Distribution patterns of contaminants in the Mogale Gold tailing dam: a case study from South Africa

This study evaluates the geochemical and mineralogical characterisation of weathering layers associated with the Mogale tailing dam in the Randfontein Cluster-Witwatersrand Basin, South Africa. In the tailing dam, it is observed that high hematite/Fe2O3 contents separate the ferruginous from the rest layers. Also, the oxidised layers with high quartz/SiO2 contents compositionally differ from the underlying (capillary zone) grey and ferruginous layers. Likewise, a combination of gypsum contents (MnO, CaO, tot S) versus pyrophyllite/muscovite (Al2O3 and K2O) distinguishes the lower grey from the upper grey layer, which corresponds to the saturated and capillary zones, respectively. Similarly, the amount of REE and most trace elements depleted in the oxidised layers is compared to the underlying layers. Factor analysis of this variation in layers related element distribution to the spatial patterns of pyrite, gypsum, hematite, and jarosite. Furthermore, analysis of the mass balance revealed an estimated net loss of between 23 and 123 ppm for uranium, zinc, nickel, and arsenic right from the inception of the tailing dam 50 years ago. A loss of between 1.5-1.8 and 2.8-2.9 % is also estimated for total sulphur and aluminium, respectively. These results underpin the sources and patterns of mobilisation of elements in the weathering zones as a major step towards the development of a predictive model for mitigation of AMD in the region.