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 35 AECOM 7.0 Mine Tailings and Tailings Management 7.1 Introduction The tailings generated from the Waihi North Project will be deposited in the proposed TSF3 and the GOP TSF. Existing tailings solids and pore fluid data have been used for comparison with the geochemistry of the proposed mine tailings generated from the Waihi North Project. Geochemical assessments involved the analysis of available ore geochemistry data for the existing mine operations and proposed mining areas, and a review of seepage and decant water quality within the existing tailings impoundments. These datasets were used to estimate tailings decant pond and seepage water quality for TSF3 and GOP TSF proposed as part of the Waihi North Project. 7.2 Laboratory Analytical Work 7.2.1 Statistical Analysis of Ore Geochemistry The objective of the statistical analysis was to determine whether any significant variation exists between the ore chemistry at the proposed GOP and WUG mine, and the ore being mined from the current and past operations. Summarised results from the ore geochemistry testing (acid generating potential, major elements and trace elements) are provided in Table 18 (proposed GOP and WUG) and Table 19 (historical ore samples from Trio, Favona, Martha and Correnso). Table 18 uses the GAI (GARD guide; INAP, 2018), to identify those trace elements that may be present at elevated concentrations relative to a typical background (mean earth crust). The specific intention of this comparison being the identification of potential contaminants of interest which may differ from current tailings at the Waihi operations. Boxplots showing the relative concentration range of mercury, arsenic, antimony, copper, selenium and manganese within the Gladstone and Wharekirauponga ore material are shown in Figure 11 to Figure 16. In general, the acid generating potential of the ore from within the proposed GOP and the WUG is similar to that of historical mining areas. This includes: • Mean sulphur concentrations in the Gladstone ore are higher than historical ore processed from Martha Pit but within the range of existing underground developments (Favona, Trio, Correnso). • Mean sulphur concentrations in the Wharekirauponga ore are comparable to the historical mining operations. • The Gladstone ore has low acid neutralising capacity relative to the historical mining operations. • WUG ore has comparable acid neutralising capacity to the historical mining operations. 7.2.2 Tailings Leaching Potential Table 18 indicates arsenic, manganese, mercury, antimony and selenium are enriched based on geochemical abundance indices. The following is noted for these trace elements with respect to ore from the existing mining operations: • Arsenic concentrations in Gladstone and WUG ore show a similar concentration and distribution to historical ore with a slightly lower mean concentration in WUG ore. • Antimony is elevated in Gladstone ore relative to historical data but is within a similar range in both datasets. WUG has slightly lower mean concentrations than historical ore with some outliers with elevated concentrations. • Mercury is more elevated in Gladstone ore and is therefore the primary focus area with regard to management of mine tailings. WUG has comparable concentrations to historical ore. • Manganese concentrations in Gladstone and WUG ore are lower than in historical ore. • Mean selenium concentrations in Gladstone and WUG ore are comparable to historical ore.
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