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 27 AECOM • Mixing of the solutions has been carried out at a range of ratios considered to reflect the maximum proportion of the working area to rehabilitated area (0.6 : 0.4) to a fully closed / rehabilitated area. (0 : 1). • A mix of the two derived leachate concentrations has been undertaken in the geochemical modelling software PHREEQC Interactive version 3.6.2 utilising the Minteq.v4 (December 2021) database in order to equilibrate the predicted chemistry based on the oversaturation and secondary co-precipitation of various trace elements. The steps undertaken in PHREEQC are as follows: 1. Equilibrate the mixed leachate solution to allow for precipitation of over saturated sulphides. 2. Calculation of the number of hydrous ferric oxides (HFO) sites that are available within the rock material - assuming they are present as ferrihydrite, for the availability of sorption reactions. Sensitivity analysis around the number of available sites has been undertaken and these analyses are included in the summarised results. 3. Determine the influence of hydrous ferric oxides on the adsorption/desorption of trace elements. In this manner a new equilibrium between adsorbed and soluble trace elements can be modelled. • Modelled results from the above process are then compared to TSF1A leachate data and where the above modelling indicates changes from the existing data, modelled results are adopted in Table 10. Results of the modelling for both the development / construction and closed / rehabilitated stages are presented in Table 10.
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