GHD | Oceana Gold NZ Ltd | 12552081 | Geochemical Assessment Wharekirauponga Underground Mine (WAI985-000-REP-LC-0013) 31 8. Conclusions Antimony, arsenic, selenium, and sulphur concentrations are elevated (geochemical abundance indexes of greater than three) in the spoil from both the Willows Access Tunnel and spoil associated with the WUG Mine. With the exception of arsenic, the geochemical abundance indexes for the Willows Access Tunnel, the WUG Mine and the existing Waihi dataset, exhibit similar ranges. Based on the available data, the WUG Mine area has a higher average arsenic concentration and geochemical abundance index and a lower average iron concentration compared to the Willows Access Tunnel and the existing Waihi dataset. Similar to the current operations at Waihi, element enrichment and a greater abundance of trace elements is apparent in both the rhyolitic and andesitic spoil material as depth increases. The acid-generating potential data for spoil from the Willows Access Tunnel and WUG Mine suggests that the spoil has similar low ANC values compared to the existing Waihi dataset and may result in acid generation and leaching of trace elements unless appropriate control measures are implemented. Appropriate management of the spoil material (from the Willows Access Tunnel and the WUG Mine) is crucial to limit potential impacts on the surrounding environment and to limit the volume of water requiring treatment. Based on existing operations at Waihi, OceanaGold has utilised a combination of methods to effectively manage spoil. It is envisaged that management of spoil sourced from the Willows Access Tunnel and WUG Mine will employ similar methods. The means of minimising acid generation of the spoil from the WUG development should consider: Limiting exposure time of rock between excavation and disposal to the WRS; Blending and/or layering the rock with limestone, in order to prevent the onset of acidification where the specific acid forming characteristics of the material being excavated suggest that the material is at risk of producing ARD; Compaction of placed spoil material in the WRS to reduce permeability, limiting oxygen and water ingress; and Covering PAF spoil with NAF material, of sufficient thickness to limit oxygen ingress to the encapsulated PAF. Water quality predictions are based on 95%ile data from the current Waihi operations. This is considered appropriate when considering the similarities between the geology of Waihi and the Willows Access Tunnel and WUG Mine, and proposed spoil management practises, but also takes into account the observed geochemical differences (i.e., increased arsenic concentration in the WUG Mine geochemical dataset). Leachate data from field columns utilising rock from the WUG Mine have enabled comparison to the derived WUG inflow water quality. The comparison suggests the assumed water quality dataset adopted for water balance modelling is considered appropriate for use in the water balance model which takes into account the water treatment capacity and availability, trace element removal rates and consented discharge requirements. Additional trace element and ABA data collection and analysis will enable further characterisation and a refinement of this assessment.
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