrite to pyrrhotite during metamorphism. Mineralogical, mineral chemical, and whole-rock chemical data for the sulfide-rich schists indicate that the pyrite → pyrrhotite transition occurs by desulfidation at constant whole-rock iron contents without participation of silicate minerals. The metamorphic reaction that converts pyrite to pyrrhotite is unusual in two respects: (a) products and reactants are quenched in at least some specimens in all metamorphic zones studied and (b) the reaction has gone to completion in some samples from each metamorphic zone. Increasing temperature (grade) is thus unlikely to exercise a dominant control over the reaction. Consideration of the amounts of iron sulfides in the schists and the composition of C–O–H–S fluids with which the rocks were in equilibrium leads to the conclusion that enormous volumes of fluid flushed through the schists during the metamorphic event (volumetric fluid-rock ratios > 1). The flow of large volumes of fluid through the schists serves as a mechanism that drives the desulfidation reaction.
