GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 70 . 4. Northern Rock Stack 4.1 Site overview 4.1.1 General site setting The current site setting of the proposed NRS is presented in Figure 4.1. The existing topography at the site slopes from the elevated rhyolite dome in the east towards the Ohinemuri River in the west. Current land use comprises the existing northern storage area (also known as the northern stockpile) and workshop in the south, with pastoral farming in the north. A Significant Natural Area (SNA) defined by the District Plan, is located east of the site beyond the mining licence boundary, which sits between the NRS and proposed TSF3. The proposed footprint of the NRS will be positioned over an existing perennial stream and its intermittently flowing tributaries, which currently discharges into the Ohinemuri River. The Ohinemuri River itself is located west of the site and flows approximately north to south. 4.1.2 Summary of proposed activity The NRS facility is proposed to be located north of the existing storage of TSF2. The NRS will be built over the existing northern storage area and workshop and extend further to the north. The existing tributary of the Ohinemuri River within this area will require diversion. A plan of the proposed NRS is provided in Figure 4.2. The proposed activity for NRS is described in the NRS geotechnical report prepared by EGL (2021a), with a summary provided below. References to relevant plans in the EGL NRS geotechnical report provided in italics: – Foundation works: Removal of topsoil and stockpiling, and installation of sub-soil and downstream toe drains (EGL NRS DWG 772). Where required, removal of weak surficial deposits and placement of NAF material to allow appropriate founding of the NRS. Establishment of an access road, collection pond, uphill diversion (clean water) and perimeter drains (EGL NRS DWGs 715-717, 730-733), which will be installed around the majority of the NRS. Construction of a low-permeability NAF soil liner (Zone A material) and installation of leachate collection drains (EGL NRS DWGs 7604-52 and 0770). – Rock stack development: Placement of rock to a maximum elevation of 1,173 mRL. This will comprise a 76 m high engineered landform over a footprint of approximately 26 ha, containing up to 7.0 Mm3 of rock. Collection pond with a capacity greater than 20,000 m3 and embankment height greater 4 m. Both NAF and PAF materials will be stored. PAF rock is only permitted in the stockpile bulk fill zone (Zones D2, D3 and E) and Zone F (EGL NRS DWGs 716-717). Lime is proposed to be applied in-situ during filling to PAF rock to delay the onset of mine rock drainage. Some of the rock will subsequently be used to backfill Gladstone Open Pit and other underground mining stopes, reducing the maximum elevation of the stack. – Closure: Any unused rock will be capped with NAF (Zone F, G and H materials) to limit oxygen and water ingress. The rock stack will have a maximum elevation of 1,148 mRL, including the capping layer. The capping layer will be covered with topsoil and contoured. After closure, the NRS would be progressively rehabilitated in accordance with the surrounding area.
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