GHD | Oceana Gold NZ Ltd | 12552081 | Geochemical Assessment Wharekirauponga Underground Mine (WAI985-000-REP-LC-0013) i Executive Summary Oceana Gold NZ Ltd (OceanaGold) are in the process of developing a new underground mine at Wharekirauponga referred to as the “Wharekirauponga Underground” or “WUG Mine”. Development of the mine initially involves developing a dual tunnel drive (the Willows Access Tunnel) to link the Wharekirauponga gold resource and a portal located on farmland near Willows Road (Willows Road Farm) approximately 5 km north of Waihi and OceanaGold’s current Martha operations. The portal area will be linked to the current Waihi site via road and an additional access tunnel. The WUG Mine is a component of OceanaGold’s Waihi North Project. This report comprises the following studies for the WUG Mine component of the Waihi North Project: Review of available geochemical data, characterisation and assessment of the acid forming properties of the Willows Access Tunnel and WUG Mine spoil material and how this will influence spoil storage, runoff and seepage; Outline of recommended strategies to minimise the onset of acidification (where applicable) and predict likely groundwater inflow, leachate and runoff water quality from the access tunnels, mine, and rock storage area in order to inform collection and treatment requirements. The Willows Access Tunnel alignment, as in OceanaGold’s current operations at Waihi, is situated in predominantly andesite material, with minor occurrences of more felsic rocks (rhyolite and ignimbrite) encountered toward the northern terminus. The WUG Mine is hosted predominantly in rhyolitic material overlain by andesitic material. Representative spoil material has been selected from exploration core in both the WUG Mine ore body and the tunnel portal area and analysed for multi-element and acid base accounting (ABA) data in order to characterise the geochemical makeup of the two areas and explore similarities / differences to material encountered at OceanaGold’s current Waihi operations. Analysis suggests that spoil material from the portal area and within the majority of the Willows Access Tunnel alignment is comparable in trace element composition and acid generating potential to spoil material currently encountered at OceanaGold’s Waihi operations based on the data collected to date. Multi-element analysis data for the WUG Mine show that arsenic is elevated and iron is depressed with respect to the current Martha dataset. In addition, both sulphur (and hence maximum potential acidity (MPA)) and the acid neutralising capacity (ANC) of spoil material within the WUG Mine is depressed compared to spoil within the WUG Access Tunnel alignment. 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. As the majority of the spoil material to be stored in a rock stack at Willows Farm (the WRS) will be sourced from the Willows Access Tunnel (spoil from the WUG Mine will form a minor component), the WRS is expected to behave geochemically similar to rock stacks at OceanaGold’s existing operations at Waihi. Groundwater inflow (into the WUG Mine and Willows Access Tunnel), WRS runoff and seepage water quality predictions are conservatively based on 95th percentile data from the current Waihi operations. This is considered appropriate when considering the similarities between the geology of the current Waihi operations and the Willows Access Tunnel, but also takes into account observed geochemical differences within the WUG Mine. The predicted water quality is utilised within the water balance assessment (reported separately), which takes into account the water treatment capacity and availability, trace element removal rates and consented discharge requirements. Field column testing utilising spoil from the WUG Mine was considered necessary to determine, inform and enable refinement of this assessment with time and give greater certainty of predicted leachate quality. The test columns have been constructed at OceanaGold’s current Waihi operations and the leachate data was utilised to confirm predictions outlined.
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