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 21 AECOM 5.4 Key Assumptions The key assumptions used in assessing and calculating the pit sump water quality are: • Rock used within the field column tests is representative of exposed pit wall rock (refer Appendix D) with the range of concentrations representing successive flushing events by rainfall • Sulphate generation rates in the columns are assumed representative of sulphate generation rates in the pit wall. This is considered conservative. • Trace element leaching is proportional to the molecular ratio of sulphate / trace element leaching observed in the field columns. • No lag has been taken into account and this data has been excluded from the column data. • Pit wall runoff is the only source of water to the pit sump. Additional water sources are likely to have a dilution effect and therefore the results are considered conservative in this respect. • The geochemical modelling assumes the entire calculated daily oxidised mass (and associated leachate metals) is dissolved into the applied daily runoff volume. 6.0 Rock Management 6.1 Introduction During development and mining, rock is to be managed to minimise the requirement for surface stockpiling and the need for externally sourced backfill materials. Some temporary surface stockpiles are however necessary hence the need for the WRS. Likewise, rock in the TSF stockpiles, or used within the NRS or TSF3 embankment will be compacted upon deposition, which will limit oxygen ingress within these storage facilities. Rock placed within the zone of oxidation for the final proposed landforms (typically within the final 2 m of directly placed and/or stockpiled material) should comprise NAF material only. Within temporary stockpiles at the Rock and Tailings Storage Area (RTSA), appropriate mitigation measures in the form of limestone amendment will be required to limit sulphide oxidation rates and/or limit the effects of already oxidised material. Similarly, the mixing and/or layering of limestone may be required for rock material used as backfill within the GOP. The actual requirements and quantities depend on the acid forming potential of the material deposited, the exposure time of material once placed and whether the material is to be placed below the groundwater table or not. A schematic detailing the overall management strategy for PAF rock sourced during the project and its final destination is illustrated in Figure 10. The schematic also refers to underground disposal of Martha PAF rock. This is covered in more detail in AECOM, 2018. To ensure geochemical management of PAF rock and to reduce adverse effects on water quality, the implementation of an appropriate management strategy is required. Commonly used methods for the management of mined rock that has the potential to impact water quality includes: 1. Oxidation effective control - Control of oxygen flux to reactive sulphides, such as by deposition under water. 2. Geochemical control - Blending rock types or addition of neutralising materials to control pH and oxidation rates. 3. Hydrological control - Placement of low permeability layers, evapotranspiration layers and rock management structures to control the release rate from the disposal facility. A combination of these methods has been effectively employed where necessary in the management of previously mined rock at Waihi, and it is envisaged that management of rock sourced from Waihi North Project will be similar. The most appropriate means of minimising acid generation is likely to include one or more of the following methods:
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