The multiple sulfur isotope composition of diagenetic and hydrothermal sulfides from the Barney Creek Formation (McArthur Basin, Australia): implications for Zn metallogenesis

The mid-Proterozoic McArthur Basin (Australia) hosts exceptionally well-preserved sedimentary rocks and giant clastic-dominant (CD-type) Zn deposits. Previous δ³⁴S-based studies aimed to reconstruct sulfide precipitation pathways; however, interpretations of δ³⁴S values are challenging due to overlapping signatures from organoclastic sulfate reduction (OSR), anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR), and thermochemical sulfate reduction (TSR).

Here, we present new multiple sulfur isotope data from mineralized and unmineralized bulk rock samples of the Barney Creek Formation, host to the Teena Zn deposit, to resolve different diagenetic and hydrothermal sulfate reduction pathways. The sulfide paragenesis includes early diagenetic fine-grained pyrite, overgrown and partially replaced by coarser pyrite and sphalerite. Sulfur was extracted as chromous-reducible sulfur (CRS; n=40) from pyrite, and as acid-volatile sulfur (AVS; n=13) from sphalerite. CRS and AVS show broadly overlapping sulfur isotope compositions, with a wide range of δ³⁴S (–2.8 to 35.7‰), Δ³³S (–0.08 to 0.05‰), and Δ³⁶S (–0.40 to 0.67‰). The multiple sulfur isotope data will be integrated with redox-sensitive trace element data to reconstruct the local depositional redox environment (anoxic, euxinic conditions) in the Teena sub-basin.

Microbial fractionation models and Rayleigh distillation trends suggest sulfides formed mainly via OSR, as the data plot on a convex array in δ³⁴S–Δ³³S space. No evidence of mixing with AOM-SR, which would result in a concave trajectory; nor for TSR, which would produce lower Δ³³S values. Importantly, the results suggest that hydrothermal sulfides most likely precipitated from sulfur that was derived from the replacement of pre-existing diagenetic pyrite.