The role of hardpan formation on the reactivity of sulfidic mine tailings: A case study at Joutel mine (Québec)

The former Eagle and Telbel mine site (hereafter referred to as Joutel mine), located near the town of Joutel in the Nord-du-Québec (Canada) houses a tailings storage facility (TSF) that has been inactive since 1996. Fresh, unweathered tailings (beneath 10–30 cm of oxidized horizon) are characterized by an average sulfide content of 6–7 wt% and an average Fe-Mn-carbonate content of 20–40 wt%. The oxidation of Joutel's tailings under atmospheric conditions resulted in the precipitation of secondary phases such as ferric oxyhydroxides and gypsum. Accumulation of these secondary phases throughout the TSF caused cementation and agglomeration of grains, which decreased the porosity of the material in a horizon below the surface. This horizon, which is referred to as hardpan, is frequently encountered within fine, reactive tailings. Characterizations showed that hardpans have a highly compact texture. The formation of hardpans limits vertical water infiltration and oxygen diffusion and these layers greatly affect the global geochemical behavior of underlying tailings in the Joutel TSF by protecting the unweathered material from oxidation. As a result, the water quality of the TSF is largely controlled by the reactivity of the upper oxidized tailings horizon. Joutel's oxidized tailings showed an acidic behavior early during laboratory kinetic leaching tests despite the near absence of sulfides and neutralizing minerals. However, when unweathered tailings were added under oxidized tailings, the water became neutral and metal leaching rates were reduced. After over a year of laboratory leaching tests, hardpans formed within the columns and the natural phenomenon was reproduced under laboratory conditions. © 2018 Elsevier B.V.