Baker, A, Colston, R & Robinson, J 2025, 'Long-term geochemistry prediction and implications for closure of the 
Doornpoort tailings storage facility, South Deep Gold Mine, South Africa ', in S Knutsson, AB Fourie & M Tibbett (eds), Mine Closure 2025: Proceedings of the 18th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-14, https://doi.org/10.36487/ACG_repo/2515_58
(https://papers.acg.uwa.edu.au/p/2515_58_Baker/)
Abstract: Long-term predictions of tailings geochemistry are of fundamental importance when developing closure plans. SLR developed a long-term prediction strategy for the Doornpoort (DPT) tailings storage facility (TSF) at the South Deep Gold Mine. Mineral scaling was identified in the underdrainage network of the DPT TSF which could result in the drainage system becoming non-operational over time. A comprehensive geochemical characterisation of the fresh and mature DPT tailings materials, assessment of all related water chemistries and a kinetic humidity cell (HC) study were developed to understand the scaling mechanisms and predict the geochemical behaviour after closure to inform engineered solutions. 
Earlier geochemical studies and SLR’s acid-base accounting results predicted the tailings materials to be potentially acid generating. Operational drainage waters, conversely, display variable pH values and supersaturation with carbonate minerals (calcite and aragonite). The same dominant mineral phases are found in the underdrainage scale. A 20-week HC experiment undertaken on composite tailings and plant slurry samples reported initial acidic values for the most recent tailings deposited in the TSF, increasing to circumneutral ranges. However, the youngest sample generated an acidic leachate over the last three weeks of the experiment. The plant slurry sample produced leachate with the highest sustained total alkalinity during the first weeks of leaching compared to the youngest tailings sample, which produced the lowest alkalinity. This difference in alkalinity was not reflected in the samples’ mineralogy. Therefore, short-term alkalinity is likely being introduced to the system by the underground ore beneficiation process.
The carbonate molar ratios indicate that, except for the plant slurry sample, the DPT tailings materials’ neutralisation potential will not be sufficient to neutralise the acidity produced by pyrite oxidation over time. However, the study has shown that aluminosilicate minerals provide secondary neutralisation capacity in the interim, with acid-generating conditions expected to dominate after closure.

Keywords: mineral scaling, underdrainage network operation, kinetic humidity cell experiment, geochemical behaviour prediction, engineered closure solutions