Environmentally Hazardous Boron in Gold Mine Tailings, Timmins, Ontario, Canada

Traditionally, cyanide, mercury, and acid mine drainage are considered critical environmental hazards associated with gold mining. To our knowledge, this is the first study of hazardous concentrations of soluble boron (B) in a gold mine impoundment tailings dam. We suggest that the B anomaly is a consequence of disposal of gold pyrometallurgical waste (slag). Borax is a common flux used during fire assaying and refining of precious metals. Vast amounts of B-rich slag may have been discarded by mining operations and precious metal refineries worldwide, but the extent and effects of B contamination have not yet been addressed. Anomalous concentrations of soluble B found in the McIntyre mine tailings, Timmins, Ontario highly exceed recommended thresholds for groundwater, freshwater, and soil leachates, and cannot be explained by natural sources alone. Boron is distributed heterogeneously within the tailings dam and correlates positively with the percentage of ≤38 μm grain-size fraction, indicating that adsorption onto silt–clay particles results in B build-up. Ephemeral, efflorescent Mg-borate found along the dam embankment suggests an outflow of B along paths of high permeability. Leachability tests indicate that slags from gold-ore refineries and fire assaying labs release ≤12 wt% soluble B in only 24 h. This high leachability suggests that slag discarded on the tailings dam is the dominant source of observed B anomalies. Altered metabasalts with <3,000 mg/kg B may be a minor B source. Gangue minerals such as tourmaline, anhydrite, calcite, and siderite cannot account for the amount of soluble B found in the tailings.