June 2022 G-14642.71-006-R-Rev0_FINAL 37 9.0 WHAREKIRAUPONGA UNDERGROUND MINE 9.1 Background WSP has completed a review of the Pre-Feasibility Study (PFS) for the proposed Wharekirauponga Underground Mine (WUG), completed by SRK Consulting (Australasia) Pty Ltd (SRK) and by OGNZL staff. This section describes the proposed mining design and the results of WSP’s review. The geotechnical section of the PFS prepared by OceanaGold is reproduced in Appendix D . 9.2 Proposed Underground Mine Development The Wharekirauponga deposit comprises a number of epithermal veins within host rhyolite domes. The rhyolite domes are overlain by pyroclastic units and post mineralisation andesite, though various streams at the site have eroded through the cover rocks to expose the underlying rhyolite. The EG and T-Stream veins that are the target of the underground mine dip to the west at between 60° and 80° (refer Figure 22) with a strike length of about 1000 m. An additional vein structure (the ‘Western Vein’) has also been identified west of the T-Stream vein. The EG and T-Stream veins are typically several metres in width and additional stringer vein sets are also present within hanging wall and footwall of the veins. The host rhyolite has been separated into two distinct units: the Eastern Rhyolite Flow and the Northern Rhyolite Flow. The nominal depth of stoping ranges between 100 m and 325 m below surface. The PFS study indicates that the underground mining method will be Modified Avoca and that mining will follow a bottom-up sequence. Stopes will be progressively backfilled with rockfill, potentially using cemented rockfill in some locations. The typical stope dimensions are 2 m to 12 m wide and 15 m along strike; with wider zones, potentially up to 20 m in width, stoped and backfilled in multiple separate blocks. Figure 23 shows a conceptual mine layout prepared by OGNZL. Key elements shown in Figure 23 include the main access tunnel (on the right), low level haulage drives, stope panels (in green), spiral declines and various ventilation shafts. It is envisaged that four ventilation shafts will be incorporated into the mine: two fresh air shafts, one of which will also allow emergency egress from the mine, and a return air shaft. Ore will be trucked back to the processing plant at Waihi via the main access tunnel. Backfill for the stopes will be carted back from the WRS at Willows Road Farm, then from the rock stack near the process plant if necessary. A key consideration in mine design will be to avoid surface disruption that could lead to significant surface water flows into the underground mine and associated adverse environmental effects. Exploratory drilling drives will be incorporated into the mine layout to allow detailed mine planning and geotechnical and hydrogeological characterisation of the ore bodies. This will enable OGNZL to more accurately predict rock mass behaviour during mining and minimise the likelihood of excessive dilation or deformation and adverse environmental outcomes.
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