Diavik Waste Rock Project: Post-emplacement investigation of sulfide weathering from a deconstructed experimental waste-rock pile

Abstract

Deconstruction of a 15 m high low-sulfide (0.035 wt% S) experimental waste-rock pile was conducted to obtain a dense, spatially located sample set for characterization of the solid-phase mineralogy, microbiological community composition, and pore-water geochemistry. The experimental test pile, located in northern Canada, was deconstructed 8 years after construction. Pore-water with low pH was generally associated with elevated concentrations of SO₄ and dissolved metals. Regions of differing pH were present within the test pile, indicating heterogeneous distribution of sulfide and carbonate minerals. Most probable number analysis showed that neutrophilic sulfur oxidizing bacteria were the most consistently prolific of populations associated with acid mine drainage at the time of deconstruction. The mineralogical evolution was examined through comparison of unweathered samples collected at the time of pile construction to weathered samples collected during the deconstruction phase. This analysis indicated that the C content of the waste rock was depleted, but the total S content of the waste rock remained relatively unchanged. Neutralization potential ratios calculated using C and total S indicate that regions of low paste pH (as low as 4.0) were generally consistent with the presence of material classified as potentially acid generating or of uncertain acid generating potential. The deconstruction study shows that variable conditions exist within the test pile (e.g., sharp changes in pH over a few meters) suggesting that waste-rock heterogeneity has influenced the geochemical evolution of the pore water within the test pile.