A Review of Viable Mineral Precipitates in Pit Lake Modeling

Mine permitting requires an analysis of reasonably foreseeable impacts. In pit lake models, precipitates may be invoked to reduce dissolved aqueous concentrations, and thereby perceived groundwater impacts in the event of a throughflow pit lake after mine closure. The phases selected by modelers are generally based on geochemical theory rather than empirical data, and in this context, a review of available information is helpful. The mineralogy of Spanish, German, Czechian, Australian, and USA acidic pit lakes includes schwertmannite, jarosite, alunite, amorphous ferric oxyhydroxide (AFH), ferrihydrite, goethite, hydrobasaluminite, gibbsite, amorphous iron and aluminum oxyhydroxysulfates, gypsum, barite, and clays (kaolinite, dickite, halloysite, and smectite). Conversely, alkaline pit lakes exhibit distinct inorganic precipitation of calcite, ikaite, gaylussite, and siderite entraining metal removal from solutions at pH > 7 with biogenic iron oxyhydroxides and carbonates. Microbial activity facilitates the formation and transformation of metal sulfide precipitates predominantly in alkaline pit lakes but also in acidic water bodies. Therefore, predictive pit lake studies should focus on selecting authigenic precipitates on an empirical basis where possible, or if observed in corresponding environments in the case of permitting a new pit lake, taking into consideration kinetic barriers to mineral precipitation. This study characterized amorphous iron, aluminum, and calcium precipitate with entrained arsenic, manganese, and zinc floc from the previously acidic, now alkaline Lone Tree pit lake. The formerly acidic Liberty pit lake generated primarily gypsum and AFH, while the basic former Cortez pit lake sediment comprised calcite with solid solution Ba, Cd, Mg, Mn, and Zn.