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 26 AECOM Table 9 Limestone Requirements for Permanent Disposal Item Mean (Martha & WUG) 95% UCL (Martha & WUG) Mean (Gladstone) 95% UCL (Gladstone) NAPP (kg H2SO4/tonne) 48 84 85 110 Assumed lag (days) – from ANC 140 140 70 70 Required lag (days) 210 210 210 210 Sulphate generation rate (mg/SO4/kg/day) 27 49 Limestone Dosing Requirement (kg CaCO3/tonne rock) (for 210 day lag) 2.3 4.0 13.3 17.2 Limestone Amendment Rate 0.2 % 0.4 % 1.3 % 1.7 % Rock placed within the zone of oxidation for the final proposed landforms (typically within the final 2 m of directly placed materials) should comprise NAF material only, as has been the practice throughout the life of mine to date. 6.3.1 NRS and Embankment Seepage Water Quality Predicted NRS and TSF embankment leachate water quality has been calculated using the following steps: • Gladstone Rock sample leachate concentrations from both the compacted and limestone blended column have been summarised in Appendix D. Both columns consist of Gladstone material with a NAPP of 103 kg H2SO4/t. The mean Gladstone NAPP is 95 kg H2SO4/t; the sulphate generation rates (SGR) based on kinetic test for Gladstone PAF rock is 49 mg SO4/kg/day. • During the life of the current proposed mining including MUG, MOP4, WUG and Gladstone, PAF rock from Gladstone is expected to comprise in the order of 62% of the total PAF rock that could be placed in TSF embankments or the NRS. The predicted PAF seepage from the NRS and TSF embankment is therefore assessed by mixing predicted seepage from the Gladstone and Martha in this 0.62 to 0.38 proportion. • The NRS could include rock from the WUG tunnels close to the current process plant. This rock is expected to have a comparable composition to existing Favona, Trio and Correnso underground developments and is not included in the above proportions. • Summarised data from the columns include data only up to a point where the lag (whether it is natural, in the case of the compacted column, or added as in the case on the limestone blended column) is exhausted. This is defined as the point at which the leachate acidity reaches pH 3.0. Data reflective of the post-lag period is considered not reflective of actual as-built conditions. • The modelled seepage is therefore derived using the following inputs:- - Working Area (62%) - Gladstone PAF seepage based on average of limestone amended and compacted columns. The use of unscaled column leach data (NAPP of 103 kg H2SO4/t) is considered a conservative representation of the seepage from Gladstone PAF rock. - Working Area (38%) – MUG, MOP4 and WUG PAF rock seepage based on average of limestone amended and compacted columns factored by the difference in SGR between Gladstone and Martha from kinetic testing (SGR of 49 and 23 mg SO4/kg/day respectively). - Rehabilitated areas – The combined Gladstone, MUG, MOP4 and WUG PAF rock factored by the reduction in sulphate generation rates achieved by capping and closure determined by Global SGR (GSGR) testing over the past two decades. The minimum GSGR for operating areas is 75 Kg SO4 per ha per day and the average for rehabilitated areas is 5.5 Kg SO4 per ha per day.
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