Mineralogical and Hydrogeochemical Characterization of Legacy Mine Wastes near Cobalt, ON

Abstract

Cart Lake, located 1.6 km south of the town of Cobalt, ON, contains elevated concentrations of As (0.95 to 18.05 mg/L Ast) resulting from uncontained mine tailings deposited during a silver mining boom from 1910 to 1983. Aqueous geochemical and investigations indicate tailings are sulfate-affected but subject to extensive carbonate buffering, with pH ranging from 6.7 to 8.6, Eh ranging from 46 to 210 mV, and alkalinity ranging from 32 to 201 mg/L HCO3-. The dominant cation species are Ca (4.9 to 192.0 mg/L), K (1.1 to 13.5 mg/L), Mg (1.2 to 8.7 mg/L), and Na (2.1 to 16.9 mg/L) and the dominant anionic species are alkalinity, As (0.003 to 18.1 mg/L), Cl- (0.3 to 16.8 mg/L), PO4 (0.04. to 9.6 mg/L), and SO42- (6.8 to 456.1 mg/L). Concentrations of minor and trace cations such as Fe, Co, and Ni are present at the g/L scale. Mineralogical investigations suggest As is mobilized from primary reactive sulfide and arsenide minerals such as arsenopyrite, skutterudite, and safflorite/löllingite/ rammelsbergite solid solutions. Primary mineral alteration products including Ca-Fe arsenates and erythrite are found in these tailings, resulting from direct alteration from primary minerals and precipitation from solution respectively. Secondary Ca-Fe arsenates and Co-Arsenates attenuate As, Co, and Fe by absorption and coprecipitation throughout the tailings profile, influenced by the presence of alteration rims around reactive mineral grains in the vadose zone. Field analysis of cation speciation and geochemical modelling of elemental equilibrium concentrations with weathering primary mineral surfaces would further inform conclusions regarding the relative contributions of each mineral to total As attenuation.