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 3 AECOM Rock produced by mining will be stored at the rock storage stacks, WRS and NRS, before being used as backfill for the mined stopes during mining. Rock produced from the tunnel between the access portal at the processing plant and Willows Farm is considered comparable to the Martha rock in terms of its geochemistry and will therefore managed in the same manner using the rock storage stacks. 1.1.3 Tailings and Rock Storage Facilities 1.1.3.1 Tailings Facilities Tailings Storage Facility 1A (TSF1A) with a current crest at174.8 m RL and Tailings Storage Facility 2 (TSF2) with a current crest at156 m RL have insufficient capacity to store the tailings volume from processing ore from both Project Martha and WNP (consented heights of 182 m RL and 160.7 m RL, respectively). The additional capacity required will be provided via the establishment of TSF3 and GOP TSF. The additional tailings storage capacity (TSF3 and GOP TSF) will also provide some capacity to allow storage of additional tailings as a result of any further ore sources being brought into production in addition to Project Martha and WNP. TSF3 will be constructed in advance of GOP TSF. TSF 3 A new TSF is proposed to be constructed immediately east of TSF1A. The proposed crest height for the embankment is 155 m RL, providing a total tailings storage volume of approximately 5,700,000 m3. A 1.5 mm thick high-density polyethylene geomembrane is assumed to be necessary to contain any initial tailings seepage within the tailings impoundment and up to the initial starter embankment height. Above the starter embankment, a low permeability layer of compacted clay is proposed. GOP TSF A TSF is proposed to be constructed in the pit void of GOP. The pit will be backfilled with rock (approximately 5.0 Mt) mixed with limestone to form a base layer for converting it into a TSF. A geomembrane will cover the base layer prior to the tailings being deposited, considered necessary to contain any initial tailings seepage within the tailings impoundment. 1.1.3.2 Northern Rock Stack (NRS) As described above, rock will be used to partially backfill the GOP and to backfill voids at the underground mines. Additionally, rock will be used to construct the new TSF3. In addition to these requirements, a rock surplus is expected. An additional temporary and permanent rock repository (NRS) is proposed. A summary of the cumulative rock disposal by location is shown in Figure 2. The proposed location of the NRS is an elevated site located immediately north of the current TSF2. Design of the NRS will be similar to the existing TSFs to limit acid and metalliferous drainage (AMD) and the entry of leachate into surface and groundwater. Design features will include: • Low permeability soil containment system beneath the NRS; • Seepage drains installed in the subsurface; • Leachate collection drains above the low permeability layer; and, • Capping of the NRS to minimise oxidation and infiltration.
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