Indium distribution in metalliferous mine wastes of the Iberian Pyrite Belt, Spain–Portugal

Indium is a rare, post-transition metal that is widely used in modern technological applications. However, despite its known toxicity the environmental processes of this element are poorly understood. This study reports on the abundance, mineralogical siting and mobility of indium in waste rock at six selected historic mine sites (Angostura, La Zarza, Rio Tinto, San Telmo, Sao Domingos, Tharsis) of the Iberian Pyrite Belt (IPB), Spain–Portugal. Sulfidic waste rock samples are invariably enriched in indium (mean 6 mg/kg In), with concentrations well above average crustal abundances (50 µg/kg In). Indium is largely present as cation substitutions in the crystal lattices of sulfide minerals, in particular kesterite (max. 0.5 wt% In), sphalerite (max. 0.27 wt% In) and chalcopyrite (max. 0.05 wt% In). The sulfidic wastes have undergone extensive oxidation prior to and after mining, with partially oxidized sulfidic waste rocks invariably enriched in indium (mean 1.5 mg/kg In). At the toe of waste dumps, oxygenation of acid rock drainage (ARD) waters leads to the development of abundant secondary metal and/or alkali (hydrous) sulfates and the formation of Fe-rich precipitates and evaporative salts (mean 1.5 mg/kg In). Consequently, indium is significantly enriched in sulfidic mine wastes of the IPB and displays mobility in ARD environments, with its mobility somewhat constrained by oxygenation of ARD waters and subsequent iron precipitation.