Feasibility Study and Technical Report - Pretivm

• The waste rock from the underground workings is predominantly PAG materialthat will be stored in Brucejack Lake and in the (flooded) underground mine.According to the established policy for ARD/ML at mine sites in BC, the selectedunderwater storage of waste rock is one of the preferred strategies to preventacid rock drainage and metal leaching.• Results from humidity cell tests indicate that the onset to ARD for the majority ofthe waste rock could potentially take tens of years before the available NP in thematerial is consumed. However, for a small proportion of the waste rock theonset of ARD may be within a year.• The onset to ARD for waste rock stored within the underground mine will likelybe delayed until the underground is flooded. This conclusion reflects NPdepletion rates of waste rock and potentially the addition of NP afforded by theuse of paste backfill (tailings with cement). Further investigation will be requiredto assess the NP of the paste backfill.• Results of kinetic tests with subaqueous columns suggest that underwaterstorage of waste rock maintains neutral pH values in the pore water and theoverlying water. Due to the reduced chemical mobility of most metals at neutralto alkaline pH values, the dissolved metal concentrations of most elements willlikely remain low.• Results from the kinetic tests with tailings in humidity cells and subaqueouscolumns show neutral pH values and low dissolved metal concentrations exceptdissolved arsenic due to its mobility at neutral pH. The flotation tailings will beNAG. This relates to their non-PAG properties and subaqueous placement.• With the current assumptions and available information incorporated into thewater quality predictions, approximately 10 parameters could exceed CCMEguidelines for aquatic life at the Brucejack Lake discharge. These parametersinclude: dissolved aluminum, dissolved arsenic, dissolved cadmium, dissolvedcopper, dissolved iron, dissolved mercury, dissolved selenium, dissolved silverand dissolved zinc.Pretium Resources Inc. 25-7 1291990200-REP-R0012-02.1Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC