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-1 AECOM When compared to historical rock trace element data, the proposed GOP rock is generally depressed in trace element concentrations with the exception of antimony, arsenic, and mercury, which are elevated. Although these elements are elevated, geochemical controls such as co-precipitation and complexion, along with the current on-site treatment facilities will control the trace element concentrations from the mine area to within the current operating limits for treated water. It is predicted that elevated trace element concentrations in mine waters arising from elevated concentrations within the rock material (with respect to historical mining areas) will not impact the site’s ability to meet the existing discharge consent conditions. Of note is mercury, which has largely been recorded below the laboratory method detection limit in site mine waters and shows an order of magnitude increase within the rock material. As such, mercury leaching potential is further assessed through kinetic leaching tests to determine the likely resultant levels that will be present in the proposed mine waters. A2.2.3.1 Distribution of Trace elements in the Rock Material Elevated trace element concentrations (antimony, arsenic and mercury) in the rock (and ore body) relative to historically mined areas need to be assessed for potential implications for consent compliance at the point of discharge (both to water and to air). An assessment of the distribution of trace elements within the rock shows that mercury is elevated in the highly clay altered Andesitic material located near the surface and associated with the Breccia material. Cross sections of the GOP profile with associated drill holes and cores showing categorised mercury concentrations are presented in Figure 24. From the figure, a pattern of decreasing mercury concentration with depth can be observed with the higher mercury concentrations (red > 10 ppm, orange 5-10 ppm) focused on the Breccia rock material or within the Andesitic material bordering the Breccia material. Core with a mercury concentration of < 1 ppm (signified as blue) is typically located within the deeper Andesitic material. This pattern is evident through the pit profiles and the distributions are consistent with those previously identified in the area (Browne, 2008). Relatively low concentrations of other trace elements are noted in areas of elevated mercury. There is also no positive or negative correlation between mercury with sulphur although the highest mean mercury concentrations are found in breccia samples which tend to coincide with the lowest mean sulphur concentrations. Both these factors may be due to the high volatility of elemental mercury and the capability of more soluble mercury forms being more distal to the ore body, especially through postmineralisation cover. This can result in the secondary mercury halo around the deposits varying significantly in size (Browne, 2008).
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