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 33 AECOM Table 17 includes the worst case backfill water quality predictions determined by this method in square brackets. The acid sulphate generation rates estimated for the waste rock stockpile and the underground mine are such that the chemistry of mine water is likely to be limited by solubilities of the species present rather than the available mass of constituents produced by sulphide oxidation of the waste rock. Concentrations calculated on the basis of the oxidised mass of contaminants as described above are therefore theoretical estimates of worst-case groundwater quality within the backfill. In addition, these estimates are unlikely to be in geochemical equilibrium depending on pH and redox conditions, which are present in the underground mine post mining. Table 17 summarises the existing groundwater quality within the WUG system, and for comparative purposes includes the pre mining chemistry of samples collected from the Waihi No.7 Shaft. The water chemistry for the Waihi No.7 Shaft was evaluated using PHREEQC, a geochemical equilibrium model which simulates chemical reactions and transport processes in natural or polluted water, based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, solid solutions, exchangers, and sorption surfaces. It is estimated from PHREEQC modelling that the water within the Waihi No 7 Shaft at 206.5 m depth was at equilibrium with iron species at an Eh of approximately –320 mV. It is assumed for the purposes of modelling that similar redox conditions would develop within the WUG Mine. Geochemical modelling of these chemistries allowing for precipitation of oversaturated species and adsorption is included in Table 17 as best estimates of backfill porewater quality post closure (values not in brackets). The PHREEQC model used the worst case predictions referred to above as inputs for a range of scenarios including the following: · • simulating iron oxy hydroxides precipitation to achieve equilibrium with mineral phases such as goethite (FeOOH), lepidocrocite (FeOOH) or ferrihydrite (Fe(OH)3); • simulating equilibrium with carbonate species using the mineral phase calcite to account for the dissolution of carbonate species associated with the residual acid neutralising capacity within the waste rock; • simulating equilibrium with sulphate species using the mineral phases gypsum or anhydrite to account for the precipitate of oversaturated sulphate species which are present; • simulating adsorption of trace elements present in the resultant solution onto hydrous ferric oxides surfaces using a two layer model with no diffuse layer (Dzombak and Morel, 1990); and, • in all scenarios it was ensured that the redox conditions remained at or about the value determined for the Waihi No.7 Shaft (-320 mV). The values presented in Table 17 reflect the range of predicted concentrations determined for the above simulations. Under strongly reducing conditions and with adequate residence time it is possible that precipitation of sulphides species will occur within the saturated backfill. In the event that these conditions do develop, extremely low levels of trace elements would be expected within groundwater.
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