Arsenic in Lakes Surrounding Yellowknife: Anthropogenic or Naturally Derived?

One of the oldest and largest mines in the Northwest Territories is Giant Mine, located 5km north of the City of Yellowknife. Over Giant Mine’s 50 years of operation from 1949-1999, it has emitted approximately 20,000 tonnes of arsenic (As) emissions through roasting of arsenopyrite to obtain the submicroscopic amounts of gold. When released into the atmosphere, the As emissions bind with oxygen to form arsenic trioxide (As2O3) which, when dissolved, releases the highly toxic As3+ species into the environment. An additional byproduct of roasting is As-bearing iron (Fe)-oxide. It has been found that high levels of As within the mine property can be attributed to anthropogenic inputs through roasting (Walker et al. 2005, Wrye 2008, Bromstad 2011). In lake sediments surrounding the Yellowknife region, collected by the Geological Survey of Canada (GSC), high levels of As have been measured. Eleven of these lake sediment samples were made into thin sections and observed through Environmental Scanning Electron Microscopic (ESEM) analysis to determine whether the As present is of natural origin or anthropogenically derived. Further Mineral Liberation Analysis (MLA) was conducted on three of the eleven samples. In all samples studied potential As-bearing phases were observed, including both natural and roaster-derived. The phases observed include arsenopyrite (FeAsS) and pyrite (FeS) of natural origin, As-sulphides and Fe-oxides containing either natural or roaster derived As, and finally As-oxides of roaster origin. MLA analysis on the remaining eight samples, as well as microXRD analysis on the synchrotron is recommended in order to develop more definitive and quantitative conclusions.