Geochemical and mineralogical characterization of arsenic in lake sediments impacted by legacy gold mining in the Yellowknife region

Abstract

Arsenic concentrations in lake sediments in the Yellowknife region, Northwest Territories, Canada are elevated from the weathering of mineralized bedrock and/or from the mining and processing of arsenic (As)-bearing ores at legacy gold mines in the area. Specifically, 50 years of ore roasting at Giant Mine (1948-2004) resulted in the release of approximately 20,000 t of arsenic trioxide (As2O3) to the atmosphere as stack emissions. Historical tailings disposal at Giant also introduced As- and antimony (Sb)-rich materials to sediments in Yellowknife Bay, adjacent to the mine property. This research aimed to understand the origin and long-term fate of As in lake sediments in the Yellowknife region.

Elevated As concentrations in regional lake sediments are largely the result of ore roasting at legacy gold mines. Radiometric dating of sediment cores indicated that the majority of As was introduced to the sediments during the period of maximum regional stack emissions. Anthropogenic As2O¬3 has persisted in lake sediments for more than 60 years, though its partial dissolution has resulted in the formation of less bioaccessible As-hosting phases including iron (Fe)-oxyhydroxide and As-bearing sulphides. The analysis of 47 sediment cores from a single emissions-impacted lake showed that sediment As concentrations and solid-phase speciation vary across the lake bottom and with sediment depth. Two distinct types of down-core As concentration profiles were identified and are interpreted to represent erosional and depositional areas. Statistical analysis revealed that water depth, as a proxy for sediment focusing, is the best predictor of As concentration in the top 5 cm of sediments. Iron concentration, as an indicator of As, Fe, and sulphur (S) co-diagenesis, is the best predictor of As concentration in deeper sediments. Sediments are a source of As to the overlying water column through diffusion-controlled release to bottom waters.

The solid-phase distribution of As and Sb in Yellowknife Bay sediments was a complex mixture of flotation tailings, roaster waste, and aerially deposited stack emissions. Arsenic and Sb in Yellowknife Bay exhibited differences in post-depositional behaviour that can be attributed to the anthropogenic controls governing the solid-phase speciation of these elements in mine waste produced at Giant.