Sulphur compounds, ubiquitous in the Hamersley Basin, provide valuable insights into the hydrological and biogeochemical function of subsurface environments. Hamersley waters are characterised by large variability in the dissolved sulphate (SO4) found in different aqueous systems (1 mg/L to 15,000 mg/L). This range, coupled with multiple potential sulphate sources with variable isotopic signatures (δ34SSO4 and δ18OSO4), results in the Hamersley region representing an ideal study site to investigate regional sulphur cycling. As pyrite represents one of the dominant source materials, extra focus was placed on identification of pyritic source materials and processes accompanying oxidative weathering of pyrite. Natural weathering of pyritic material was also compared to accelerated oxidation induced by the various mining operations in this region, some of which are known to display localised signs of Acid and Metalliferous Drainage (AMD). The primary aim of this study was to identify the origin of dissolved sulphate delivered to Hamersley aqueous environments and characterise the dominant hydrological and geochemical processes leading to variability in groundwater chemistry, with a particular focus on processes driven by the sulphur cycle.
Coupled, isotope and geochemical analysis, with a focus on dissolved sulphate signatures (δ34SSO4 and δ18OSO4) was used to distinguish the origin of dissolved sulphate and enabled identification of water impacted by pyrite oxidation. Aqueous environments containing pyrite within subsurface material had high sulphate concentrations (>1000 mg/L) and low δ34SSO4 signatures (+1.2‰ to +4.6‰) which reflected the signatures of solid pyritic rock samples (-1.9‰ to +4.4‰). In addition, groundwater δ34SSO4 signatures were also able to distinguish between SO4 in groundwater and minerals derived from oxidation of the different pyritic source rock. Parent pyrite originating from the Roy Hill Shale of the Fortescue Group was 34S-depleted relative to pyritic Mount McRae Shale of the Hamersley Group and this was also reflected in groundwater and sulphate minerals. Oxygen isotope signatures of sulphate (δ18OSO4) and water (δ18OH2O) was further able to constrain pyrite oxidation pathways and identified ferric iron as having a more dominant role in the oxidation process in AMD settings (~70% contribution) relative to natural settings (30 − 40%). End-member modelling, using SO4:Cl ratios and end-member signatures was able to identify an additional atmospheric source of sulphate in Hamersley waters characterised by a δ34S value of ~+7.5‰.
Sulphate concentrations and acidity, both by-products of the oxidation process was found to largely depend on aquifer lithology and availability of carbonate minerals. Groundwater deriving from subsurface lithological units containing both pyrite and carbonate minerals, was found to have high alkalinity, a neutral pH, and were saturated in respect to gypsum. High magnesium in the majority of samples suggested dedolomitisation was the dominant process buffering the acidity generated by oxidation. However, in a small number of aqueous systems, weathering of silicate minerals was also identified to play a dominant role in acid neutralisation. This study demonstrated that, in the Hamersley region, sulphur isotopes when coupled with geochemical analysis provide a robust tool to investigate weathering and hydrological processes related to sulphur cycling.
|Publication status||Unpublished - Oct 2014|