Mobilization and attenuation of heavy metals within a nickel mine tailingsimpoundment near Sudbury, Ontario, Canada

The oxidation and the subsequent dissolution of sulfide minerals within the Copper Cliff tailings area have led to the release of heavy metals such as Fe, Ni, and Co to the tailings pore water. Dissolved concentrations in exc ess of 10 g/l Fe and 2.2 g/l Ni have been detected within the shallow pore water of the tailings, with increasing depth these concentrations decrease to or near analytical detection limits. Geochemical modelling of the pore-w ater chemistry suggests that pH-buffering reactions are occurring within th e shallow oxidized zones, and that secondary phases are precipitating at or near the underlying hardpan and transition zones. Mineralogical study of t he tailings confirmed the presence of goethite, jarosite, gypsum, native su lfur, and a vermiculite-type clay mineral. Goethite, jarosite, and native s ulfur form alteration rims and pseudo-morphs of the sulfide minerals. Inter stitial cements, composed of goethite, jarosite, and gypsum, locally bind t he tailings particles, forming hardpan layers. Microprobe analyses of the g oethite indicate that it contains up to 0.6 weight I % Ni, suggesting that the goethite is a repository for Ni. Other sinks detected for heavy metals include jarosite and a vemiculite-type clay mineral which locally contains up to 1.6 weight % Ni. To estimate the mass and distribution of heavy metal s associated with the secondary phases within the shallow tailings, a serie s of chemical extractions was completed. The experimental design permitted four fractions of the tailings to be evaluated independently. These four fr actions consisted of a water-soluble, an acid-leachable, and a reducible fr action, as well as the whole-rock total. Twenty-five percent of the total m ass of heavy metals was removed in the acid-leaching experiments, and 100% of the same components were removed in the reduction experiments. The data suggest that precipitation/coprecipitation reactions are providing an effec tive sink for most of the heavy metals released by sulfide mineral oxidatio n. In light of these results, potential decommissioning strategies should b e evaluated with the recognition that changing the geochemical conditions m ay alter the stability of the secondary phases within the shallow tailings.