Iron-hydroxide, iron-sulfate and hydrous-silica coatings in acid-mine tailings facilities: A comparative study of their trace-element composition

Abstract

Abstract Surface alteration-layers often coat minerals in acid-mine drainage systems and the characterization of their chemical composition is required to understand the uptake or release of potentially toxic elements. Samples with micrometer-thick rock coatings were collected from bedrock in contact with three acidic tailings ponds and a small lake, all located within the Copper Cliff mine tailings disposal area in Sudbury, Ontario, Canada. Distribution and concentration of trace-metals in the rock coatings were characterized with Laser-Ablation Inductively-Coupled Plasma Mass Spectroscopy and Micro X-ray Fluorescence Spectroscopy. The rock coatings are composed of goethite, ferrihydrite, schwertmannite, jarosite and amorphous silica. The latter phase is a product of the non-stoichiometric weathering of the underlying siliceous rock. Layers within the coatings are distinguished on the basis of their atomic Fe:Si ratios: FeO x coatings have Fe:Si > 4:1, Si–FeO x coatings have Fe:Si = 4:1 to 1:1 and SiO x coatings have Si > Fe. Iron-rich coatings (FeO x ) in contact with acidic tailings ponds (pH x in contact with lake water at near neutral pH have similar trace-metal concentrations than Si–FeO x and SiO x , most likely the result of higher adsorption rates of metals at near neutral pH conditions. High trace-metal concentrations in Si–FeO x and SiO x are explained by the presence of jarosite-group minerals, which formed within Si-rich alteration layers through mixing of leached alkaline cations and trace elements from the underlying rock and Fe 3+ -sulfate solutions from the pond. Calculated enrichment factors for trace metals and metalloids in the coatings (relative to the pond) indicate that the mobility for Pb, As, Cr and Cu in the upper part of tailings ponds is commonly lower than the mobility for Zn, Mn, Co and Ni. The environmental significance of these findings is discussed in terms of the attenuation of trace metals in the coatings and the widespread occurrences of silica gels and jarosite-group minerals.