The sign of Δ33S is independent of pyrite morphology

Abstract

Previous bulk sulphide analyses have suggested a correlation between the morphology of pyrite in Archean metasedimentary rocks and the sign of associated Δ33S. However, it remains to be determined whether such a correlation exists and what the underlying mechanism is. This study measured the multiple sulphur isotopic compositions of pyrite nodules and disseminated pyrite grains from two typical Neoarchean shale samples with SHRIMP-SI, following detailed textural investigations by sodium hypochlorite etching and electron microscopy. The Roy Hill Shale sample shows two generations of pyrite in both nodules and disseminated grains reflected by textures. The first generation is Δ33S-weakly negative while the second generation is Δ33S-highly positive. Both generations of pyrite show linear relationships between δ34S and Δ33S, although generation two displays wider ranges of δ34S and Δ33S compared with generation one. In the Δ33S-Δ36S diagram, generation two plots along the Archean Reference Array (ARA, Δ36S/Δ33S ≈ −1) whereas generation one deviates from the ARA in a trend similar to the Biological Fractionation Line (Δ36S/Δ33S ≈ −7). The Nammuldi shale sample also exhibits two generations of pyrite, both of which are Δ33S- and δ34S-positive albeit of different magnitude. For these two shale samples, the sign of Δ33S is not associated with the pyrite morphology. A plausible explanation for the morphology-specific Δ33S reported in previous studies could be related to bulk analyses involving pyrite of different generations with opposite Δ33S. Furthermore, based on the distribution of the earlier nodular and disseminated pyrite in both samples, a model involving four stages is proposed for the formation of pyrite nodules studied here: (1) formation of pyrite framboids composed of microcrystalline pyrite, (2) dissolution of framboidal pyrite and reprecipitation to generate larger single pyrite crystals, (3) aggregation of single pyrite crystals into nodules, and (4) injection of later hydrothermal fluids.