Stable isotope evidence for thermochemical sulfate reduction in the Dugald River (Australia) strata-bound shale-hosted zinc-lead deposit

Sulfur isotopes vary along strike and down-dip in the mid-Proterozoic Dugald River strata-bound shale-hosted zinc-lead ore deposit, Australia. Although the metal grade has been substantially increased at its southern end by tectonism, geological relationships indicate that mineralisation occurred during early diagenesis, probably in an organic-rich, shallow-water to evaporitic setting (Muir, 1983). Cu, Cu/(Cu + Pb + Zn) and Pb/(Pb + Zn) decrease in the mineralisation northwards independent of the main structural thickening, and so are likely to represent a pre-deformational primary geochemical dispersion. Average δ34S values in pyrite and sphalerite range from ∼ + 8‰ in the south, coincident with the highest copper content of the ores, to ~ + 8‰ in the north, accompanying deposit thinning. In contrast, adjacent footwall and hangingwall iron sulfide does not exhibit isotopic zonation, and has a δ34S peak between +3 and +4‰ (n = 33), with some values as light as -14.5‰, whereas overlying dolomites contain δ34Spy = +5.5 to + 17.5‰; each population is attributed to varying degrees of closed-system biogenic sulfate reduction. Two populations of carbonate carbon isotopes are present. The first (δ13C = - 11 to -5‰) mainly characterises graphitic Footwall Limestone of the Lady Clayre Dolomite, and is interpreted to reflect the metamorphic equilibration of carbonate and organic carbon isotopes. The second (δ13C = -25 to -15‰) characterises the sulphide lens and its immediate sedimentary host, and is most consistent with formation by the oxidation of organic carbon. Thermochemical sulfate reduction (TSR) by organic matter is preferred to account for the δ34S zonation, because of: (1) mass-balance calculations; (2) the δ13C values of ore and near-ore carbonate; (3) the high temperatures likely for ore formation (150-250°C), which would have prevented biogenic sulfur reduction but promoted TSR; and (4) the trend toward δ34Sseawater. The proposed mineralisation model involves an H2S - and metal-bearing fluid with δ34S = -3 to 0‰. This fluid ascended into the Dugald basin in the south, permeating laterally northward through carbonaceous sediments, below an evaporitic to shallow marine carbonate platform. During this migration, the hydrothermal fluid reacted with organic matter and sulfate to form 34S-rich sulfide (δ34S = +8 to +10‰) by TSR. Reaction was catalysed by deep sourced H2S, biogenic H2S and H2S that evolved by hydrothermal-cracking of organic matter. The sources of sulfate for the reaction were: (1) evaporites locally within the sequence; and (2) dissolved sulfate diffusing from surface waters into the diagenetic zone.