Oceana Gold Waihi North Project Waihi North Project Geochemical Assessment – Geochemistry of Tailings and Overburden, Treatment and Mitigation Revision 0 – 17-Jun-2022 Prepared for – Oceana Gold (New Zealand) Limited – Co No.: 2274246 57 AECOM 8.0 Conclusions Mercury, antimony, and arsenic are generally elevated in Gladstone rock relative to the historical Waihi dataset and mean concentrations in the earth’s crust. These elements are enriched in both the Breccia and Andesitic rock material, with mercury showing the greatest difference. Within the Andesitic material, mercury is more elevated in the highly clay altered rock, in rock located closer to the surface, and in rock located near the borders of the Breccia material. The acid-generating potential data suggests that the rock from the GOP has a lower neutralising capacity compared to the existing dataset. The static field tests confirmed that in general, rhyolite material from the WUG mine is elevated in arsenic but is comparable to the historical Waihi dataset in terms of acid generating capacity. Kinetic laboratory and field column tests suggest that due to the low neutralising capacity of the Gladstone material, the natural lag time is short and material has the potential to exhibit depressed pH and associated elevated sulphate and trace element concentrations in leachate within 10 weeks of placement. Treatment methods employed on some of the columns (limestone dosing, saturation, and compaction) suggest they are valid strategies to avoid and/or mitigate the effects of AMD from exposed and placed rock during operations associated with the Waihi North Project. The specific method and strategy employed to limit the onset of acidification depends on the rock destination, method of placement, length of exposure, and specific ABA characteristics of the rock. The overall rock management strategy is based on temporary storage of material in the existing Rock and Tailings Storage Area (RTSA) area or at Willows Road combined with permanent rock disposal to the GOP area, NRS and the embankment of TSF3, or as backfill in the underground mine. The specific recommended mitigation outlined depends on the rock’s source and ultimate end state and is broadly discussed in four broad categories: • Temporary storage of rock sourced from WUG in the WRS prior to placement underground as backfill or within permanent disposal structures; and • Disposal of rock to GOP with placement and compaction; and • Disposal of rock to both the NRS and TSF3; and • Placement of rock as backfill as part of MUG and WUG mine operations. PAF rock placed into temporary stockpiles at the RTSA should be amended with limestone to ensure the introduction of a 30-week lag period. This will ensure that PAF rock material is neutralised until the material is placed permanently outside of the zone of oxidation. For PAF rock that will potentially be exposed for a period in excess of this, additional amendment may be required, as is currently practised. Permanent disposal of PAF material sourced from both WUG and GOP will be undertaken in a manner that will limit the rate of oxygen ingress and limit the potential of the material producing acidity until saturation and/or consecutively placed material (on top) will remove the placed material from oxygen exposure. For exposure periods exceeding the materials natural lag, the addition of limestone will extend this lag period. The general mitigation measure for the placement of PAF rock within GOP is for material to be placed and compacted in lifts. Saturation with perched water will provide appropriate control until the returning groundwater table will secure long term control. For material above the recharged groundwater level, compaction will provide long term control. Limestone amendment for compacted material will be dependent on the required lag period. The amendment rates outlined are considered conservative and within the current operating limits of the site. Monitoring of the PAF rock material will enable refinement to those calculated and outlined rates, and will be part of ongoing operations. Geochemical modelling has predicted water quality associated with specific disposal locations and mitigation strategies in order to inform the impact of the planned operations on the wider environment and short and long term treatment requirements. The assessments undertaken are considered conservative based on a number of factors. These include (but are not necessarily limited to); selection of rock material for kinetic / column tests focussed on non-weathered, high pyritic material; scaling
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