Geochemical Evolution of Froth Treatment Tailings at an Oil Sands Mine in Northern Alberta, Canada

Froth Treatment Tailings (FTT) are generated during water-based bitumen extraction in the oil sands mining operations in Northern Alberta, Canada. Acid generation and metal(loid) release during sulfide-mineral oxidation is an emerging consideration for management and reclamation of FTT deposits. This pyrite-bearing tailings stream is deposited sub-aerially where post-depositional hydraulic segregation of FTT slurry generates beach deposits susceptible to oxidative weathering. Recent studies have demonstrated the potential for acid generation and metal(loid) release during oxidative weathering of FTT however, the geochemical evolution of commercial scale FTT beach deposits is not fully understood. Refining the geochemical conceptual model of FTT beach deposits will provide insight into tailings management strategies and landform reclamation planning. This research examines the geochemical evolution of a commercial-scale sub-aerial FTT beach deposit. A sampling campaign was completed on the Plant 6 FTT beach deposit at the Mildred Lake mine operated by Syncrude Canada Ltd (Syncrude). Continuous core sampling was conducted on five boreholes collected along a transect of the P6 FTT Beach deposit which approximately followed historic FTT deposition points. Boreholes were drilled to a depth ranging from approximately 30 to 46 m and tailings solid samples were collected along the borehole depth profiles at each location. Tailings pore-water was extracted by mechanical squeezing from solid samples and tailings solids were sub-sampled for subsequent analysis. Mineralogical analysis revealed that FTT beach solids are generally dominated by quartz plus various phyllosilicates, sulfides, carbonates, oxides, phosphates, and additional silicates occurring at minor to trace amounts. Sulfide and carbonate mineral contents each range from approximately 5 to 10 wt. % but their abundance can exceed 20 wt. % at discrete locations. Pyrite and minor marcasite are the only sulfides detected in FTT beach samples. Siderite is the dominant carbonate in both weathered and non-weathered FTT beach samples, which commonly contain lesser amounts of dolomite, ankerite, and calcite. Pore-waters in non-weathered regions of the FTT beach deposit are generally characterized by circum-neutral pH (7–8) and relatively low dissolved SO4, Fe, and metal(loid) concentrations.