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The treatment of a copper sulphide-bearing gold ore by direct cyanide leaching, ammonia pretreatment and ammoniacal cyanide leaching was investigated. Dissolution behaviour of gold and copper in these leaching systems was demonstrated. Severe interference by the copper containing sulphides with cyanide leaching of gold is observed at ρ(NaCN)≤5 g/L. This is consistent with speciation calculations. Ammonia pretreatment is shown to readily eliminate the copper interference, allowing almost complete extraction of gold with concomitantly low reagent consumption in subsequent cyanide leaching. In ammoniacal cyanide system, Box-Behnken experimental design shows the main and interaction effects of NH₃, NaCN and Pb(NO₃)₂. The concentrations of NH₃ and NaCN are statistically confirmed to be significant factors affecting extraction of gold while the effect of Pb(NO₃)₂ is limited. Increasing the …