Memo. Detour Lake Mine: Assessment of Rock Types for ML/ARD Selective Handling FINAL

Transcription

1 SRK Consulting (Canada) Inc West Hastings Street Vancouver, BC V6E 3X2 T: F: Memo To: Jim Robertson Client: Detour Gold Corp. From: Chris Kennedy Stephen Day Project No: CD0.006 Cc: Veronika Shirokova, SRK Date: January 20, 206 Subject: Detour Lake Mine: Assessment of Rock Types for ML/ARD Selective Handling FINAL Introduction This memo provides SRK s assessment of selective handling opportunities to manage metal leaching and acid rock drainage (ML/ARD) potential at the Detour Lake Mine (DLM), located 40 km north-east of Cochrane, Ontario. The contents of this memo have been adapted from the presentation provided to Detour Gold Corp (DGC) on June 8, Background 2. Current Waste Handling DLM currently separates waste rock based on ARD potential as determined using an NP/AP criterion of.5. This is resulting in two waste categories potentially ARD generating (PAG) and non-ard generating (NAG). Waste rock at DLM is considered to have low reactivity because typical sulphide content is less than %. It is not certain that PAG rock will generate ARD and if it does, acidity may not be observed for decades and well into closure. However, DLM rock includes some rare components that contain elevated sulphide that are more reactive and may impact water quality disproportionately when compared to the other rock types. Segregation and selective handling of these small volumes of rock could be considered to mitigate their potential to impact overall water quality for the stockpiles. 2.2 Criteria for Selective Handling Geochemical criteria defined herein for selective handling of rock types could include: Rocks that are rapidly reacting, which influences drainage chemistry and triggers acid generation in nearby waste rock; and Early generation of acid with ARD onset occurring during operations.

2 SRK Consulting Page 2 Factors that can affect reactivity include elevated sulphide content, low carbonate mineral content, and occurrence as volumes that remain intact when mined and dumped (potentially becoming hotspots within the PAG dump). These considerations form the basis of evaluating selective handling approaches of waste rock from the DLM. 2.3 Previous Assessment for Rock Type Selective Handling at the DLM The geochemical studies that were part of the environmental assessment (EA) process for the DLM (AMEC 200 ) identified two rock types that were recommended for further assessment to determine if they were candidates for selective handling. These two rock types were talc chlorite (TC) and volaniclastic (VC). Talc chlorite was not identified as an ARD risk, but noted to potentially have higher rates of cobalt and nickel leaching due to the solid phase enrichment of these two elements when compared to other rocks types expected in waste rock. Volcaniclastic was identified as an ARD risk due to higher sulphide content of some samples (up to 2.2% sulphur) and acidification in a humidity cell test after approximately one year of testing. 3 Assessment for Selective Handling SRK re-assessed the ABA database to evaluate the significance of these two rock types and determine if other rock types might be candidates for selective handling. The following steps were taken as part of the selective handling assessment: The geochemical database from the feasibility study was the primary source of data; Humidity cell testing that was started during the EA and continued until 205 provided element leaching information and rates of acidification (where applicable); Only waste rock samples in the database were considered; Percentile composition results (acid-base accounting and element scan data) for each rock type was calculated and compared by using Box and Whisker plots; and Ten times average global basalt values (Price ) were used as an enrichment level to compare with DLM rock types. South of the chert horizon has not been assessed in the past or current ARD program due to lack of exploration holes in this area. AMEC 200. Detour Lake Project, Metal Leaching and Acid Rock Drainage Characterization Report Vol I and II. Report Prepared for Detour Gold Corporation by AMEC Earth & Environmental, August Price 997. Guidelines and Recommended Methods for the Prediction of Metal Leaching and Acid Rock Drainage at Minesites in British Columbia. Prepared for the Reclamation Section, Energy and Minerals Division, Ministry of Employment and Investment. April 997.

3 SRK Consulting Page 3 4 Findings 4. Talc Chlorite Rock Type In summary, TC has been dismissed as a rock type that would benefit from selective handing. This is on account of its relatively low solid phase element concentrations when compared to global averages of similar rock types in addition to low solubility of cobalt and nickel under neutral ph conditions. Talc chlorite does have the highest cobalt and nickel of the rock types evaluated in the dataset, but concentrations are not considered elevated compared to the enrichment levels as illustrated in Figure 4- and Figure 4-2. None of the samples exceeded the cobalt level and only the maximum value for nickel exceeded the level. The enrichment levels provide the basis for potential leaching, whereas humidity cells yield the basis for rates of leaching. After four year of humidity cell testing, TC was shown to leach metals at low rates compared to samples that had gone acidic (Figure 4-3 and Figure 4-4). This is because the solubility of both cobalt and nickel will constrain concentrations from TC leaching so long as the ph is greater than seven. For the non-potentially ARD generating (NAG) waste rock at the DLM, contact water is expected to have a ph closer to eight, which would support low solubility of cobalt and nickel as compared to more acidic conditions n=38 n=46 Cobalt (mg/kg) n=00 n=65 n=0 n=2 n=23 n=6 --- = 0x global basalt average 0 Z:\0_SITES\Detour_Gold\CD0.006_204_DLM_MLARD_Support\Problem_RocksforARD\[DLM_MASTER_980_ABA Database_stats_CD0.006_REV00_CBK.xlsx] Figure 4-: Box and whisker comparison of DLM waste rock cobalt concentrations.

