WAI-985-000-REP-LC-0056 Revision 0 Page 14 of 21 Waihi North Project – Tailings and Rock Storage – Functional Need Assessment In summary, the scale and footprint of co-disposal required at Waihi would bring novel technical and operational challenges associated with the geochemical and geotechnical characteristics of the ore that would save little in surface area and would not avoid impacts on the affected tributary. Slurry tailings impoundments have been successfully used at Waihi for over 30 years and remain OGNZL’s preferred option. Placement of dry stack of tailings and rock into Martha Open Pit In pit placement of dry tailings and rock into Martha Open Pit has been considered. Given that in pit disposal avoids the geotechnical stability issue associated with dry stack tailings this option is feasible from a technical perspective. However, this would sterilise the gold resource associated with the Martha Open Pit Project. It may be feasible to do Martha Open Pit Project first however this project requires TSF3 also. For these reasons, placement of dry stack tailings and rock into Martha Open Pit is not a viable option for OGNZL. Whilst Martha underground is operating it will be undesirable to place dry tailings into the pit from a health and safety perspective, saturation at the base of the pit could potentially liquify the dried tailings unless they could be effectively sealed which is difficult given the limited access to the base of the pit. Dry stack tailings facility A tailings “dry stack” was considered for TSF3 to avoid the Significant Natural Area vegetation within the proposed TSF3 impoundment. A dry stack places dry tailings with dump trucks or conveyor and then compacts the tailings in a stack or stockpile. Waihi has an average annual rainfall of 2100mm and for the scale of the project “dry stack” tailings facilities would not be practical due to the complexities around the placement and compaction controls required to place the tails and manage surface and subsurface drainage. Dewatering and dry stacking of tailings was examined in 2020 as an alternative to wet disposal in TSF3. Geotechnical test-work on current tailings and dewatering (filtration) test-work on fabricated tailings demonstrate that a dry cake can be produced. However, research showed most dry stack facilities are constructed in relatively arid environments and that due to the high rainfall conditions in Waihi several risks were posed including: • Tailings dry stacks can saturate in higher rainfall climates and can have geotechnical stability risks unless proper surface water management and subsurface drainage is implemented. • A large collection pond would be required to manage surface water at the toe of the dry stack to manage runoff and erosion from the dry stack. • Erosion of the dry stack in high rainfall. • Production performance of dewatering plant may be inadequate such that dry cake is not dry enough to support stack construction. • Ability to construct with dry cake in wet weather will likely stop production. • A traditional slurry TSF would be required to take tailings during wet periods or during breakdown of dewatering plant. • Dry stack may not be as geochemically stable long-term as the traditional slurry tailings in TSF1A and TSF2.
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