Feasibility Study and Technical Report - Pretivm

Stopes that will not be re-exposed by adjacent mining may be backfilled withunconsolidated waste and/or by paste fill with sufficient binder to remove any risk offuture liquefaction (low-strength paste fill). High-strength paste fill will be required in thelower portion of all primary and secondary stopes that will be undercut by sill extractionfrom below. Table 16.3 tabulates the total projected paste fill volumes over the LOM bystrength requirement and by binder dosage.Table 16.3LOM Paste Fill RequirementsPaste TypeLOMQuantity(m 3 )28-dayStrength(kPa)BinderDosage(%)DensityDry Paste(t/m 3 )MassDry Paste(t)BinderRequired(t)High-strength Paste 139,000 800 5.5 1.46 203,000 11,000Regular Paste 4,166,000 300 3.9 1.46 6,082,000 234,000Low-strength Paste 1,714,000 100 2.8 1.46 2,503,000 69,000Total 6,019,000 - - - 8,788,000 314,00016.3.5 PASTE BACKFILL TEST WORKPretivm engaged AMC to undertake the first stage of a high-level study on the suitabilityof using mill flotation tailings for paste fill at the Brucejack mine. The results showed ahigher-then-expected cement requirement for the range of required paste fill strengths.The density of the paste was low and resulting strengths required higher-than-expectedcement content to achieve the target strengths.Pretivm also engaged AMC to undertake second-stage laboratory testing. Stage 2 testwork aimed to identify other classes of binders that would achieve target strengths atlower dosing rates. In particular, Stage 2 investigated the use of blended blast furnaceslag and fly-ash with cement, as possibly better paste mix recipes.The Stage 2 test work program included the following:• material characterization tests in areas such as specific gravity and particle sizedistribution• determination of paste fill density at a yield stress of 250 Pa as the benchmarkfor the paste fill mix• unconfined compressive (UC) tests of mixes using General Purpose (GP) cement,slag, and fly-ash blend cements to look at the effect of adding fine ground ironblast furnace slag and fly-ash to the GP cement binder. Two slag blends weretested: MineCem (MC) containing 55% slag and Sunstate Slag Blend (SS)containing 35% slag. Medium-size fly-ash (FA) was also used.As shown in Table 16.4, the Brucejack tailings paste fill mixes responded very favourablyto the slag-based and fly-ash binders. The test program demonstrated a significantdifference in the strength values for the paste fill mix with GP cement compared to theslag-based (MC and SS) and FA mixes. The following differences were noted:Pretium Resources Inc. 16-16 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC