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 51 AECOM TSF Receiving Tailings Martha Ore (MUG, MOP4 etc) GOP Ore WUG Ore Mean all sources 8% - 92% Maximum WUG (by quarter) - - 100% Water quality assessment for TSF3 and GOP TSF considers two key components, surface water chemistry and pore-fluid chemistry. The surface water component (representing the tailings pond supernatant) is also likely to reflect the composition of the decant solution, pumped back to the processing plant. The pore-fluid component represents the water within pore spaces of the tailings material which will report as seepage to underdrains. For the purposes of this assessment the TSF decant water quality scenarios are based on mean and maximum component of the GOP or WUG ore that could be placed in TSF3 and GOP TSF from Table 23. The predicted porewater is, however, only assessed based on the mean from Table 23 as the seepage will be controlled by the overall mix of tailings placed within a tailings storage facility. 7.3.1 TSF Pond Water Quality Water chemistry associated with TSF1A, TSF2, TSF3 and GOP TSF pond water (decant) was predicted using the following steps: • The samples of tailing liquor from laboratory leach tests (Table 20) indicate an initial porefluid solution. Mean raw liquor data measured from processed Gladstone ore material was adjusted using mean elemental concentrations representing the entire Gladstone ore sample dataset (Table 18). The chemistry of this solution was taken to represent the initial fluid portion of the tailings slurry prior to deposition in the tailings facility. • The SPLP results are used to assess the relative leachability of trace elements in tailings once placed within the storage facility • Only trace elements elevated with respect to previously placed tails and with increased trace element leachability are assessed in this way (arsenic). For other trace elements the default values in existing TSF underdrains and decant are adopted. • The above steps define the adjusted liquor shown in Table 21. • Geochemical modelling is undertaken using geochemical modelling software PHREEQC Interactive version 3.6.2 , utilising the Minteq.v4 database. This assesses the speciation of chemicals present and any oversaturation of phases that would potentially precipitate out of solution as a result of the above adjustments. This modelled solution (Table 24) was taken to represent tailings’ pond water (decant) chemistry. • Rainfall into the TSF (and associated dilution), and evaporation from the TSF (and associated concentration) of pond water were not accounted for making the assessment of any trace elements with elevated total concentrations conservative. PHREEQC modelling predicts that as water pumped into the TSF reaches equilibrium with expected TSF mineral species, pH will reduce towards neutrality, and Iron and Aluminium will largely precipitate out of solution. This is expected to occur primarily in response to the formation of the Gibbsite (Aluminium hydroxide), and hydrous ferric oxide minerals including Ferrihydrite. When compared with average decant chemistry from TSF1A and TSF2 from 2014 to 2020, modelled equilibrated pond water exhibits similar chemical trends to measured averages from existing facilities (Table 24). Calcium, potassium, sodium and sulphate remain elevated relative to other components, and concentrations of iron and aluminium are low relative to their concentration in the liquor.
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