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 8 AECOM Elevated trace element concentrations (antimony, arsenic and mercury) in the rock (and ore body) relative to historical mined areas need to be assessed for potential implications for consent compliance at the point of discharge (both to water and to air). An assessment of the distribution of trace elements within the rock shows that mercury is elevated in the highly clay altered Andesitic material located near the surface and associated with the Breccia material. Relatively low concentrations of other trace elements are noted in areas of elevated mercury. There is also no direct correlation between mercury with sulphur (the highest mean mercury concentrations are found in the hydrothermal breccia samples which tend to coincide with the lowest mean sulphur concentrations). Both these factors may be due to the high volatility of elemental mercury and the capability of more soluble mercury forms being more distal to the ore body, especially through postmineralisation cover. This can result in the secondary mercury halo around the deposits varying significantly in size (Browne, 2008). Further assessment of mercury is therefore necessary as sulphide content and management of sulphide alteration will not necessarily limit mercury release in the same way that it will control other elevated trace elements (arsenic and antimony). Wharekirauponga Underground (WUG) As noted in Appendix B (Section 4.2.3) arsenic, iron, selenium and antimony are elevated relative to mean concentrations within the earth’s crust. Iron, selenium and antimony concentrations in WUG rock are comparable to concentrations observed in the existing mine operation at Martha. Arsenic in T Stream and EG ore bodies is elevated with respect to historical Martha rock, but comparable to the concentrations in the Gladstone rock. Rock from the WUG mine will be managed within the Willows Rock Stack (WRS) and will primarily be used as backfill within the mine. The only WUG rock being disposed of in the NRS will be from the transport tunnel from the plant and this material will be more similar to the Martha rock.
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