Feasibility Study and Technical Report - Pretivm

including samples with low head grades. Gold recovery varied from 82.8 to 99.8%,averaging 97.2%, while the head gold grade fluctuated from 0.5 to 200 g/t, averaging21.5 g/t. At the silver head grade range of 3.9 to 1,897 g/t , the silver recovery variedfrom 51.2 to 99.1%, averaging 88.5%. Gold and silver reporting to the tailings increasedwith head grade, which implies that more aggressive procedures may be used to improvegold and silver recoveries during periods of the higher-grade feeds (e.g. finer primarygrinding and reagent modifications).A substantial fluctuation was also noted in the gold and silver gravity concentrationrecoveries (Figure 13.24). Recoveries varied substantially, from 2.8 to 81.7% for goldand from 0.2 to 78.9% for silver. The variation is typically a result of the nugget goldeffects. In general, gold recovery by gravity concentration increased with head grade;however, silver recovery reduced with head grade. On average, gold and silver recoveriesreporting to the concentrate of the gravity separation procedure were 45.3% and 21.2%,respectively.There is a good relationship between head gold grade and gravity concentrate gold grade(i.e. concentrate grade increased with feed grade). The average gold grade of theconcentrate produced from gravity concentration tests was 21.7 kg/t. However thesetests did not show a good correlation between head grade and gravity concentrate gradefor silver. The average silver grade of the gravity concentrate was 15.4 kg/t. Theserelationships are shown in Figure 13.25 and Figure 13.26.Figure 13.25 Gravity Concentrate Grade versus Head Grade – Gold (2012)Pretium Resources Inc. 13-43 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC