How to grow trees on the wastes of a boreal gold mine – identification of the main physico-chemical limitations

The Osisko gold mine in Malartic, Quebec (Canada) is an open-pit mine producing low grade ore and is designed to operate at 55,000 metric ton per day. Its non-acid generating wastes will eventually cover large surfaces that will need to be reclaimed. To minimise mine impacts on water consumption, milling wastes will be deposited in the form of thickened tailings (68% DM). To our knowledge, no study of vegetation establishment on this type of mine waste has yet been published. The Osisko mine is located on the edge of a small town. Consequently, visual aspects of mine waste reclamation are a major concern with respect to social acceptability of the company. Since the surrounding region is covered by boreal forest, tree colonisation appears essential. Tree plantations may accelerate the conversion of degraded lands into forests and also add biodiversity to the area. Two studies were conducted, one on overburden waste rock and a second on milled tailings. In the first study, plantations were established on compacted waste rock (10:1 slope) covered with overburden topsoil or subsoil at two compaction intensities. The second study was conducted in a glasshouse to evaluate the thickened tailings’ capacity to sustain tree growth (tamarack, jack pine, black spruce, basket willow, hybrid poplars, and green alder). Tailings alone or mixed with several amendments were tested (overburden soils, vermicomposts from food wastes, chicken manure, peat). In addition, we compared the use of a thin or a thick layer of overburden topsoil. We showed that direct planting in the thickened tailings was not suitable for boreal trees under glasshouse conditions. This substrate high water retention capacity and low macroporosity (or air-filled porosity, ≤ 8%) may be responsible for tree death by limiting 02 availability required for root respiration. To overcome this aeration stress, an organic matter-rich amendment should be used to raise macroporosity to levels suitable for tree growth. Peat (8% DM in the mixture) was the most effective amendment for improving macroporosity (27%) compared to other amendments, including two different composts (22 and 35% DM in the mixtures leading to 14 and 17% macroporosity, respectively) and mine soils. The use of composts produced appropriate macroporosity levels but also increased electrical conductivity to levels (3.4 to 4.1 mS cm-1) limiting broad-leaved species survival and conifer biomass production. No trace metal contamination of the trees occurred in the mixtures, probably due to the near-neutral pH conferred by the tailings. Moreover, the presence of underlying alkaline tailings limited Mn, Zn, and Al phytotoxicity of the acidic overburden topsoil layer from occurring in tree leaves. Consequently, growth of jack pine and all broad-leaved species was improved with a thin layer compared to a thick layer of overburden soil, although these trees also showed Cu accumulation in their fine roots (>100 mg.kg-1). This study demonstrated that the use of thin layers of acidic overburden topsoil above alkaline thickened tailings could improve tree growth and accelerate landscape reclamation.