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 24 AECOM • The application of limestone (by spreading on each placed lift) may be required at the base of the pit as it is progressively backfilled. This will depend on the rate of back filling and hence exposure period of the placed rock and the specific acid forming characteristics of the material deposited. • Regular (monthly) PAF slurry testing and regular surface limestone application after placement have proven to be effective at managing material. 6.2 RTSA Temporary Disposal and WRS While OGNZL prefers to place PAF rock directly into the permanent repositories, there are times of the year (generally during the winter) when that is not possible, and the material needs to be stored temporarily prior to permanent placement. It is proposed that where possible only material from the MOP4, MUG and WUG (due to the higher acid neutralising capacity and different physical properties compared to the GOP material) is stored in temporary repositories. Where it is unavoidable, the temporary placement of high sulphur low ANC Gladstone PAF will need to be compacted, limestone amendment based on duration of exposure and delineated such that any rehandling can manage the higher acid generating potential of this material. Sulphate generation rates for rock from MOP4 and MUG have been conservatively estimated by utilising the adopted static geochemical results from the Martha Phase 4 cutback and kinetic testing of Martha rock collected within the open Martha Pit. The adopted values for the PAF rock from Martha operations to determine limestone dosing rates are as follows: • NAPP mean of 48 kg H2SO4/t and a 95% Upper Confidence Limit (UCL) of 84 kg H2SO4/t. • Sulphate Generation Rate mean of 27 mg SO4/kg/day and a 95% UCL of 47 mg SO4/kg/day. 6.2.1 Limestone Dosing Requirements The limestone amendment required for PAF rock placed within the temporary stockpiles at the RTSA and the Willows Rock Stack (WRS) will be dependent upon the results of testing of the material prior to it leaving the open pit or WUG. Current operational practises see daily testing of the rock for NAG pH and NAPP. Depending on the results, material is then classified as PAF or NAF, a limestone amendment rate is calculated, and the application rate is adjusted accordingly. These same practices are proposed for calculating limestone amendment rates for rock produced by the WUG and placed within the WRS. PAF rock placed in temporary storage structures should not be left exposed for a period of more than 30 weeks. This PAF rock should be blended with crushed limestone at a rate designed to provide a lag period appropriate to mitigate generation of AMD for a 30 week period (Table 8). Monitoring of placed rock will ensure these dosing rates are appropriate and enable refinement as part of ongoing operations. The dosing rate given in Table 8 is considered conservative based on the assumptions outlined in Section 6.2.2. Table 8 Martha PAF rock Limestone Dosing Requirements – Short Term Exposure Item Mean 95% UCL NAPP (kg H2SO4/tonne) 48 84 Adopted Sulphate Generation Rate (kg SO4/tonne/day) 0.027 0.047 Limestone Dosing Requirement (kg CaCO3/tonne rock) (for 210 day lag) 2.3 4.0 Limestone Amendment Rate 0.2 % 0.4 % 6.2.2 Key Assumptions The calculated limestone dosage rates given in Table 8 are considered to provide a high degree of conservatism in that it is assumed that all sulphur is present as acid generating pyrite (FeS2), with no allowance for other sulphide minerals that do not generate equivalent acidity, such as sphalerite. In
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