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 1 AECOM 1.0 Introduction Waihi North Project (WNP) is a multifaceted development proposed by Oceana Gold (New Zealand) Limited (OGNZL) that has the potential to extend the current life of mine to at least 2038 by adding gold production to its Waihi operations. The project consists of six key and linked components: • The new Gladstone Open Pit (GOP); • The new Wharekirauponga Underground Mine (WUG); • A new tailings storage facility (TSF3) with a crest height of 155 m RL; • A new rock storage facility (Northern Rock Stack or NRS) as well as a temporary rock stack at Willows Road for the WUG referred to as the Willows Rock Stack (WRS); • Increasing the throughput capacity of the existing Processing Plant by 1 million tonnes per annum (MTPA) to 2.25 MTPA; and • Upgrading of the existing water treatment plant and reconsenting of the existing treated water discharge to the Ohinemuri River. In addition, the project will run concurrently with the already consented Project Martha. OGNZL has engaged AECOM New Zealand Limited (AECOM) to carry out a geochemical study of the ore, tailings, and rock expected to be recovered from the proposed mine areas, and to assess the potential influence these materials may have on the environment. The purpose of this study is to determine the following, through statistical and geochemical assessment and predictive modelling, in order to support the consenting process: • Whether rock and ore to be recovered as part of the proposed mining operations will be statistically different in elemental composition from that recovered from the current Waihi operations. • Whether trace elements are present in rock and ore that may constitute contaminants requiring particular management to ensure they do not give rise to any environmental concern. • The likely geochemical composition of tailings produced from the ore that will be deposited in tailings storage facilities and the associated effect on decant and seepage water quality. • The potential for sulphide oxidation and acidification of the rock during stockpiling and the corresponding mass of leachable contaminants released to porewater on mine backfilling and flooding. • Effectiveness of various treatment requirements to mitigate potential acid generating conditions.
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