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 i AECOM Executive Summary The Waihi North Project (WNP) comprises the proposed Gladstone Open Pit (GOP) and Wharekirauponga Underground Mine (WUG), together with the construction of a new tailings storage facility (TSF3), a Northern Rock Stack (NRS), and a temporary rock stack at Willows Road (WRS) and upgrading the existing water treatment plan. The WNP will extend the current Waihi life of mine plan (LOM) by eight years, ceasing production in 2038. The WNP will include the recovery and processing of approximately 15,750,000 tonnes of ore and approximately 28,400,000 tonnes of rock. For GOP, the geology and mineralogy of the project area, as with the greater Waihi epithermal vein system, is expected to be generally consistent with that encountered in the existing Martha, Favona, Trio, and Correnso mines, with the exception of the greater influence of the breccias which overlie the andesite rock in the GOP. Geology and mineralogy at WUG varies from other ore bodies in that areas of mineralisation are hosted within rhyolitic materials. Characterisation of the likely geochemical effects as a result of rock utilisation in temporary stockpiles and permanent repositories has been undertaken by both static and kinetic testing, as well as geochemical modelling. An extensive analytical programme of multi-element testing and acid base accounting testing has been undertaken on rock associated with the project components in order to understand the contaminants that may leach from the material with the potential to adversely influence water quality, and to explore whether this differs from those from current site operations. In addition, laboratory kinetic testing and field columns have been utilised to assess the rate of potential acidification and contaminant release, and to assess the effectiveness of appropriate control measures and management practices to limit the effect of rock placement to the receiving environment. The static field tests confirmed that in general, Andesite and Breccia material from GOP is elevated in mercury, antimony, and arsenic relative to the historical Waihi dataset and mean concentrations in the earth’s crust. Other trace elements were either present in similar or lower concentrations when compared to the existing datasets. In terms of acid generating potential, the rock from GOP was found to have 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 in terms of acid generating capacity. Kinetic laboratory and column field tests of potentially acid forming (PAF) rock material from GOP area exhibited depressed pH and associated elevated sulphate and trace element concentrations. ‘Treatment’ of these PAF rock columns included saturation, limestone blending, and compaction. These treatment strategies were shown to be effective for controlling the leachable trace element mass when compared to the ‘non-treated’ columns. The degree of effectiveness was related to the timing of the treatment (pre or post column inception), the introduced lag period, and the stage of the acid rock drainage allowed to develop. The overall rock management strategy is based on temporary storage of material in the existing Rock and Tailings Storage Area (RTSA) and WRS combined with permanent PAF rock disposal to the GOP area, northern rock stack (NRS), and the embankment of Tailings storage facility 3 (TSF3), and as backfill in underground mines. The specific recommended mitigation outlined depends on the rock’s source and ultimate end state and is discussed in four broad categories: • Temporary storage of rock sourced from WUG in the proposed WRS prior to placement underground as backfill or within permanent structures; • Disposal of rock to GOP with placement and compaction; • Disposal of rock to both the NRS and TSF3; and, • Placement of rock as backfill as part of Martha Underground Mine including the Rex Vein (MUG) and WUG mine operations. PAF rock from WUG placed into the temporary rock stack should be amended with limestone to ensure the introduction of a 30 week lag period. This will ensure that PAF rock is neutralised until the material is placed permanently outside of the zone of oxidation.
RkJQdWJsaXNoZXIy MjE2NDg3