Neutral Mine Drainage prediction for different waste rock lithologies – Case study of Canadian Malartic

Neutral Mine Drainage (NMD) can become a problem if not properly addressed when low sulfide waste rocks are disposed of at mine sites. However, NMD, as opposed to acid mine drainage (AMD), is difficult to predict using classical kinetic tests due to the contaminant immobilization processes that occur, namely sorption and precipitation. A method using modified ethylenediaminetetraacetic acid leaching procedure and sorption tests on a positive control was proposed, which allowed the method to be validated. However, this method needed to be applied to different lithologies to consider the geological variation within orebodies. The risk assessment method was therefore applied to four different lithologies from Canadian Malartic mine. Two lithologies from the Canadian Malartic pit (carbonated porphyry: CPO and carbonated greywacke: CGR) were shown to have sufficient zinc sorption capacity to accommodate the total potential contaminant load. The other two lithologies from the Barnat pit (altered ultramafic: AUM and talc and chlorite schist: TCH) had sorption capacities and potential contaminant contents that were relatively close for Ni, which occurs within talc minerals. The modified kinetic experiments showed that Ni was leached at concentrations <1 mg/L. When Zn is the only metal considered for risk assessment of AUM and TCH, the risk of NMD generation is low. However, if all ions that could potentially occupy the same sorption sites as Zn (Ni, Co, Cu, Mn) are considered, the leaching risk increases. This study indicates that mineralogy should be considered in risk assessment and that further work is needed to include a release factor in the risk assessment of NMD. • NMD prediction methodology was applied to four lithologies from Canadian Malartic. • Characterizations and field data suggest main elements of concern are Ni and Zn. • Mineralogy suggests low risk of Ni and Zn leaching. • EDTA kinetic tests confirmed the low leachability of elements. • Lithologies displayed significant sorption capacity, reducing NMD risk related.