Jarosite Solid Solution Associated with Arsenic-Rich Mine Waters, Macraes Mine, New Zealand

Abstract

Jarosite is one of the principal precipitates from process waters of the pressure-oxidation autoclave system at the Macraes gold mine, and this jarosite is discharged with the process waters to a tailings impoundment for long-term storage. Dissolved sulfate and arsenate concentrations in the autoclave, inferred from mineralogy, both exceed 0.1 mol/L (≈10,000 mg/L). Coarse-grained (≈1 mm) jarosite crystals, precipitated as mineral scales, provide insight into the mineralogical nature of the jarosite. The jarosite crystals are zoned, with some zones having up to 12 wt% Al, as part of the jarosite-alunite solid solution series. Substitution of up to 1.7 wt% Na for K also occurs. Both the monovalent and trivalent sites in the jarosites have apparent deficiencies (≈10 atm.%) because of protonation of the hydroxyl groups. There is <0.3 wt% As substitution for S in the jarosite anionic site, despite the abundant dissolved As in the autoclave solutions. Instead, jarosite is intergrown with ferric arsenate, and most jarosite growth zones contain numerous small (<20 μm) inclusions of this ferric arsenate. Ferric arsenate precipitation with jarosite occurs as dissolved As decreases by several orders of magnitude during the latter stages of oxidation in the autoclave slurry. The lack of structural As and only minor Na substitution limits the destabilization effects of solid solution within the jarosite in contact with the tailings waters, and jarosite dissolution is expected to be slow. Dissolution is further limited by high (up to 8,000 mg/L) dissolved sulfate in the tailings impoundment. Both jarosite and associated ferric arsenate dissolve incongruently in the surficial environment, yielding ferric oxides/hydroxides, which adsorb As, so that dissolved As is generally <10 mg/L in the tailings water.