Abstract
The environmental impact of the Robinson Deep mine residue deposit in Johannesburg was assessed by evaluating the water chemistry and surface precipitates, thereby identifying the processes responsible for forming the contamination footprint. Precipitates were compared to a documented paragenetic sequence of mineral formation associated with sulphide rich mine waste. PHREEQC, a geochemical modelling tool, was utilized to predict the formation of precipitates from evaporation ponds. The chemistry of the leachate was analysed and compared to water quality standards in order to determine the possible environmental impact.
The chemistry of the water emanating from the mine residue deposit reveals that an Fe-SO4 dominant chemistry persists, this is consistent with acid mine drainage environments. The most toxic cations and anions contained in the water are Fe, Al, Mn, Ca, As, Mg, Cu, Zn, Pb and SO . It is predicted that the impact of toxic metals identified in the water decreases further from the mine residue deposit due to dilution and co-precipitation with different mineral phases such as goethite.
Precipitates identified include jarosite group minerals, goethite, melanterite, copiapite, Mg-copiapite, halotrichite, pickeringite, gypsum and alunogen. These secondary minerals may be used as indicator minerals of acid mine drainage. Assessment and prediction of the stage of contamination and possible environmental impact, may, therefore be pursued when comparing the indicator minerals to predicted paragenetic sequences. For example, the precipitation of melanterite is consistent with an early stage of acid mine drainage development. The presence of melanterite thus suggests that oxidation of sulphides is an ongoing process on the Robinson Deep mine residue deposit.
Evaporation of water in the evaporation ponds aids in increasing the concentration, hence allowing the predominant precipitation of jarosite group minerals. Alternatively, rainfall dilutes the water allowing dissolution of minerals located on the banks of the evaporation ponds to predominate. This mechanism of precipitation and dissolution is seasonal; formation of precipitates predominates during the dry season, while dissolution is most prominent during the wet season. The development of hardpans indicates that the main mechanism of formation of the associated phases is through capillary action at the sediment surface. Leaching of Fe2+-rich water from the mine residue deposit containment area is indicated by the presence of copiapite, while jarosite and goethite formation tend to form part of a hardpan layer. Goethite is an indication of a late stage mineral predominant at lower sulphate and higher pH conditions.
Results of predicted formation of precipitates by PHREEQC are not in very good agreement with actual field observations. This is mainly due to the lack of thermodynamic data for many of the sulphate minerals observed.
Hence, precipitates associated with acid mine drainage may be utilized as indicator minerals. Consequently, there identification may facilitate in environmental monitoring and risk assessment.
Prof. J. M. Huizenga
Prof. J. Gutzmer