Feasibility Study and Technical Report - Pretivm

with lesser components supplied by surface water infiltration in lakes. Groundwaterdischarge zones are generally restricted to lakes, creeks, gullies, and breaks in slope.The hydrostratigraphy of the site is composed of a thin, discontinuous layer of glacial tillor colluvium underlain by bedrock. Thicker overburden deposits are confined to localsections of the valley bottom and are not present in the vicinity of the proposedunderground mine.Bedrock of the Project area can be broadly divided as follows:• Triassic marine sedimentary and volcanic rocks of the Stuhini Group• Jurassic sediments and volcanics of the Hazelton Group• early Jurassic dikes, sills, and plugs of diorite, monzonite, syenite, and granite,the most common of which are grouped as the “Sulphurets Intrusions”.A general trend of decreasing bedrock permeability with depth is observed site wide,though permeability varies by two to three orders of magnitude at any given depth. Thereis no apparent relationship between hydraulic conductivity and the major structure in theimmediate vicinity of the Project area (the Brucejack Fault).16.6.3 NUMERICAL MODEL DEVELOPMENT AND CALIBRATIONThe conceptual model described in Section 16.6.2 was used as the basis for thedevelopment of a numerical hydrogeologic model, built using the graphical user interfaceGroundwater Vistas (ESI 2011), and MODFLOW-Surfact code (Harbaugh et al. 2000;HydroGeoLogic 2011). The numerical model was calibrated in stages to availablehydrogeologic data collected within the study area, comprising 67 packer and slug testswithin bedrock, 40 hydraulic head targets, and approximately 3 months of volumetricdischarge data available from mine dewatering activities that occurred in 2011 to 2012.Prior to predictive simulation runs, an additional run was completed to account forongoing dewatering at the site; dewatering activities are expected to continue until thestart of mining operations, with the water level in the existing adit maintained at anapproximate elevation of 1,300 masl.16.6.4 PREDICTIVE SIMULATIONS AND INFLOW ESTIMATESPredictive simulations are based on a 21-year mine life, as stipulated in the undergroundmining plans provided by AMC on February 27, 2013. The underground mining stopesand associated developments were simulated using head-dependent boundariesconstrained to outflow (i.e. drains). Drains representing the development (i.e.underground working access and egress ramps and declines) were activated according tothe mining plan, and remained active throughout mining operations, while mining stopeswere deactivated after a period of one year, at which point the stopes were assumed tobe backfilled with paste. The conductance of Brucejack Lake was adjusted throughoutthe simulated operations to reflect tailings deposition.Pretium Resources Inc. 16-37 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC