GEOCHEMICAL, MINERALOGICAL AND MICROBIOLOGICAL CHARACTERIZATION OF A SULFIDE-BEARING CARBONATE-RICH GOLD-MINE TAILINGS IMPOUNDMENT, JOUTEL,QUEBEC

The results of an integrated geochemical and mineralogical study condu cted at the Agnico-Eagle gold-mine tailings impoundment, Joutel, Quebe c, are correlated with bacterial populations determined from an enumer ation of 3 groups of Thiobacilli. The tailings were determined to cont ain approximately 5 wt.% sulphide-S, predominantly as pyrite, and up t o 30 wt.% carbonate minerals, chiefly as dolomite-ankerite and siderit e. The objective of the study was to evaluate the potential for the de velopment of acidic drainage and dissolved-metal migration in carbonat e-rich tailings impoundments, and to compare the results of the geoche mical and microbiological characterization of the tailings. Sulphide-o xidation reactions have proceeded to a depth of 20-100 cm below the ta ilings surface. Pyrrhotite consistently shows more alteration than pyr ite and arsenopyrite. Pyrrhotite is altered mainly through the replace ment by goethite. The most abundant Thiobacilli are neutrophilic bacte ria of the Thiobacillus thioparus type. The maximum most probable numb er values for these bacteria occur 20-40 cm below the tailings surface , a depth that coincides with the disappearance of oxide coatings. Thi s observation, coupled with the sharp decline in gas-phase O-2 concent ration, suggests that rapid bacterially-mediated S-oxidation is occurr ing at this depth. The pore-water pH throughout the tailings varies be tween 6.5 and 8.5; nd low-pH waters were observed in the impoundment. These neutral pH conditions are attributed to the effect of acid-consu ming carbonate-mineral dissolution reactions, which are also indicated by increased concentrations of Mg and Ca and alkalinity in the shallo w zone of the tailings. As a result of these acid-neutralization react ions, dissolved metal concentrations are low. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.