Geochemical controls on the environmental mobility of Sb and as at mesothermal antimony and gold deposits

Abstract

Antimony and arsenic are commonly mobilised into the environment from mesothermal mineral deposits. Both these metalloids are potentially toxic in the environment when dissolved in water at low levels (<0·01 mg/l). Mobility of antimony, in comparison to that of arsenic, is documented at mine sites in four different mesothermal systems in low grade Palaeozoic-Mesozoic metamorphic terranes of New Zealand (Globe Hill, Reefton; Macraes, Otago; and Endeavour Inlet, Marlborough) and Australia (Hillgrove, New South Wales). Dissolved antimony can reach ∼50 mg/kg in mine waters where evaporative concentration occurs in oxidised near-neutral pH mine waters in contact with stibnite. Such waters are chemically saturated with respect to antimony oxides, and antimony oxide precipitation occurs locally. Most mine waters have lower dissolved Sb concentrations, especially where high rainfall causes dilution. However, high rainfall areas have widespread diffuse mobilisation of both Sb and As to give elevated background levels (0⋅1–0⋅01 mg/kg) in downstream waters. Dissolved Sb is decreased by adsorption of Sb to hydrated iron oxide (HFO) precipitates in streams. Bulk distribution coefficient, K_d, for this Sb adsorption ranges up to at least 10⁵, and is similar to that of As adsorption in the same settings. Attenuation of dissolved Sb by HFO results in an order of magnitude decrease in Sb concentrations on a scale of metres. In the absence of HFO, dissolved Sb can be transported in streams for many kilometres. Elevated dissolved Sb concentrations can arise distant (tens of kilometres) from a mine site because of dissolution of antimony-bearing minerals physically transported downstream. Hence, without HFO attenuation, environmentally toxic levels of Sb can be readily leached from mine sites and distributed widely.