The former Sherritt–Gordon sulfide deposit in Sherridon, Manitoba, Canada, was mined for zinc, copper, silver, and gold between 1930 and 1951. Since mine closure, two high-sulfide tailings impoundments underwent extensive oxidation, resulting in the release of very high concentrations of dissolved sulfate and metals to the tailings porewater. During precipitation events and Spring freshet, surface seeps develop along the flanks of the impoundments, discharging groundwater with a pH as low as 0.39 and dissolved concentrations of sulfate and iron up to 203 g L−1 and 68 g L−1, respectively, along with other metals at elevated concentrations. Several efflorescent minerals were observed within groundwater-seepage areas. Using a combination of powder X-ray diffraction and energy-dispersive spectroscopy, secondary efflorescent sulfate minerals at seepage zones were identified, including melanterite, rozenite, halotrichite, chalcanthite, alpersite, copiapite, hexahydrite, jurbanite, pickeringite, jarosite, and gypsum. According to geochemical modeling of seepage waters using a modified WATEQ4F database for ferrous and ferric sulfate minerals, the occurrence of most of the observed sulfate salts was favored. In the geochemical modeling, we used data available for seepage waters that were representative of discharging water after mineral precipitation had already occurred. Discrepancies between some of the observed and modeled results likely reflect the use of water compositions representative of the later stages in the reaction sequence rather than the composition during actual formation of the minerals. The formation of these secondary efflorescent minerals resulted in a temporary removal of sulfate and metals from solution. These minerals rapidly dissolved during laboratory dissolution tests; this would result in increased loading to receiving surface waters during precipitation events and Spring freshet.

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