Controls on the mobility of mercury from gossan mine tailings Murray Brook mine, New Brunswick

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University of New Brunswick


Between 1989 and 1992, gold was recovered, by cyanide heap leaching, from a gossan overlying the Murray Brook massive sulfide deposit in Northern New Brunswick. In 1991, elevated mercury and cyanide concentrations were discovered in groundwater below the tailings. Those data indicated that residual cyanide from the gold leaching operation contributed to the increased mercury mobility from the tailings through solubility enhancement by cyanide complexation. The concentration of cyanide in the groundwater has declined since 1993, with a coincident decline in mercury concentration. Residual cyanide in the tailings is expected to continue to degrade and the present study is designed to evaluate the degree of mercury mobility within the tailings pore water that could occur in the future as a result of complexation with natural ligands such as humic acid, OH and Cl. Based on X-ray diffraction, optical and scanning-electron microscopy, and electron microprobe examination of the tailings, mercury is distributed, probably through coprecipitation, within the secondary ferric oxy-hydroxide and sulfate minerals such as goethite [FeO(OH)] and beudantite [PbFe3(As04)(S04)(0H)6]. Batch leaching experiments were conducted on the tailings by varying the concentrations of humic acid, OH and Cl to determine the effect on mercury concentration in the leachate. Analyses of the leachate indicate that residual cyanide is still present in the tailings, even where tailings are exposed at the surface. Equilibrium geochemical speciation modeling of the leachate indicates that the mercury is complexing with cyanide as Hg(CN)2 , and natural ligands such as Cl and OH have no effect in controlling mercury concentrations. The pH of the water can influence the mercury concentrations in solution due to the effect of mercury adsorption to solid surfaces. These results may have implications with respect to plans for rehabilitation of the gossan tailings