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 14 AECOM Figure 6. Box plot showing range of Sulphur concentrations in rock material Box plot shows from highest node to lowest node: maximum / 75%ile / Median / 25%ile / minimum concentrations. Data with a value below 0 on the Y-axis are not shown. In general, the acid generating potential of the rock from within the proposed GOP and WUG is similar to that of historical mining areas. This includes: • Mean sulphur concentrations in the Gladstone rock ranging from 2.7% in the andesite to 3.2% in the hydrothermal breccia. • Mean sulphur concentrations in the Wharekirauponga rock ranging from 0.91% in the WUG mine to 3.7% in the WUG access tunnel. • These are comparable to the historical rock sulphur concentrations (including Martha, Trio, Favona ranging from 2.1 to 3.0%. As a result the acid potential (AP) in kg CaCO3 per tonne of the Gladstone and WUG rock is considered comparable. The acid base accounting of the rock from within the proposed GOP and the WUG mine has the following differences: • The largest difference between the datasets is the lower neutralising capacity within the GOP. This is highlighted with respect to a lower overall mean acid neutralising capacity (ANC), ANC/MPA, and neutralisation potential (NP) values within the proposed GOP rock when compared with existing mined rock. • The acid neutralising capacity of the WUG rock is comparable to existing mining operations as shown in Figure 3. • As a result of the similar AP and ANC in WUG rock to historical mined rock, the ANC/MPA is also comparable to previously mined rock. When plotted on a net potential ratio (NPR) / net acid generating (NAG) pH diagram (Figure 7), the majority of the samples from WUG and Gladstone are potentially acid forming (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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