Feasibility Study and Technical Report - Pretivm

The conceptual layout is preliminary in nature and reflects numerous assumptions.Additional investigations and assessments are required to address assumptions anduncertainties regarding the waste rock’s layout and its impact on Brucejack Lake.25.4.5 BRUCEJACK LAKE SUSPENDED SOLIDS OUTFLOW CONTROLIt will be necessary to control the TSS concentrations at the outlet of Brucejack Lake tomeet the MMER regulations. The following three possibilities were proposed anddiscussed, and an allowance has been included in the capital cost estimate:• washing waste rock before depositing in the lake• turbidity curtain surrounding tailings deposit and waste rock• outlet control structure for temporary control of the lake outflow.Further work regarding the TSS mitigation strategy is required during subsequent stagesof design.25.4.6 WATER MANAGEMENT PLANContact runoff is expected from three sources during construction and operations:• the upper laydown area where the waste rock transfer and pre-production orewill be stored• the mill building and portal site which requires an extensive cut into bedrock,some of which is currently assumed to be potentially acid-generating material• groundwater seepage to the underground mine tunnels.Runoff from the former two sources will be managed by storage and treatment. Contactwater ponds will be sized to contain runoff from the 24-hour, 10-year return periodrainfall event (102 mm). The contained runoff will be pumped to the water treatmentplant for treatment prior to release into Brucejack Lake.The average water requirement for the Brucejack process plant is 3,134 m 3 /d based ona mill throughput of 2,700 t/d. This water is required for the tailings slurry to the lake,the underground paste backfill, the concentrate slurry, and minor evaporative losseswithin the plant (approximately 7 m 3 /d). Process water will be sourced from:• treated underground seepage water• ore moisture (approximately 3% by weight)• reclaim from the lake.Groundwater seepage into the underground workings is expected to vary fromapproximately 2,140 to 4,080 m 3 /d throughout the LOM. Seepage water will be sent to awater treatment plant, and then the process plant, where its use will be maximized inprocess. With a settled dry density of 1.46 t/m 3 and a slurry consisting of 65% solids byPretium Resources Inc. 25-4 1291990200-REP-R0012-02.1Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC