Oceana Gold Waihi North Project Waihi North Project Geochemical Assessment – Geochemistry of Tailings and Overburden, Treatment and Mitigation Revision 0 – 17-Jun-2022 Prepared for – Oceana Gold (New Zealand) Limited – Co No.: 2274246 29 AECOM The rate of limestone amendment for both the rock material is dependent on the required lag period. Both saturation of the placed PAF rock and the sealing effect of the overlying liner and tailings will be highly effective long-term controls on oxidation of this material. The degree of oxidation during filling will therefore be the primary factor influencing the post closure water quality within this fill. As the rate of material placement and hence burial beneath the oxidation zone will vary, the lag of placed rock will need to be adequate to limit oxidation of material for this time. The working lift of placed PAF rock material will be compacted to limit oxidation. The lag required will depend on the rate of infilling, the compacted nature of the PAF material after successive lifts of rock are placed, and the time period until the material is either saturated from groundwater recharge or at a sufficient depth below the active surface to limit oxygen ingress. Groundwater recharge in the pit is expected to take an extended period from when underground mining and dewatering cease. This includes a period of approximately 3.5 years to fill the underground voids and an additional 6 years for recharge from base of pit to natural groundwater levels. It is therefore expected that low permeability layers created from rock placement in lifts with compaction and depth below the active surface will be the primary control on oxidation and hence porewater quality in the short term (during backfilling and prior to tailings placement). Based on oxygen probes in existing TSF embankments the effective depth of oxidation in uncovered PAF rock is likely to be in the order of 8 m. A conservative average period of exposure for limestone amendment has therefore been estimated based on the maximum duration that any placed rock will be within 8 m from the active filling surface. The majority of the GOP rock backfill is placed in a period of less than 2 years beneath the 13 ha lined area. Based on the rate of backfilling and the assumed 8 m oxidation profile, Table 11 and Table 12 show the lag required for PAF material during backfilling of the GOP in the rock placed immediately beneath TSF Liner would be in the order of 250 days. Table 11 Martha Rock Acidity Neutralisation Requirements Year (of backfilling) Total lag Required (days) Additional lag required Limestone Amendment Rate (Mean NAPP) Limestone Amendment Rate (95% UCL NAPP) 1 70 - - 2 250 110 0.4% 0.6% Table 12 Gladstone Rock Acidity Neutralisation Requirements Year (of backfilling) Total lag Required (days) Additional lag required Limestone Amendment Rate (Mean NAPP) Limestone Amendment Rate (95% UCL NAPP) 1 70 - - - 2 250 180 1.0% 1.4% The calculated exposure period and hence required limestone amendment increases rapidly in year 2 of backfilling. This is due to the increased timeframe for placed lifts of waste to achieve a depth of 8 metres from the active placement as a result of the greater base pit surface area relative to the volume being deposited. It is also noted that while Gladstone PAF rock could be placed in the base of the pit in year 1 of backfilling without limestone amendment, the inability to stockpile the material limits the volume that could be placed in this manner to material that could be placed and compacted within the 70 days lag period. Additional limestone requirements for the neutralisation of acidity produced from the pit walls during backfilling have also been calculated, based on variables outlined in Table 13.
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