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 17 AECOM 4.4.2 Wharekirauponga Underground Mine Field column testing of Wharekirauponga rock samples has been undertaken to further refine the assessment of trace element leaching and sulphate generation rates. Details of the column testing data available is provided in Appendix D and provides the basis for assessment of leaching behaviour. 5.0 Gladstone Sump Water Quality During development, the pit walls of GOP will be subject to oxidation and pit wall run-off which will contribute a significant dissolved solids load to the base of the pit. The area of pit wall will increase as the pit progresses and decrease as the pit is backfilled. Water chemistry of the run-off is dictated primarily by the mineralogy of the pit wall in the run-off flow path. Distinct differences in resultant water quality reporting to the sump will depend on whether infiltrating water passes over PAF rock or NAF rock. Key to the AMD process is the oxidation of sulphide minerals, which produces sulphuric acid and increases leaching of trace elements. During episodes of rainfall, oxidation products are dissolved by rainfall, forming an acidic solution that contains trace elements at potentially high concentrations. Where elevated sulphide concentrations are present in the pit wall rock (PAF rock), the run-off is likely to have low pH, high sulphate concentrations, and detectable levels of trace elements. The degree to which pit wall rock is likely to be acid forming is dictated primarily by the minerals present, which is a function of pit wall lithology (alteration) and the degree of mineral oxidation. As such, the pit wall can be divided into areas referred to as pit wall associations, grouped by analogous mineralogy and weathering profiles, which generally produce similar run-off chemistry. In order to estimate sump water quality, a conservative approach has been adopted in which two distinctive runoff water qualities have been devised and mixed accordingly. The method adopted is outlined and summarised in the sections below and shown in Figure 9. Figure 9 Gladstone Open Pit – Water Quality reporting to sump
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