Physical and Chemical Investigation of Waste Rock From Past Operations at the Detour Lake Gold Mine

In August 2011 researchers from the University of Waterloo, the University of Alberta
and Detour Gold Corp. began a research program at the Detour Lake Gold Mine (DLM).
This research program involves extensive field and laboratory studies and data analysis
(e.g., internal structure, grain size, porewater, pore gas) focused on four waste-rock
stockpiles deposited at the DLM site during operations between 1983 and 1999, and
which had 13 years of post-closure with covers. The goal of the study is to understand
the processes that control the movement and composition of porewater in the historic
waste-rock stockpiles. Although some of the waste rock at DLM is potentially acid
generating (PAG) and has the potential to release dissolved constituents to the
environment, most (83%) of the waste-rock is non-acid generating (NAG). The current
investigation showed that the distribution of PAG and NAG waste-rock in the historic
stockpiles and the degree of oxidation are variable, and zones of oxidized waste rock
are found throughout these piles. The degree of oxidation appears to be related to the
waste-rock type and amount of sulfide minerals rather than the location in the stockpile.
Elevated concentrations of dissolved sulfate and depleted concentrations of pore-gas
O2 indicate that sulfide oxidation is occurring, but the concentrations of dissolved metals
in the matrix porewater are generally low. A predominance of neutral pH porewater and
paste-pH results for the waste-rock matrix suggest that ARD has not set in, however,
acidic paste pH values in about 20% of the samples indicates that once the pH
neutralization capacity of the waste rock is depleted, acidic conditions can develop. A
thin soil cover placed on the stockpiles in the 1990s restricts the rate/extent of sulfide
oxidation. Analysis of these decades-old waste-rock stockpiles will provide DLM with
insight into how to design the new waste-rock stockpiles in a manner that will minimize
the risk of seepages that may evolve during operations as well as for the post closure
period decades into the future. This knowledge will also improve Detour’s ability to
manage new waste-rock stockpiles that are being designed and permitted for the new
West Detour developments. The research project is continuing with additional financial
support from DLM and NSERC and technical support from the staff and graduate
students from the Universities of Waterloo and Alberta, and Carleton University.