Environmental mineralogy and geochemistry of processing residues at Prohibition historic gold mine site, Waiuta, Westland, New Zealand

The Prohibition mine processing site was abandoned in 1951. Part of the site was remediated in 2016, but some remaining processing residues exposed at the surface have elevated As, Cu, Hg, Pb and Zn. Most of these residues consist of haematitic sandy tailings with oxidised As material (As = 4000–9000 mg/kg), and that As is largely immobile in the environment. Minor As-rich flue residues have As contents of 600–4000 mg/kg, from which As may be mobilised into the environment, but dissolved As <0.3 mg/L prevails in associated waters. Lead-rich residues (locally >5 wt% Pb) from an assay office including arsenic-rich glassy slags (locally >3 wt% As) were also deposited. Naturally colonising plants have absorbed low As contents, with <2 mg/kg (dry weight) in shrubs and up to 20 mg/kg (dry weight) in grasses. This natural revegetation appears to be adequate for site rehabilitation.