GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 32 . Table 3.1 Stratigraphy of the Gladstone area Unit Depth encountered (m bgl) (thickness where present) Description Topsoil Surficial (0.25 – 3.4 m) Occurs across the site. Ash/regolith(1) and RT (rhyolitic tuff) 0.4 – 3.4 (3.1 – 13.1 m) Occurs across the site, reducing in depth and thickness to the south where it overlies the dacite. Sandy ignimbrite 4.7 – 43.5 (5.7 – 38.8 m) Occurs west of the proposed pit area. Typically separated from andesite by RT unit, however a small section is inferred to have direct contact with andesite beneath the Ohinemuri River west of the site. (1) Dacite 3.5 – 8.9 (> 21.1 m) In the southeast of the proposed pit area, dacite separates the ash/regolith and RT units from the underlying breccia. Hydrothermal breccia (Coromandel Group) 2.5 – 28.5 (3.2 – 24.5 m) Overlies the andesite in the eastern and southern areas of the proposed pit. Not present in the northern and western areas of the proposed pit. Andesite (Coromandel Group) 2.0 – 29.0 (> 118 m) Andesite host rock. Encountered at shallowest depths beneath the central and western area of the proposed pit. Occasional outcrops of weathered altered andesite in elevated areas. 1. OGNZL leapfrog model, 2021. 3.3 Conceptual groundwater model (Gladstone CGM) 3.3.1 Overview Two groundwater systems (shallow and deep) are inferred to be present at the Gladstone site, with this consistent with the broader Waihi area. The shallow system refers primarily to a series of units including surface sediments, ash layers, RT, VC and dacite. This system is characterised by rainwater infiltration, run-off and interactions with surface water. The deep system refers to groundwater within the andesite rock. The weathering of the andesite surface and hydrothermal breccia, prior to deposition of the dacite and other younger volcanics, provides hydraulic separation between the shallow and deep groundwater systems. The existing Favona portal infrastructure is constructed in the andesite rock and intersects the shell of the proposed GOP. Dewatering of the andesite currently occurs to enable underground mining, which extends to an approximate depth of 800 mRL (OGNZL, 2019). Complex faulting in the area has the potential to provide either conduits or barriers to groundwater flow between ore bodies, with these demonstrating a southwest-northeast orientation. Dewatering of Favona mine commenced in 2005, with further mining in the Waihi area providing hydraulic connection between the Martha area to the northwest and the Favona-Moonlight-Gladstone area. The dewatering for Favona mine, and ongoing dewatering for Correnso and MUG mine activities, has resulted in the andesite in close proximity to the veins and underground workings being dewatered, with the shallow groundwater system being locally under-drained. The low permeability of weathered andesite and breccia has allowed maintenance of a shallow groundwater system. This situation is evident across Waihi, where mining and dewatering is occurring at depth. The Ohinemuri River to the southwest of the GOP has demonstrated losing conditions, suggesting loss of water to shallow groundwater. These losing conditions have occurred concurrently alongside upwards vertical hydraulic gradients from the deep andesite adjacent to the river, providing evidence of the hydraulic separation of the shallow and deep groundwater systems local to the proposed GOP. The Gladstone CGM is presented in the following cross-sections (Figure 3.4 to Figure 3.6).
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