Authigenic iron oxyhydroxide rims attenuate deleterious element fluxes during sulphide oxidation in historical gold mine tailings

Historical gold tailings pose environmental hazards globally, leading to acid mine drainage and the release of harmful elements due to the oxidation of unrecovered sulphides. The metastable secondary iron oxyhydroxides formed during this process influence the mobility of these elements, although the specific mass flows between the sulphide cores, iron oxyhydroxide rims and the environment remain poorly understood. Using a gold tailings storage facility in the Klerksdorp goldfields (South Africa) as a natural laboratory, this study investigates the role of secondary iron oxyhydroxide rims in controlling the mobility of Co, Ni, As, Pb, Zn, Au, and Cu after prolonged exposure of tailings to surface conditions. The release versus retention of these deleterious elements is characterised and quantified using a multi-method approach including LA ICP-MS, automated mineralogy, and wet chemistry. Although iron oxyhydroxide rims make up less than 1% of the mineralogy, they strongly retain As, Ni, Cu, and Zn fluxes that emanate from the precursor sulphides. In contrast, Co, Au, and Pb show limited compatibility (< 3% deportment) with these rims, suggesting their preferential mobilisation into the environment. The measured immobilisation of some of these elements is insufficient to meet environmental standards, highlighting the need for direct remedial measures, including metallurgical reprocessing and effective capture mechanisms for deleterious elements. These strategies are essential for reducing environmental risks and can simultaneously recover valuable (and critical) metals such as Au, Cu, Co, and Ni.