Hydrogeochemical, Mineralogical and Microbial Processes Occurring in Old Sulfide-Rich Tailings

The former Sherritt-Gordon Mine, located in Sherridon Manitoba, processed approximately 7.7 Mt of sulfide ore between 1931 and 1951. The Zn-Cu volcanic massive-sulfide ore body yielded Zn and Cu concentrates with minor amounts of Au and Ag. The tailings derived from the ore extraction activities contain up to 60 wt. % as sulfide, mainly as pyrrhotite [Fe1–xS] and pyrite [FeS2], with pyrrhotite equaling or exceeding pyrite in all samples. Other primary sulfide minerals in the tailings include minor amounts of sphalerite [(Zn,Fe)S], chalcopyrite [CuFeS2], and trace amounts of arsenopyrite [FeAsS] and galena [PbS]. Tailings were discharged into three separate impoundments; the Camp tailings deposited between 1931 and 1932; the Woods tailings deposited between 1937 and 1951; and the subaqueous Fox tailings deposited near the end of mining operations in 1951. During the past ~80 years, extensive oxidation of sulfide minerals in the Camp tailings has resulted in extremely high concentrations of dissolved SO4 and metals in the tailings porewater, among some of the highest reported in the literature. \nMineralogical analyses of the Camp tailings found that the pyrrhotite was the first mineral consumed during oxidation followed by sphalerite, then pyrite. Dissolved Ni and Co released from pyrrhotite, and dissolved Zn and Cd from sphalerite, were attenuated when they were sequestered in early-formed Fe oxyhydroxides. The sorptive capacity of Fe oxyhhydroxides decreases over time, and with continued exposure to low-pH porewater, the Ni, Co, Zn and Cd initially sequestered in the Fe-oxyhydroxides may eventually be released back to the porewater. Elements such as Cu, Pb and Cr were observed to form distinct secondary mineral phases or remained incorporated into mature Fe oxyhydroxides, however, Ni and Zn were mobile. Magnetite was relatively stable except under extremely low-pH conditions, where it became a source of Fe(II), Fe(III) and potentially Cr. \nMineralogical and geochemical characterization of the submerged Fox tailings focused on two contrasting areas of this deposit: sub-aerial tailings with the water table positioned at a depth of 50 cm; and sub-aqueous tailings stored under a 100 cm water cover. The sub-aerial tailings showed a zone of extensive sulfide-mineral alteration extending 40 cm below the tailings surface with moderate alteration restricted to depths >60 cm. In contrast, sulfide-mineral alteration within the submerged tailings was limited to a <6 cm zone located below the water-tailings interface. Porewater within the upper 40 cm of the sub-aerial tailings was characterized by low pH (1.9–4.2), depleted alkalinity, elevated dissolved SO4 and metals, and abundant populations of acidophilic sulfur-oxidizing bacteria. Conversely, pore-water in the sub-aqueous tailings was characterized by a circumneutral pH, moderate alkalinity, and low concentrations of dissolved SO4 and metals. The primary control of water chemistry in the sub-aqueous tailings appeared to be dissimilatory sulfate reduction (DSR), evident from elevated concentrations of pore-water H2S, large shifts in δ13C-DIC and δ34S-SO4 consistent with fractionation during DSR, and elevated populations of sulfate reducing bacteria. In addition, mineralogical investigation revealed the presence of secondary sulfide coatings on primary sulfide minerals that may control metal mobility within the sub-aqueous tailings. \nSurface water discharging from the Fox tailings flowed into the Woods tailings. Similar to the Camp tailings, extensive sulfide oxidation had occurred in the Woods tailings during the past 60 years resulting in elevated concentrations of sulfide oxidation products and low pH values in the unsaturated tailings porewater. During precipitation events and the spring freshet, surface seeps developed along the flanks of the tailings impoundment discharging groundwater with very low pH values (e.g., as low as 0.39) and elevated concentrations of dissolved metals (e.g., SO4 up to 203 g L-1; Fe up to 68 g L-1). Efflorescent minerals, including melanterite, rozenite, halotrichite, chalcanthite, alpersite, copiapite, hexahydrite, jurbanite, pickeringite, jarosite and gypsum were observed within these groundwater seepage areas. The formation of these secondary efflorescent minerals removes SO4 and metals from solution. Laboratory