LABORATORY INVESTIGATION OF ARSENIC MOBILITY FROM GOLD MINE TAILINGS

Nova Scotia, Canada has a long history of gold mining; more than 64 mining districts in the province have produced a total of 1.2 million ounces of gold since 1860 from carbonate- and arsenopyrite-bearing flysh deposits of the Meguma Supergroup. These small scale operations have resulted in several high arsenic tailings areas (up to 31 wt% As) that are characterized by pH-neutral surface water. Many of these sites are easily accessible to local residents, and have been used as recreational areas for ATVs and dirtbikes. A laboratory column study was used to investigate arsenic mobility from tailings solids under three different scenarios: 1) unremediated as-is tailings; 2) tailings remediated with a crushed limestone cover; and 3) tailings remediated with a vegetative cover. The overall objective of this project is to reduce human exposure through the use of physical barriers and to limit potential future degradation of groundwater quality.

Four tailings types including an oxidized arsenic-rich end-member, an unoxidized high calcium to arsenic ratio end-member, and two samples representing bulk oxidized and unoxidized tailings with chemical compositions between the two end-members were leached weekly with three solutions: a synthetic acid rain solution intended to simulate current unremediated conditions; a calcium carbonate-saturated acid rain solution designed to simulate rainwater percolation through a limestone cover; and an organic acid solution projected to simulate plant root environments in revegetated tailings.

Multiple arsenic-bearing phases including primary arsenopyrite (FeAsS) and its oxidation products scorodite (FeAsO₄•2H₂O) and yukonite (Ca₇Fe₁₂(AsO₄)₁₀(OH)₂₀•15H₂O) have been identified as arsenic-hosting phases. The variable stabilities of these primary and secondary minerals have been shown to greatly influence arsenic mobility under the range of geochemical conditions investigated. Notably, the synthetic leachate solutions from each of the three scenarios appear to have less of an affect on arsenic mobility than the original tailings mineralogy for each of the four tailings types.