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 30 AECOM Table 13 Pit Wall Acidity Neutralisation Requirements Item Value NAPP (95% UCL Gladstone Rock) 110 kg H2SO4/tonne Adopted oxygen ingress depth 0.1 m PAF Pit Wall Area 136,050 m2 Limestone required to fully neutralise acidity 112 kg CaCO3/tonne Pit wall neutralisation requirements 3,500 tonne CaCO3 Volume of Gladstone Backfill 2,619,260 m3 Limestone amendment rate for pitwall neutralisation <0.1% Due to the volume of material being backfilled into the pit and the limited oxygen ingress into the pit walls, the actual calculated amendment rate (to account for this acidity) is proportionally small. It is therefore recommended that additional limestone addition to account for the pit wall oxidation is adequately provided for by the calculated neutralisation requirements conservatively estimated for backfill lag extension. Notwithstanding the values and assumptions provided, the key assumptions used in assessing and calculating the amendment requirements are as follows: • That all PAF rock placed has NAPP characteristics reflective of the mean and 95% UCL figures given. • PAF rock used within the column tests is representative of rock from the proposed mining areas (with calculated sulphate generation rates proportional to calculated NAPP values). • No scaling factors have been applied to the sulphate generation rates calculated from the column leachate tests. This is considered a conservative assumption as it does not take into account likely differences in porosity and particle size distribution. • That exposure periods given are conservative based on the backfilling schedule, the rate of perched water saturation and assumed oxidation depth. • For pit wall acidity, it is assumed that exposed PAF material has a NAPP value based on the Gladstone rock of 95% UCL of 110 kg H2SO4/t. This is considered conservative. 6.3.3 Placement and Compaction of Rock Disposal of rock via placement and compaction within the GOP (and within TSF3 embankment and the NRS) will be undertaken in a manner that will limit the rate of oxygen ingress and limit the potential of the material producing acidity until consecutively placed material (on top) is of sufficient thickness to prevent ongoing oxidation. Requirements for limestone amendment, required lag and derivation of the dosing rate for PAF are as outlined in Section 4.3 for other permanent repositories. For material sourced from the GOP it is recommended that limestone spreading be applied atop the placed layer and then compacted in order to introduce alkalinity (via infiltrating rainfall) into the material and to extend the natural lag of the PAF rock after placement. Alternatively, for material sourced from Martha, this material should be blended on the conveyor at the appropriate rate before placement and compaction within the pit. It is proposed that limestone spreading is based on current site management practises, where regular (monthly) slurry testing and regular surface limestone application is undertaken. However, it is proposed that the amendment rate be adjusted to account for the required lag period (the period from placement and being outside of the zone of oxidation). This rate of amendment should be related to the sulphur % of the rock material, the NAG pH, the classification of the material based on the sulphur % and the NAG pH, and the required lag period.
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