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 25 AECOM addition, the use of a 95th UCL NAPP provides a degree of conservatism that on average, the given dosing rate requirement should provide sufficient excess limestone to account for any variation encountered. The mean values given are considered to be more realistic over the entire mass of rock, however the 95th UCL derived dosing rates are considered a worst-case scenario and may be required periodically when high PAF material is being extracted from Martha or WUG and stored within the RTSA or WRS respectively. Refinement of the predicted dosage rates for rock will be undertaken as additional information from routine monitoring becomes available. Specific information requirements to allow such a review may include the period of exposure and changes in NAPP values outside the initial values adopted. The key assumptions used in assessing and calculating the amendment requirements are as follows: • That collected static data and calculated NAPP (based on the mean and 95th UCL of the Martha dataset) is representative of the mass of the PAF rock material to be excavated as part of Waihi North Project. • PAF rock used within the kinetic 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. No inherent lag within the PAF rock material has been taken into account and it is assumed that sulphides within rock will oxidise immediately upon removal. This is considered a conservative approach as it is likely an inherent lag period will exist within the PAF rock. 6.3 Permanent Rock Disposal A large quantity of PAF rock material sourced from both the Martha and GOP is assumed to be stored permanently within the proposed NRS. Material will also be utilised within the embankment of the TSF. The PAF rock from WUG will be placed in the WRS and NRS prior to return as backfill within the WUG. Disposal of PAF rock to the NRS and TSF embankments 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) will remove the underlying material from oxygen exposure. It is proposed that no limestone amendment will be required for: • Gladstone PAF material likely to be exposed for a period of less than 10 weeks. • Martha and WUG PAF material likely to be exposed for a period of less than 30 weeks. This difference is due to the greater natural lag in the Martha material, which contains higher ANC than the Gladstone rock material (refer Section 4.3). The WUG rock is inferred to have a comparable lag to Martha PAF rock which will be further verified by ongoing column testing. Lime amendment rates for WUG are therefore expected to be comparable to Martha PAF rock material which will be confirmed by column testing results. For material likely to be exposed for a period of greater than its defined natural lag period, it is recommended that limestone be either blended (via conveyor during transport to the RTSA) or applied atop the placed rock prior to compaction in order to introduce alkalinity via infiltrating rainfall into the backfill/embankments. 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 is 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 and the required lag period. For illustration, Table 9 shows an example where rock material with ABA characteristics as per the average and 95% UCL Martha and Gladstone Rock material (Appendix A) is placed and compacted and left uncovered for a period of 210 days. The calculated amendment rates of between 0.2 and 1.7 % are conservatively recommended in order to extend the lag period to the required 210 day period.
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