Technical Report for the Fort Knox Mine - Kinross Gold

Fort Knox Mine Technical Report16.2.2 Heap Leach Recovery – Fort Knox DepositProjected heap leach recoveries were based on laboratory testing of eight bulk sampleswhich are representative of the material to be leached. Gold recoveries from the ROMfeeds ranged from 28% (samples 103A and 200A) to 93% (sample 205B). Average goldrecovery was 61.2% with an average of 224 days of leaching and rinsing. Goldextraction was progressing from all eight feeds at a slow rate when leaching wasterminated, though a longer leaching cycle would increase gold recoveries somewhat.From this testwork, the following recovery relation was derived and used in mineplanning:Gold recovery at time t (days) = 0.65 - 1/ (0.03t + 1/0.65)16.3 Past Considerations16.3.1 True NorthThe True North deposit is hosted in altered metamorphic rocks, and generally crushesand grinds at a more rapid rate than material from Fort Knox. Blending of True North andFort Knox mineralization increased the throughput of the mill over feeding Fort Knoxmineralization alone.The True North mineralization locally contains concentrations of stibnite, which has thepotential to decrease the pH, lower the dissolved oxygen content, and increase theconsumption of cyanide. To offset the detrimental attributes of True North ore the feedrates were limited and additional mill reagents were added.16.3.2 Fort KnoxBased on the varying hardness of the ore, crushing and grinding rates can changesignificantly within the deposit having a direct impact on mill throughput and ultimatelyunit costs. The mill tonnage rates are adjusted to reflect crushing properties based onwhere mining has been scheduled in the pit.51