dissolution experiments found that these minerals rapidly dissolved in water, indicating that this removal would only be temporary, and that there is still the potential for increased loadings of dissolved metals and SO4 during precipitation events. Below the vadose zone in the saturated tailings, concentrations of dissolved metals and SO4 decreased, but remained elevated in deeper groundwater. Along the groundwater flowpath, near the border between the Woods tailings and the discharge area into Woods Lake, concentrations of metals and SO4 abruptly decreased, coinciding with strong shift towards more positive δ34S-SO4 values, consistent with the influence of DSR on water chemistry rather than simple dilution. DSR currently occurring at the tailings edge appears to lead to a substantial decrease in loading of dissolved metals, SO4 and low pH groundwater to Woods Lake. \nGroundwater and surface water discharging from the Sherridon tailings impoundments discharge directly into Camp Lake. A 5-year hydrological and geochemical sampling program was initiated at Camp Lake to identify the mass-loadings and seasonal distributions of dissolved metals and SO4 discharging from the lake to the receiving Kississing Lake. Outflow from Camp Lake was sampled weekly and biweekly and dissolved ion concentrations and pH showed a strong seasonal cycle in mass-loadings. During winter months when the lake was ice-covered, discharge water from Camp Lake had a neutral pH with low concentrations of dissolved metals and SO4 similar to background concentrations. The Camp Lake discharge had an abrupt increase in dissolved metal and SO4 concentrations and decreases in pH during the spring freshet that remained relatively constant until fall freeze-up, when dissolved metal concentrations and pH returned to winter values. The annual and interannual variations in loadings measured in Camp Lake were different from those measured at the two streams feeding Camp Lake, revealing the contribution of high-salinity groundwater discharge from the tailings to the lake during dry years and the potential for significant loadings due to the dissolution of efflorescent minerals or enhanced transport of solutes through the thick unsaturated zone in the tailings during relatively wet years. The abrupt changes in pH, metal and SO4 concentrations, and the timing of these changes with the appearance and disappearance of ice-cover on the lakes, suggests a combination of physical and geochemical controls related to shifts in sources of water, mixing and changes in solubility. Despite fairly low average annual metal concentrations measured in Camp Lake discharge, concentrations of Zn and Cu were elevated above background in bottom sediments of Kississing Lake in a zone extending 9.5 km2 from the location of Camp Lake inflow. \nThe insight gained from studying metal release from sulfide-rich tailings was used to identify processes and controls on the release of metal(loid)s from Quaternary sediments and influence on background water quality in Alberta’s Southern Oil Sands Regions. A survey of over 800 groundwater wells completed in sand and gravel aquifers found that 50% of the wells contained As concentrations exceeding drinking water guidelines of 10 μg L. The same geochemical, mineralogical and isotopic methods used to investigate sulfide oxidation processes in the Sherridon tailings were applied to these glaciofluvial sediments to identify As sources. Unoxidized sediments collected from below the water table contained abundant arsenian framboidal pyrite and As-bearing Fe oxyhydroxides. Speciation model calculations showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite, suggesting that reductive dissolution of As-bearing Fe oxyhydroxides may be the source of some As in deeper reduced groundwater. In contrast, the oxidized sediments above the water table did not contain framboidal pyrite, but exhibited spheroidal Fe oxyhydroxide grains with elevated As concentrations. The habit and composition suggest that these Fe oxyhydroxide grains in the oxidized sediment were an alteration product of former framboidal pyrite grains. The results of the mineralogical analyses indicate that the oxidation of framboidal pyrite during weathering may be the source of As released to shallow aquifers and reductive dissolution of Fe oxyhydroxides may be the source of As in deeper aquifers.

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