Abstract
The sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana) are located in a similar semiarid environment but have a contrasting mineralogical composition. The Matchless tailings are pyrite-rich, whereas the Selebi-Phikwe tailings are dominated by pyrrhotite. Hydrochemical models are established with computer codes for water-balance, sulphide oxidation rate and hydrochemical equilibrium calculations. The data input is based on detailed mineralogical, chemical and kinetic investigations carried out on the core of boreholes drilled in 2000 and 2003. The oxidation of pyrrhotite proceeds at a much faster rate than the oxidation of pyrite. The PYROX code, which is used for kinetic calculations, can take these differences into account by applying different oxide-coating diffusion coefficients (D2) for pyrrhotite and pyrite. Humidity-cell testing is widely used to predict the post-mining composition of drainage water in humid climates. However, the semiarid conditions at Matchless and Selebi-Phikwe only allow a minimal water flux within the dump. Under such conditions, humidity-cell testing is likely to overestimate the seepage-water pH. This is suggested by the hydrochemical equilibrium calculations for the post-mining period at Selebi-Phikwe, which predict a seepage-water pH about one unit lower than the pH at the end of the 26-weeks humidity-cell testing period. The acidity of the seepage water can be reduced by about half a pH unit, if an oxygen barrier below the evaporation zone is installed. A clay layer 0.5 m thick covered by >1.5 m tailings represents the optimal design for a wet barrier. All three computer codes used for water-balance calculations (HELP3, UNSAT-H and HYDRUS-1D), predict >85% average water saturation for such a layer, which diminishes the diffusion of oxygen into the pile and production of SO −24 and H+. The alternative design for a dry barrier consists of a vegetated silt layer 1 m thick on top of the tailings. This barrier does not significantly influence the diffusion of oxygen although it reduces the net infiltration to ≤11 mm/year.
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Acknowledgments
The investigation of the Matchless and Selebi-Phikwe dumps was carried out within projects of Technical Cooperation of the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR, Germany), the partners of which were the Geological Survey of Namibia (GSN) and the Department of the Geological Survey of Botswana (DGS). The German contribution to the projects was financed by the German Federal Ministry for Economic Cooperation and Development. The authors would like to express their sincere thanks to all the institutions and individuals who contributed to the project, in particular to T. Machacha (Director of the DGS), H. Vogel (Leader of the cooperation project in Botswana), G.I.C. Schneider (Director of the GSN), V. Petzel (Deputy Director of the GSN) for their coordinating support. Many thanks to the DGS drilling team and to K. Keipeile for their engaged sampling. S. Mwiya (GSN) organized field trips and contributed to vivid scientific discussions. ICP-MS analyses were performed by U. Siewers and H. Lorenz, XRF analyses by J. Lodziak and F. Korte, and S total analyses by H. Wehner (all BGR). M.E. Böttcher (Max Planck Institute for Marine Microbiology, Bremen, Germany) analyzed S sulphide . W. Sand and his team at the University of Hamburg, Germany, carried out microcalorimetric measurements. BC Research (Vancouver, Canada) performed the humidity-cell testing.
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Schwartz, M.O., Schippers, A. & Hahn, L. Hydrochemical models of the sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana). Environ Geol 49, 504–510 (2006). https://doi.org/10.1007/s00254-005-0033-z
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DOI: https://doi.org/10.1007/s00254-005-0033-z