The chemical and isotopic characterization of the pyrite to pyrrhotite desulfidation reaction across the metamorphic gradient of a metasedimentary basin

During prograde metamorphism, sulfur is released through the pyrite to pyrrhotite desulfidation reaction. This process is believed to supply sulfur and metals to metamorphic fluids, potentially contributing to the formation of orogenic gold deposits. A deeper understanding of this reaction can therefore help unravel the mobility of sulfur, gold and other metals in metamorphic fluids during prograde metamorphism. We present a novel in situ study of multiple sulfur isotopes combined with trace elements, focused on the texturally-constrained pyrite to pyrrhotite desulfidation reaction. This reaction is documented in detail along a prograde, Barrovian metamorphic field gradient − increasing from the biotite through the garnet, staurolite, and sillimanite zones − recorded in the Pontiac Group metasedimentary sequence, which is fault-bound to the southern Abitibi Subprovince and its orogenic gold deposits. The prograde pyrite to pyrrhotite reaction texture, where pyrrhotite (Po) replaces corroded pyrite (Py₁), is identified in rocks from the biotite and garnet metamorphic zones. Py₁ and Po coexist in the staurolite zone without a clear textural relationship, and past the sillimanite isograd, Py₁ is completely absent with Po in abundance. Py₁ and/or Po are replaced by euhedral to subhedral, inclusion-free, late/retrograde pyrite (Py₂) in all metamorphic zones. In situ trace element analysis of the sulfide assemblage reveals a decrease, with increasing metamorphic grade, in metal contents such as Au, Ag, Te, Bi, and Se, associated with the prograde replacement of Py₁ by Po. Arsenic and Co are preserved and enriched in Py₂, while Ni is concentrated in Po, and Cu is redistributed to chalcopyrite. In situ, multiple sulfur isotope analyses show no analytically distinguishable fractionation (δ³⁴S shift of −0.1‰ ± 0.9 and a Δ³³S shift of +0.06‰ ± 0.17) between Py₁ and Po. These results indicate that the H₂S produced by the desulfidation reaction retains the δ³⁴S-Δ³³S signature of Py₁ across the biotite, garnet, and staurolite zones. This detailed examination of the textural and chemical evolution of sulfides highlights the role of desulfidation in metal mobility. The pyrite to pyrrhotite reaction can be an efficient mechanism for supplying sulfur, Au, Ag, Te, Bi and Se, and if synchronous with the release of fluids and deformation, mobilizing Au through the Earth's crust.