GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 16 Figure 8 was provided by OGNZL and shows the different overburden types from GOP, categorised by geochemical composition. The NAF/PAF criteria are as per the OGNZL rock classification procedure for managing mine waste and the High (Mercury) Hg is based on >3.5mg/kg Hg Overall, GOP will produce 18.7 Mt of overburden, of which approximately 11.9 Mt is PAF. Aside from acid forming potential, the GOP waste rock also has elevated levels of mercury (Hg). NAF-High Hg, HPAF-Low Hg and HPAF- High Hg are all treated as per PAF. Overburden used in construction of the GOP TSF will primarily come from the NRS or MOP4 cutback and will be predominantly PAF. The overburden will be neutralised with limestone, as required, at a rate to be confirmed prior to construction. AECOM (2021) suggests the following requirements with respect to geochemical treatment: – No additional limestone amendment required for Martha and WUG PAF material likely to be exposed for less than 20 weeks. – Limestone amendment rate of 0.6% for Martha and WUG PAF material where a lag period of 250 days is required. – Overburden placed within the zone of oxidation for the final proposed landform (typically within the final 2 m of placed material) should comprise NAF material only. Figure 8 Geochemical properties of the GOP waste rock 2.5.3 Pit walls AECOM (2021) indicates that approximately 73% of the GOP walls are PAF with a total volume of 13,605 m3 available for oxidation, assuming an oxidation depth of 0.1 m. 2.5.4 Tailings The geochemistry of the GOP TSF tailings (likely from the processing of WUG ore) are expected to have similar geochemical properties to the tailings stored in TSF1A and TSF2. Modelled pore water chemistry for the GOP TSF is presented in AECOM (2021). The pH of the porewater post closure is estimated to be 5.6, indicating slightly acidic conditions.
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