Feasibility Study and Technical Report - Pretivm

In the high-grade domains (including West Zone)−with the separation of the lower- andhigher-grade populations−the lower-grade population is amenable to the ordinary krigingmethod of grade interpolation.Multiple indicator kriging was selected for estimation of the higher-grade populationwithin the high-grade domains, to control the skewness of the data. Multiple indicatorkriging involves modelling variograms at a series of grade thresholds, which allows therange of continuity to be reduced at the higher grades. A mathematical model was thenused to define the top end of the grade distribution. The result of this estimation methodis that, while no top cut is used to limit the higher grades, the higher grades are limited intheir influence using a mathematical model based on the higher-grade data to estimategrades in the top class. This is based on probability rather than using the individualextreme grades in the dataset for grade estimation.Figure 14.4Log Probability Plot Showing the Threshold Between Lower- and Higher-gradeVOK Population99.99Log Probability Plot for AUM200.0,100.0M99.999.89998959080706050403020105210.50.20.10.050.020.01 0.001 0.01 0.1 1 10 100 1000 10000AUPretium Resources Inc. 14-8 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC