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 ii AECOM Permanent disposal of PAF material sourced from GOP will be undertaken in a manner that will limit the rate of oxygen ingress and potential for acid generation until saturation and/or subsequently placed material (on top) will cover the placed material at a depth below the zone of oxidation. For exposure periods exceeding the material’s natural lag, the addition of limestone will be required to extend this lag period. The general mitigation measure for the placement of PAF rock within GOP is that material will be compacted in lifts and over time be below the depth of oxidation. This will provide appropriate control until the permanent groundwater table is re-established to provide secure long term control. For material above the recharged groundwater level, compaction and cover will provide long term control. Limestone amendment for both end-tipped and placed and compacted material will be dependent on the required lag period. The amendment rates outlined are considered conservative and within the current operating limits of the site. Monitoring of the rock material will enable refinement to those calculated and outlined rates and will be part of ongoing operations. Geochemical modelling of the Gladstone Sump water quality, NRS and TSF3 embankment seepage water quality, and GOP pore water quality has been undertaken in the geochemical modelling software PHREEQC version 3.4. The Minteq.v4 database was utilised in order to equilibrate the predicted chemistry based on the oversaturation and secondary co-precipitation of various trace elements. The results inform the potential impact of the planned operations on the wider environment and consequently the short and long term treatment requirements so that adverse effects are avoided. The assessments undertaken are considered conservative based on a number of factors. These include (but are not necessarily limited to); selection of rock material for kinetic / column tests focussed on non-weathered, high pyritic material; scaling factors to the laboratory / field tests to full scale operations have not been applied; and use of Mean and 95% upper confidence level (UCL) data to calculate applicable limestone dosing rates. Geochemical testing on GOP ore was found to exhibit similar geochemical characteristics to that of the existing material from MUG with the exception of mercury, which was found to be elevated. The analytical results of the Leach Tests and Synthetic Precipitation Leaching Procedure (SPLP) analysis of the GOP ore material combined with the geochemical modelling undertaken suggest that arsenic, calcium, cadmium, cobalt, chromium, copper, iron, mercury, magnesium, nickel and lead concentrations in porewater will be reduced via adsorption to hydrous ferric oxides, while potassium, sodium and sulphate will remain more mobile. The resultant predicted leachate water quality will be similar in nature to that stored within the existing TSF facilities, with potentially less mixing with groundwater due to the presence of a geomembrane in the basal structure. As with the existing TSFs, seepage will be collected and treated. In summary, the mitigation and management practices put forward are considered suitable based on the extensive and targeted datasets collected and on more than thirty years of successful application of these practices at Waihi. The assumptions applied during the calculation of amendment requirements and the predictive geochemical modelling undertaken ensure that the assessment is considered conservative and within the current (or planned) operating limits of the site. Monitoring of the rock material will enable refinement to lime amendment rates calculated, outlined, and presented in this report, and will be part of ongoing operations.
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