Geomicrobiological and geochemical investigation of a pyrrhotite-containing mine waste tailings dam near Selebi-Phikwe in Botswana

An acid mine drainage (AMD) generating tailings dam consisting of waste from about 32 years of Ni-, Cu-, Zn- and Co-sulfidic ore processing near Selebi-Phikwe, Botswana, was investigated to quantify the AMD generation potential for developing a remediation strategy within the mine closure plan. The climate in the region is semiarid with an average annual temperature of 21 °C. The approximately 40 m high dam covers an area of ca. 1 km2. The surface of the central part of the dam is water covered, but the periphery is dry. Three holes were drilled through the water unsaturated periphery down to the saturated zone at about 25 m depth. Altogether 65 solid samples were taken in 1 m intervals and geomicrobiologically and geochemically analyzed. Brown precipitates of iron(hydro)oxides were discernible throughout the entire unsaturated zone with a paste pH in the range of 3–4. The tailings had sulfur contents in the range of 2–8 wt.% (average 4%). The total sulfur consisted mainly of reduced inorganic sulfur. Pyrrhotite was the main metal sulfide, pyrite, Ni- and Co-sulfides occurred in minor amounts. Metal sulfide oxidizing Acidithiobacillus ferrooxidans-like bacteria were present in high numbers (mean value of most-probable-number method: 3 × 106 cells g− 1 dw) throughout the whole unsaturated zone. Mean numbers of living bacteria (CARD-FISH) and total microorganisms (SybrGreen direct counts) were 1 × 107 cells g− 1 dw and 8 × 107 cells g− 1 dw, respectively. The average potential pyrrhotite oxidation rate measured by microcalorimetry was 1.9 × 10− 9 molpyrrhotite m− 2 s− 1. The ratio of biological to chemical pyrrhotite oxidation was ca. 50:50. For the assumed 0.1 m thickness of the oxidation zone within the 25 m unsaturated zone the potential average pyrrhotite oxidation rate was 3.4 × 10− 5 mol m− 2 s− 1 (94 kg FeS m− 2 year− 1). At this rate, all pyrrhotite in the tailings dam would be oxidized within 80 years.