Origin of two distinct multiple-sulfur isotope compositions of pyrite in the 2.5 Ga Klein Naute Formation, Griqualand West Basin, South Africa

Quadruple sulfur isotope ratios (32S/33S/34S/36S) were measured by a high precision SF6 method for early diagenetic pyrite in the 2.5 Ga upper Prieska facies of the Griqualand West Basin from the Agouron drill core GKP01. Measured Δ33S (≈ δ33S–0.515δ34S) values range between −2.3 and +8.5‰ with no apparent meter-scale stratigraphic trend for 70 m section. It was found that early diagenetic pyrites occur in two texturally and isotopically distinct populations such that bulk rock isotope data would reflect mixing of these two sulfur pools. Nodular and layered pyrite yield mostly negative (down to −2.3‰) and relatively constant Δ33S values over a few tens of cm scale. Disseminated pyrites, in contrast, yield positive (0.0–8.5‰) Δ33S values that are heterogeneous in sub-cm scales. The former pyrite sulfur was likely derived from seawater sulfate through microbial sulfate reduction, and the latter preferentially sampled aqueous H2S-S0 reservoirs in late Archean oceans. Relatively constant Δ33S values for layered pyrite suggest Archean sulfate reservoirs had residence times of a few million years. Most data yield a well-defined Δ36S/Δ33S ratios of −0.9, suggesting predominance of a single S-MIF source reaction. Four analyses of a two cm length sample (306.5 m) yield Δ36S/Δ33S slope of −1.6. This slope is consistent with the relationship expected for a symmetry-breaking isotope effect during the S + S2 reaction, but deviations from the main Archean array (i.e., Δ36S/Δ33S = −0.9) may also result from various combinations of SO photolysis and SO2 oxidation. High spatial resolution-high precision SF6 measurements of Archean pyrite provide significant new insights into the origin of Archean sulfur mass-independent fractionation (S-MIF) and the late Archean sulfur cycle.