Oil sands tailings are important, globally relevant, S reservoirs, known to contain active and diverse microbial communities. However, the potential for S bigeochemical cycling occurring within composite tailings (CT), a mixture of fluid fine tailings, post-processed sand and gypsum (flocculant) used in dry reclamation approaches, has not been examined prior to this study. Biogeochemical characterization of CT solid and porewater samples taken from 5 depths over a 40 meter CT deposit (Mildred Lake, Syncrude Ltd, Fort McMurray, AB, CANADA) revealed distinct depth dependent zones of surficial iron reducing and more extensive sulphate reducing microbial activity. Porewater H2S was detected below 6 meters in the deposit ranging in concentration from 14–300 µM, while much lower concentrations, 1-40 µM, of porewater Fe2+ were restricted to surficial CT samples. Metagenomic (454 pyrosequencing) characterization revealed highly diverse CT microbial communities, with 21 different phyla encountered overall. However, consistent with the porewater geochemical profiles, two depth dependent, structurally distinct communities emerged from multivariate statistical analyses of phylogenetic data (UniFrac http://bmf.colorado.edu/unifrac): a surficial CT zone of Fe3+ reduction and an underlying, more extensive zone of SO42- reduction. These microbial zones were linked to DOC, redox and salinity conditions within the CT deposit. Consistent with the notion that accessible organic carbon was limiting IRB and SRB activity, available SO42- and Fe3+ sources were evident and abundant throughout the deposit. Pilot reclamation is currently focusing on capping CT with a freshwater wetland, reflecting the original boreal forest landscape. However a wetland may provide a more labile organic carbon source for underlying CT associated bacteria, which may stimulate greater H2S generation and/or aid sequestration through IRB driven FeS formation. Recent field and laboratory results from the adjacent CT pilot fen reclamation site suggest that stimulation of H2S generation associated with downwelling fen organic carbon inputs is occurring. These results identify the need to consider the potential for microbial biogeochemical transformations of waste materials to hinder the efficacy of proposed reclamation scenarios.