Oceana Gold Waihi North Project Waihi North Project Geochemical Assessment – Geochemistry of Tailings and Overburden, Treatment and Mitigation Revision 9 – 17-Jun-2022 Prepared for – Oceana Gold (New Zealand) Limited – Co No.: 2274246 A-4 AECOM Figure 25 Box plot showing calculated Acid Neutralising Capacity range in rock material Box plot shows from highest node to lowest node: maximum / 75%ile / Median / 25%ile / minimum concentrations. Data extending below 0% on the Y-axis are not shown. Samples tested for acid base accounting have been collected from existing drillcore. As such low ANC samples with highly reactive sulphur may have consumed acid neutralising capacity and be acid producing before sampling. Total sulphur is used to calculate the maximum potential acidity (MPA) and acid potential (AP) and as such these values accurately representative of the acid producing potential of the rock material. Acid neutralising capacity based on titration may however be understated. This makes calculation of NAPP (NAPP=MPA-ANC) conservative for the rock materials assessed. Selected samples have also been analysed for Total carbon which is used to calculate neutralising potential (NP), net potential ratio (NPR= NP/AP) and net neutralisation potential (NNP=NP-AP). For drill core samples with negative ANC values the total carbon is assumed to be at the limit of detection. It is also noted that some of these negative ANC’s are a function of sulphides in drillcore oxidising post drilling. When plotted on a net potential ratio (NPR) / net acid generating (NAG) pH diagram (Figure 26), the majority of the samples from WUG and Gladstone are PAF exhibiting a low pH (pH < 4.5) and low NPR (NPR <1). Of the remaining samples, approximately one third are classified as ‘uncertain’. In general, the Martha Pit material (MOP4) exhibits a lower acid producing potential.
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