Quadruple sulfur isotopic fractionation during pyrite desulfidation to pyrrhotite

Abstract

The chemical and mineralogical conversion of pyrite to pyrrhotite has been studied in many aspects, but the associated quadruple sulfur isotopic fractionation between these phases has not been investigated previously. In this study, Ruttan pyrite and pyrite from a Neoarchean carbonaceous phyllite drill core were heated to 675 °C, and the released sulfur was reacted with iron metal, forming compact sulfide rims. The products of both desulfidized pyrite and sulfidized iron metal are pyrrhotite, following the equations n(1-x)FeS2 = nFe1-xS + (1-2x)Sn and Sn + n(1-x)Fe = nFe1-xS, respectively. The mean δ34S of the Ruttan pyrite experiment products has shifted slightly with the residue becoming slightly heavier and the sulfidized iron becoming slightly lighter than the starting Ruttan pyrite. However, the shift is small (0.5‰ and 1.2‰, respectively), yet the range of δ34S increases in the pyrrhotite products, suggesting that the evaporation of sulfur during pyrite breakdown is not occurring uniformly. A shift in δ34S between the initial Archean pyrite and resultant pyrrhotite products is likely a real difference in composition of the analyzed pyrite and the pyrite extracted for the experiment. The compositions of the pyrrhotite run products are quite close with the difference in Δ33S and Δ36S being within ca. 0.1‰. They keep constant with the starting Ruttan pyrite while differ slightly from the initial Archean pyrite, which is likely due to heterogeneity in the original pyrite. The experiments indicate that the fractionation in Δ33S and Δ36S during pyrite desulfidation, sulfur evaporation transfer and condensation and iron metal sulfidation is negligible. Therefore the preservation of Δ33S and Δ36S between initial pyrite and released sulfur during pyrite desulfidation is demonstrated, establishing Δ33S and Δ36S as effective tools of tracing sulfur sources of Archean gold deposits and opening up pyrrhotite as an appropriate phase for studying Archean S-MIF.