WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. 1.0 INTRODUCTION Engineering Geology Limited (EGL) has been appointed by Oceana Gold (New Zealand) Limited (OGNZL) to undertake a technical report for the new Northern Rock Stack (NRS) for resource consent for the Waihi North Project (WNP). The NRS is located at the Development Site. This is the name given to the area occupied by the Processing Plant, Water Treatment Plant, the existing Tailings Storage Facilities (Storage 2 and Storage 1A), and the proposed Gladstone Open Pit, the new TSF (Storage 3) and the NRS. They are identified in Figure 2. The proposed rock storage facility has a maximum working crest level of RL173 (see Drawings 0715, 0716, 0717 in Appendix A) and will be an operational stockpile which will be closed as an engineered landform with a maximum crest of RL148 (Drawings 0780, 0781, 0782). It will permanently store surplus overburden earth and rock from underground and open pit mining that is not used for Tailings Storage Facility (TSF) construction or underground and open pit backfill. This technical report has been prepared for resource consent and details a design for the assessment of environmental effects as required under the Resource Management Act 1991. This report is Volume 4 of a 4-part series of reports prepared on the tailings storage and rock disposal for the WNP. Volume 1 is an overview of the tailings and rock disposal strategy (Ref. 1), Volume 2 is a technical report on Gladstone Open Pit (GOP) Tailing Storage Facility (TSF) (Ref. 2) and Volume 3 is a technical report on the new TSF, Storage 3 (Ref. 3). Some of the rock at Waihi is potentially acid forming (PAF) if left exposed to air and water and therefore the NRS will have design and operational features to prevent and to contain any acid drainage. They include earth liners (base and cap), subsurface drainage beneath the liner (subsoil drains), subsurface drains on top of the liner (leachate drains) and the addition of limestone to provide both a geotechnically and geochemically stable landform of earth and rock during operation and closure. The NRS includes an extension of the Northern Uphill Diversion Drain (Drawings 0715, 0730 to 0733). This drain diverts clean run-on water around the site so the water does not need to be treated. This extension includes diversion of a tributary stream (labelled TB1) of the Ohinemuri River. To make room for the NRS, the existing Development Site Workshop will need to be relocated onsite. The proposed new location is adjacent to the bottom of the existing haul road Workshop Ramp (Drawing 0715). 22 June 2022 OCEANA GOLD (NEW ZEALAND) LIMTED TAILINGS STORAGE AND ROCK DISPOSAL VOLUME 4 NORTHERN ROCK STACK RL173 PROPOSED ROCK DISPOSAL FACILITY TECHNICAL REPORT EGL Ref: 9018
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