Evaluation of the Potential Impacts of Submerging the Oxidized Tailings and Adding a Cover Layer at Mine Principale

Abstract

Oxidation of sulfide minerals generates acid and forms various secondary phases including Fe(III) (oxy)hydroxide phases, jarosite, and gypsum, which can attenuate hazardous metal(loid)s by precipitation, co-precipitation, and adsorption reactions within tailings. The abandoned Cu-Ag mine at Mine Principale, Chibougamau, Québec, has three tailings impoundments (Parcs, A, B, and C) that have oxidized for more than 50 years. This project focused on Parc B, where tailings are stored to an average depth of 8.0 meters below the ground surface (mbgs). The study involved mineralogical, chemical, and microbiological analyses to evaluate the potential impacts of the proposed elevated water table (EWT) technique on the partially oxidized tailings. The EWT involves raising the water table and maintaining it above reactive tailings. Increasing the water content limits sulfide-mineral oxidation by minimizing O2 ingress into the tailings, owing to its low diffusivity in saturated media. However, the EWT can cause direct dissolution of soluble secondary phases, and also can potentially induce reducing conditions that promote microbially catalyzed reductive dissolution of Fe(III) (oxy)hydroxide phases fueled by organic C compounds. In this study, mineralogical and selective chemical extraction techniques indicate the presence of various secondary phases including Fe(III) (oxy)hydroxides that are associated with significant amounts of hazardous metal(loid)s including Cu, Ni, Co, Zn, As and Pb in the oxidized zone, which is limited to a depth of about 1.5 mbgs in Parc B. Results of amplicon sequencing of 16S rRNA genes show significant abundances of Fe- and S-oxidizing bacteria and S-reducing bacteria but no presence of Fe-reducing bacteria. Solid-phase analyses indicate a low amount of total C in the oxidized tailings. These findings suggest that EWT technique can be an effective reclamation method for Mine Principale tailings if use in conjunction with another suitable method to address potential low-quality drainage resulting from the inducement of reducing conditions post EWT application.