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 42 AECOM Laboratory leach tests were undertaken on ore material (head) from the proposed GOP to produce a liquor and residue that would make up the tailings from ore processing. Laboratory results for these ore (head), liquor (porewater) and residue (tailings solids) components are summarised in Table 20. Ore was processed in three different grind sizes (53, 75 and 90 µm) to investigate relative recovery rates. Data from the 75 µm tests has been presented here as it is most representative of likely ore processing during mining. The liquor data is used to calculate predicted porewater quality and seepage quality from the consolidated tailings. The mineral extraction process introduces considerable alkalinity as can be seen in the liquor and residue samples which is consistent with actual data for the TSFs at site as shown in Table 24. In addition, samples of tailing solids were sent for Synthetic Precipitation Leaching Procedure (SPLP) analysis (USEPA method 1312). This method is designed to determine the mobility of analytes present in processed tailings and partially simulates conditions within the tailings facility where water passes through the consolidated tails and adsorption-desorption of soluble trace elements can occur. The results of the SPLP analyses undertaken on three Gladstone and three WUG ore are presented in Table 21. The results suggest that the trace elements of concern present in the liquor that have been mobilised during the processing of the ore rapidly reduce and ongoing leaching of trace elements from the tailings after the initial process water release are orders of magnitude lower. The SPLP data is used to assess relative leachability of trace elements from respective ores which is used in the predictions of porewater quality and seepage quality. The partition coefficient for the tailings residues were calculated using the ratio of residue geochemistry to SPLP extraction concentration correcting for the 20:1 liquid to solid ratio in the SPLP test method. For trace elements where the leachability has increased in Gladstone and Wharekirauponga ore from the historical Waihi dataset, the difference in leachability has been summarised in Table 22, where the increases are greater than an order of magnitude and are therefore included in the assessment of decant and porewater.
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