Metalloid Attenuation from Runoff Waters at an Historic Orogenic Gold Mine, New Zealand

Abstract

The metalloids arsenic and antimony are mobilised through the decomposition of arsenopyrite, pyrite, and stibnite at the historic Big River Mine. Their content in the solid substrates reach levels up to ≈25 wt% As and 3.6 wt% Sb, but their concentrations in the mine water are only elevated locally, up to 0.85 mg/L dissolved As and 0.007 mg/L dissolved Sb. Water draining the waste rock are characterised by neutral pH, but at the mine’s former processing site, the oxidation of sulphide concentrates has produced acidic conditions, both in the water (pH 3.8) and solid mining residues (paste pH 2–4). Dissolved metalloid concentrations are being effectively attenuated on site through the formation of secondary As phases in the mining residues, including scorodite, iron sulphoarsenate, and amorphous iron arsenate. Attenuation is also occurring by co-precipitation and adsorption onto iron oxides or hydroxides, and adsorption onto clay mineral surfaces. These processes are complemented by dilution of the dissolved As and Sb by the Big River. Therefore, despite the extremely high levels of As and Sb in the mining residues, metalloid mobilisation is limited, resulting in relatively low concentrations in water leaving the site.