Investigating sulfur pathways through the lithosphere by tracing mass independent fractionation of sulfur to the Lady Bountiful orogenic gold deposit, Yilgarn Craton

As sulfur is the primary complexing ligand for gold transport in aqueous fluids, recent work has demonstrated that the Archean systems can preserve mass independent fractionation of sulfur (MIF-S) to yield near-zero values (Δ³³S₁ = 0 ± 0.5‰). In such cases, interpretation of the geological significance of such isotopic signatures may be compromised as it is known that mass dependent fractionation of sulfur (MDF-S) can yield small Δ³³S values (herein as Δ³³S_B) often owing to geobiological reactions involving sulfur. In this study, we reassess the range of possible values of Δ³³S_B from the traditionally accepted value of 0 ± 0.2‰, to demonstrate that this range is not constant but rather directly linked to the degree of δ³⁴S fractionation. We present a multiple sulfur isotope dataset from the Neoarchean Lady Bountiful orogenic gold deposit from the Yilgarn Craton, Western Australia. This deposit contains pyrite with an isotopic composition of δ³⁴S = ~+2.1 ± 0.8‰ and Δ³³S = +0.14 ± 0.06‰ (2SD on the mean). The analysis of these results, as well as of a data compilation from other ~2.65 Ga orogenic gold deposits of the Yilgarn Craton, indicates that Δ³³S₁ values are too large to be the result of purely MDF-S processes. In addition, when compared to a compilation of >3400 Δ³³S₀ measurements of Archean sedimentary rocks, which shows highly variable positive and negative signals, the remarkably constant positive MIF-S₁ signature of gold deposits from the Yilgarn Craton, with Δ³³S₁ values between 0 and +0.6‰, is evidence that the source of the auriferous fluids equilibrated at depth by mixing with a non MIF-S₀-bearing sulfur reservoir. By enhancing the application of MIF-S₁ (Δ³³S₁) as a powerful and indelible tracer to understand the source and emplacement pathways of gold-bearing fluids through the lithosphere, the outcomes from this study demonstrate the recorded +Δ³³S₁ signature in the auriferous fluids reflects the presence of sulfur that was at least partially sourced from an Archean sediment reservoir.