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 16 AECOM Figure 8. NAG pH distribution in Gladstone Pit. Cores: Red, NAG ph >5; Orange, NAG pH 2.5-5; Yellow, NAG pH 2-2.5; Blue, NAG pH < 2. 4.4 Rock Leaching Potential 4.4.1 Gladstone Open Pit Kinetic and field column testing of Gladstone rock samples has been undertaken to further refine the assessment of PAF materials, as well as to characterise the influence of reaction rates. Details of the kinetic and column testing is provided in Appendix D, with a summary provided below. The kinetic testing was undertaken on six rock samples representative of the Gladstone vent breccias, with one additional control blank of the testing equipment. Test results showed that rock samples with depressed pH and elevated trace element concentrations coincided with the highest acid producing potential. Conversely, the rock with the highest NAG pH showed depressed trace element concentrations and a pH similar to the control sample. Mercury, which has elevated total concentrations in the breccia rock compared to the wider site, was only detected in 3.8% of the kinetic leachate samples analysed throughout the testing period. As such, the rock samples tested do not leach elevated levels of mercury under acid generating conditions simulated in the kinetic tests. In general, the kinetic test samples indicated leaching characteristics consistent with the waste from the historical and existing mine rock samples. There were two samples that represented high sulphur and very low ANC that rapidly became acid generating and leached trace elements at levels greater than rock from the wider site. The field column testing results suggest that the application of limestone, compaction of material, and saturation are all valid strategies to mitigate AMD from both exposed and placed rock during the proposed mining operations. Specific strategies depend on the rock destination, method of placement, length of exposure, and specific ABA characteristics of the rock. This is explored and discussed in Section 6.0. A component of the high sulphur and very low ANC samples identified in the kinetic testing were included in these column samples in proportions equivalent to their overall proportion in the total rock. As a result, the column results are considered to reasonably represent the leachability of the total GOP rock once placed in a rock storage area.
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