Carbon sequestration potential of tailings dams: a case study of Freda Rebecca gold mine

Abstract

Mine tailings are increasingly recognised as potential substrates for mineral carbonation, but they may also contain environmentally important heavy metals. This study assessed the carbon sequestration potential of the tailings storage facility at Freda Rebecca Gold Mine, Zimbabwe, using tailings deposited between 2022 and 2026. Twenty surface samples were collected, with four samples representing each deposition year. Acid-reactive mass loss was assessed using a hydrochloric acid fizz procedure, thermal mass loss was measured after heating samples to 800 °C, and selected heavy metals (Pb, Cu, Cd, As and Zn) were determined after acid digestion. Acid-reactive mass loss attributed to carbonate presence decreased from a mean of 4.2% in the 2022 and 2023 deposits to 2.9% in 2026. Thermal mass loss decreased from 7.7% in 2022 to 5.4% in 2026. The annual mean concentrations of Pb, Cu, Cd and Zn were 0.3061–0.5575, 0.6167–0.9984, 0.0054–0.0420 and 0.4307–0.5348 mg/L, respectively, while the annual As means reported in the dataset ranged from 0.0024 to 0.0230 mg/L. The older deposits therefore showed higher acid-reactive mass loss and thermal mass loss than the more recent deposits. These patterns are consistent with increasing weathering and possible mineral carbonation with longer exposure, although atmospheric CO₂ uptake was not measured directly and carbonate mineral identity was not confirmed mineralogically. The heavy-metal results demonstrate that potentially toxic elements remain present and require monitoring. The study concludes that the Freda Rebecca tailings storage facility has measurable indicators consistent with passive carbon storage potential, while further work is required to quantify actual CO₂ uptake and confirm the mineralogical mechanisms involved.