Sulfur isotope effects associated with the conversion of pyrite to pyrrhotite in a dry hydrogen atmosphere were measured at temperatures from 500° to 600°C. H2S was the primary fluid species produced, although at temperatures of 500° and 525°C native sulfur in amounts up to 30 percent of the total liberated sulfur condensed in the collection system. At temperatures of 550°, 575°, and 600°C, more than 90 percent of the liberated sulfur was in the form of H2S. At all temperatures pyrrhotite was the only residual iron sulfide. Native sulfur was characterized by a δ34S value of –0.1 per mil relative to the starting pyrite, whereas H2S varied from –0.2 to +0.1 per mil (most values were within analytical error of 0‰). The produced pyrrhotite ranged from 0.3 per mil at 500° and 525°C, to 0.2 per mil at all higher temperatures. Pyrrhotite-H2S fractionation factors are near equilibrium values for these temperatures and suggest that potential kinetic isotope effects are suppressed at low fO2 conditions where H2S is the principal fluid species. Both experimental and empirical studies suggest that very little sulfur isotope fractionation accompanies the conversion of pyrite to pyrrhotite in carbon-rich metapelites. For this reason the isotopic composition of pyrrhotite in graphitic metapelites may serve as a reliable indicator of the isotopic composition of sulfur that was available for assimilation by magmas or hydrothermal fluids, and that may have been involved in later ore genesis.

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