Oceana Gold Waihi North Project Waihi North Project Geochemical Assessment – Geochemistry of Tailings and Overburden, Treatment and Mitigation Revision 9 – 17-Jun-2022 Prepared for – Oceana Gold (New Zealand) Limited – Co No.: 2274246 A-4 AECOM A2. Rock A2.1 Introduction A combination of rock composition analysis, assumptions around rock management, and acid-base accounting is used to determine the likely geochemical effects of rock stockpiling. The degree of sulphide oxidation and corresponding leaching of trace element contaminants is a function of not only rock composition but also the proposed stockpile size, construction, duration of stockpiling, and molar ratio of trace elements to sulphur. The mass of soluble trace elements generated as a result of sulphide oxidation is used to predict likely effects to groundwater on flooding of the mine at closure. Samples of rock from borehole core were selected to be spatially representative of the rock produced with mining. A2.2 Rock Composition Summarised results from the whole rock geochemistry testing (Acid Generating Potential, Major Elements and Trace Elements) are given in Table 31 (proposed Gladstone, WUG), Table 32 (current Martha Mine, MUG, Rex and MP4), and Table 33 (Historical Rock samples from Trio, Favona, Marhta and Correnso). A2.2.1 Sample Selection Samples for trace element testing for the GOP were selected from the following 14 drillholes within the proposed pit void to provide spatially representative sample of the mineralised andesite and hydrothermal breccia (Table 29). The locations of these drillholes are shown in Figure 17. Table 29 Sample Selection Drillhole Sample Number (n) Depth Interval (m) UW540 23 1.6 to 25.9 UW542 53 4 to 56.4 UW549 55 5 to 70.9 UW557 85 18.2 to 110.2 UW558 27 14 to 49.2 UW563 44 7.9 to 164.5 UW566 57 10.1 to 107.5 UW567 67 12.8 to 94.1 UW568 65 56.7 to 156.6 UW569 29 19.7 to 94.1 UW570 27 8.1 to 80.4 UW571 38 5 to 52.9 UW574 58 5.4 to 101.2 UW601 73 6 to 118.5 A2.2.2 Trace Element Digestion Method Two digestion methods were used for trace element analysis; a 2-acid and a 4-acid digestion method. The 2-acid digestion method has been adopted for elements susceptible to volatilisation, such as mercury, as digestion using the 4-acid method can potentially result in under reporting of these elements.
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