4 SRK Consulting Page Nickel (mg/kg) n=00 n=38 n=65 n=0 n=2 n=23 n=46 n=6 --- = 0x global basalt average 0 Z:\0_SITES\Detour_Gold\CD0.006_204_DLM_MLARD_Support\Problem_RocksforARD\[DLM_MASTER_980_ABA Database_stats_CD0.006_REV00_CBK.xlsx] Figure 4-2: Box and whisker comparison of DLM waste rock nickel concentrations. Co Release (mg/kg/week) Acidic HCs PF - HC-3 (HW) PF - HC-7 (HW) PF - HC-27 (HW) KPF - HC-4 (HW) MF - HC- (HW) MF - HC-5 (HW) MF - HC-6 (HW) MF - HC-5 (HW) MF - HC-23 (HW) KMF - HC-2 (HW) TC - HC- (FW) VC - HC-24 (FW) VC - HC-25 (FW) FMV - HC-8 (FW) FMV - HC-22 (FW) CG - HC-20 (FW) FI - HC-26 (Intr) MI - HC-9 (Intr) TC (HC) Co Content (mg/kg) Z:\0_SITES\Detour_Gold\CD0.000_MLARD\Waste Rock\HCs\Calculations\[DetourOutcomes_CD0.000_rev02.xlsx] Figure 4-3: Cobalt release rate versus solid phase content.

5 SRK Consulting Page 5 Ni Release (mg/kg/week) Acidic HCs PF - HC-3 (HW) PF - HC-7 (HW) PF - HC-27 (HW) KPF - HC-4 (HW) MF - HC- (HW) MF - HC-5 (HW) MF - HC-6 (HW) MF - HC-5 (HW) MF - HC-23 (HW) KMF - HC-2 (HW) TC - HC- (FW) VC - HC-24 (FW) VC - HC-25 (FW) FMV - HC-8 (FW) FMV - HC-22 (FW) CG - HC-20 (FW) FI - HC-26 (Intr) MI - HC-9 (Intr) 0.02 TC (HC) Ni content (mg/kg) Z:\0_SITES\Detour_Gold\CD0.000_MLARD\Waste Rock\HCs\Calculations\[DetourOutcomes_CD0.000_rev02.xlsx] Figure 4-4: Nickel release rate versus solid phase nickel content. 4.2 Volcaniclastic Rock Type The VC rock type was identified in the EA studies as potentially having higher ARD potential due to higher sulphur values and a humidity cell that went acidic after one year of testing. Review of the available data for VC waste rock found that the average sulphur was 0.4%, whereas the 75 th percentile was 0.5% and the 95 th percentile was.2%, which are all much lower than the acidic humidity cell tested that had 2.2 % sulphur. The sample tested was therefore not a useful indicator of the range of reactivity because it was exceptionally unusual. A comparison with the other rock types (Figure 4-5) shows that VC contains the same range of sulphur concentrations as other rock types. The distinctive feature of VC is that it contains somewhat lower carbonate concentrations (Figure 4-6) which results in lower NPRs than other rock types (Figure 4-7). The sulphur concentrations indicate that VC is not distinctively reactive compared to other rock types but the lower NPRs show that ARD onset may occur sooner for this material thereby meeting one of the criteria for special handling.

6 SRK Consulting Page 6 0 n=23 n=6 n=00 n=38 n=65 n=2 n=46 n=0 Sulphur (%) --- = 0x global basalt average Z:\0_SITES\Detour_Gold\CD0.006_204_DLM_MLARD_Support\Problem_RocksforARD\[DLM_MASTER_980_ABA Database_stats_CD0.006_REV00_CBK.xlsx] Figure 4-5: Comparison of rock type sulphur percentile statistics. However, spatial distribution of a rock type is important to consider for special handling because isolated and thin layers of PAG VC are not amenable to segregation during mining and will become mixed with less reactive materials during handling. In comparison, larger scale zones of the material that remained intact could become an ARD hotspot in the waste rock dump. The EA predicted that approximately 4% of the total waste rock would be comprised of VC. Currently, understanding of how the 4% VC is distributed is limited as the resource model has low resolution for this rock type and VC has not yet been encountered during operations. Based on discussion with DLM geologists, it is likely that VC is present in relatively thin units compared to other rock types. DLM geologists will be evaluating this further. In summary, volcaniclastic rocks are not candidates for special handling because their characteristics merely represent the spectrum of ARD potential at the DLM rather than a uniquely reactive material. However, they appear to contain a higher proportion of PAG rock than other rock types and may provide an indicator to operations that higher proportions of PAG material may be encountered when the occurrence of VC can be forecast.

7 SRK Consulting Page n=00 n=38 00 n=65 n=2 n=23 n=46 n=6 n=0 TIC (kg CaCO 3 /t) 0 0. Z:\0_SITES\Detour_Gold\CD0.006_204_DLM_MLARD_Support\Problem_RocksforARD\[DLM_MASTER_980_ABA Database_stats_CD0.006_REV00_CBK.xlsx] Figure 4-6: Comparison of rock type carbonate (TIC) percentile statistics n=00 n=38 n=65 n=2 n=23 n=46 n=0 0 n=6 NPR (ratio) --- = Z:\0_SITES\Detour_Gold\CD0.006_204_DLM_MLARD_Support\Problem_RocksforARD\[DLM_MASTER_980_ABA Database_stats_CD0.006_REV00_CBK.xlsx] Figure 4-7: Comparison of rock type NPR percentile statistics.

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