Feasibility Study and Technical Report - Pretivm

26.5.6 BRUCEJACK OUTLET CONTROL STRUCTUREBGC (2013) completed a scoping level study and provided material take-off quantities foran outlet control structure for Brucejack Lake. BGC recommends completing the ongoingstudy to evaluate the potential TSS issue, which includes:• hydrodynamic modelling of the lake to evaluate stratification in the lake andturn over events for various conditions• analyses of the potential for re-suspension of settled tailings during laketurnover events• methodology of waste rock deposition in the lake and estimation of anticipatedTSS concentrations released into the water column by the waste rock and fromdisturbance of the lake bottom sediments and potentially tailings duringplacement of waste rock• estimation of potential TSS concentrations at the lake outlet based on the aboveinputs.If the estimated TSS concentrations at the lake outlet are within the anticipatedregulatory limits, no active TSS mitigation measures may be required. If otherwise,further design of appropriate TSS mitigation measures will be required. If, following theadditional analyses outlined in the points above, an outlet control structure is deemed arequired, practical and preferred method of TSS mitigation, a site investigation,bathymetric survey, geotechnical, structural, hydrotechnical and hydraulic design will berequired to bring the proposed outlet structure to an appropriate design level. In additionto the TSS concentrations, the ongoing study to estimate the dissolved water qualityshould also be evaluated to confirm that parameter concentrations will remain withinregulatory limits.The estimated cost for the above-mentioned recommended work is $400,000.26.5.7 WASHING WASTE ROCK PRIOR TO DEPOSITION IN BRUCEJACK LAKEWaste rock deposition in Brucejack Lake presents the possibility of introducing finesuspended solids to the surface water and to the outflow from the lake. It isrecommended that development of a washing mechanism to remove fine suspendedsolids from the waste rock be undertaken. Initial investigations suggest that waste rockcould be fed to bar screens with the fine material (less than 1 in) directed to a screwclassifier. Overflow from the classifier would be reintroduced to the tailings stream fordisposal on the deep lake bed under the deposit of tailings. All other material could bedumped directly to the lake with the knowledge that the settling rates of these coarsersolids will ensure that they are trapped in the lake. The estimated cost for this designwork is $60,000.Pretium Resources Inc. 26-9 1291990200-REP-R0012-02.1Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC