Résumé - Abstract
Prior to final shutdown of the Faro Mine Complex, YT, formerly the largest open pit Zn-Pb mine in the World, 320 million tonnes of sulfide-bearing waste rock was deposited in 31 waste-rock piles and low-grade stockpiles at the site. The extent and duration of low-quality drainage from waste-rock piles is controlled by complex interactions between physical, geochemical and microbiological processes, which are affected by the waste-rock characteristics, climate and waste-rock pile construction characteristics. The Faro Waste Rock Project, an interdisciplinary investigation of the sulfide-bearing waste rock contained in the Main and Intermediate Waste-rock Piles at Faro, includes comprehensive measurements of the characteristics of the waste rock. This waste rock characterization indicates heterogeneous distribution of sulfide and carbonate minerals, resulting in zones of potentially acid generating (PAG) rock adjacent to zones of non-acid generating (NAG) rock. Measurements of water isotopes (?2H, ?18O) indicate that water flow through the waste-rock piles is predominantly derived from rainfall, with lesser contributions from snowmelt. Exothermal oxidation of sulfide minerals has resulted in evaporation within the waste piles. Gas transport within the waste-rock pile is complex; although O2(g) transport within much of the waste pile is controlled by diffusion, thermally driven advective gas transport is actively promoting sulfide oxidation in zones deep within the pile. The oxidation of sulfide minerals is catalyzed by neutrophilic and acidophilic sulfur- and/or iron-oxidizing microorganisms, including, among others, members of the Alicyclobacillus, Acidithiobacillus and Thiobacillus genera. Synchrotron-based determination of sulfur speciation indicates that oxidation occurs through both polysulfide (dominant) and thiosulfate oxidation pathways. The dissolution and precipitation of primary and secondary phases control the pore-water pH and the release and attenuation of sulfate and dissolved metals. The depletion of carbonate minerals has resulted in the development of low-pH zones within the waste-rock pile. The effluent from the Main and Intermediate Waste-Rock piles currently is near-neutral pH, and characterized by high concentrations of dissolved sulfate (mean concentration 8,970 mg L-1; maximum 14,200 mg L-1) , iron (mean concentration 6.3 mg L-1; maximum 32.1 mg L-1) and other metals (e.g., zinc: mean concentration 620 mg L-1; maximum 1,160 mg L-1). Data analysis and numerical modelling studies will provide a basis for improved predictions of future effluent water chemistry and for assessment of remediation strategies, which can be used to assist in the development of closure plans.