Experimental simulations of bacterially-mediated magnetite oxidation and observations on ferricrete formation at the Salobo IOCG mine, Brazil

Abstract

Previous data provides evidence that magnetite-rich rock material can sustain large populations of acidophilic, iron-oxidising bacteria. Here we investigate the role of microbially-mediated oxidation of magnetite in more detail, using a strain of Acidithiobacillus ferrooxidans cultured from weathered, iron-rich rocks at the Salobo iron-ore copper-gold (IOCG) deposit, Brazil. The bacterially-mediated oxidation experiments provide evidence for bacterial colonisation and biogeochemical interaction with magnetite mineral surfaces. Experiments were run with different substrates and varying geochemical conditions in three laboratory scenarios: (1) Oxidation of ultrafine chemical grade nano-particulate magnetite over 60 days; (2) oxidation of near-pure magnetite blocks over 90 days; and (3) oxidation of magnetite-rich ore over 180 days. Attachment strategies of bacterial cells to mineral substrates varied with substrate, pH and availability of ferrous iron in solution; colonisation of minerals was generally non-selective. Under acidic conditions (pH 2–3), magnetite oxidation was driven by chemical dissolution that caused crystallographically-controlled differential corrosion of mineral surfaces. At pH > 4, iron-oxidising bacteria that attached to magnetite surfaces did not cause visible surface changes, and we infer that magnetite oxidation may have been occurring via internal electron diffusion, forming pseudomorphous maghemite. The activity of acidophilic bacteria can create local acidic microenvironments in ambient circumneutral pH settings, especially on Fe2+-bearing silicates and sulphides that accompany magnetite. These microenvironments can affect magnetite bio-oxidation mechanisms, causing some magnetite dissolution and/or alteration. Bacterially-oxidised ferric iron, derived from dissolution of magnetite and associated minerals, is precipitated as ferric (oxyhydr)oxide at circumneutral pH. We compare the results of these laboratory studies to observations on magnetite alteration textures in ferricrete overlaying the ore body at the Salobo mine. We suggest that magnetite is also bioavailable to iron-oxidising bacteria in other surficial, acidic to circumneutral pH settings, such as mine tailings and can contribute to varying alteration and dissolution/precipitation processes involving magnetite.