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 18 AECOM 5.1 Sulphate Generation Rate of Gladstone Open Pit Walls The unmodified leachate data from the field column Col1 (refer to Appendix D) has been summarised as it is considered the most representative field column in terms of ratio of Andesite and hydrothermal breccia material compared to the total exposed wall areas of the final GOP. The raw leachate data has been summarised from week 10 to week 79, in order to exclude the initial lag period which was present in the column tests. The approach is considered appropriate as by the time the pit reaches its maximum extent (and exposed surface area), the bulk of the exposed walls would have been exposed to the atmosphere for a period exceeding any inherent lag period. The mean sulphate generation has been calculated based on the volume of the material in the column. This value has been adjusted to account for the Net Acid Producing Potential (NAPP) differences between the material in the field column and the median and 95% UCL of the rock material within the GOP (Table 4). Table 4 Sulphate Generation Rate for Pit Walls Item NAPP (kg H2SO4/t) Sulphate Generation Rate (mg SO4/kg/day) Column 129 34 Gladstone Rock (95th % UCL) 109 29 Gladstone Rock (Median) 95 25 The calculated sulphate generation rate of between 25 to 29 mg SO4 per kilogram of oxidising material per day of exposure is therefore assumed for oxidation of the pit walls. In order to account for the total volume of oxidising material and therefore the total mass of oxidation products, the total pit wall area has been divided into PAF and NAF areas based on the volumes of PAF and NAF material from the rock waste schedule. An oxidation depth of 0.1 m has then been applied to this area to calculate the total volume of oxidising pit wall which is conservative based on observation of the existing pitwalls. The adopted values are outlined in Table 5. Table 5 Total Oxidising Volume in Gladstone Open Pit Wall Item Units Value Total Exposed Wall Area Ha 18.7 Density t/m3 2.1 Percentage of PAF % 73 Depth of Oxidation Profile m 0.1 Total Volume Available for Oxidation m3 13,605 Tonnes of material Available for Oxidation t 28,570
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