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 28 AECOM Table 10 Predicted NRS / Embankment Leachate Water Quality DEVELOPMENT CLOSURE Min Med Max Min Med Max pH 4.5 4.6 7.9 pH 4.8 6.9 7.9 Al 0.003 12 25 Al 0.003 0.7 1.4 As 0.001 0.002 0.07 As 0.001 0.002 0.01 Ba 0.013 0.020 0.044 Ba 0.013 0.02 0.04 Ca 31 480 590 Ca 31 480 590 Cd 0.00005 0.007 0.012 Cd 0.00005 0.0005 0.002 Co 0.0002 1.9 3.0 Co 0.0002 0.33 0.60 Cr 0.0005 0.001 0.13 Cr 0.0005 0.001 0.003 Cu 0.0005 0.027 0.36 Cu 0.0005 0.008 0.009 Fe 0.02 300 470 Fe 0.02 90 120 Hg 0.00008 0.0001 0.0005 Hg 0.00008 0.00011 0.00047 K 4 14 22 K 4 14 22 Mg 15.4 115 280 Mg 15.4 115 280 Mn 0.3 15 61 Mn 0.3 15 61 Na 12 37 101 Na 12 37 101 Ni 0.003 0.78 1.4 Ni 0.003 0.063 0.5 Pb 0.0001 0.002 0.005 Pb 0.0001 0.0002 0.0007 SO4 198 1,440 2,400 S 198 1,440 2400 Sb 0.00002 0.002 0.003 Sb 0.00002 0.0006 0.0025 Se 0.001 0.002 0.005 Se 0.001 0.002 0.005 Zn 0.003 1.2 2.8 Zn 0.003 0.043 0.4 Notes: Bold denotes numbers based on modelling, italics denote TSF1A data. Development modelling numbers are based on six scenarios. Closure modelling numbers are based on two scenarios. All concentrations reported in mg/L. 6.3.2 Permanent Rock Disposal to Gladstone Open Pit The closure of GOP involves backfilling with approximately 5 Mt of rock placed and construction of a liner over this rock to enable tailings placement. NAF/PAF rock material sourced from both Martha and the GOP will be utilised to backfill the GOP. Furthermore, the neutralisation capacity of NAF / low PAF material from the Martha together with amended limestone will introduce a buffer to acidity from the exposed pit walls. The key mitigation measures for the placement of PAF rock within the GOP are as follows: • For rock placed below GOP TSF liner, material will be paddock dumped and spread and compacted in lifts in the order of 1m in height, lime amendment of these layers will be completed to provide lag to limit acid generation until subsequent lifts are placed and compaction via track rolling vehicle movement across each of these lifts. • For rock above the GOP TSF liner the continued placement and encapsulation of material will be in the same manner as other RTSA permanent storage facilities.
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