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 7 AECOM 4.0 Rock 4.1 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 and rock placement, duration of stockpiling, and trace element ratio to sulphur content. 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 GOP and WUG mine at closure. Samples of rock from borehole core were selected to be spatially representative of the rock produced with mining of the respective ore bodies. 4.2 Rock Composition Summarised results from the whole rock geochemistry testing for Acid Generating Potential, Major Elements and Trace Elements are presented as follows:- Table 1 (proposed GOP), • Table 2 (Martha Mine), • Table 3 (Historical Rock samples from Trio, Favona, Martha and Correnso). Appendix A outlines the statistical analysis of rock geochemistry datasets for the proposed GOP. Appendix A also includes the rock geochemistry results as follows: • Appendix A, Table 31 (proposed GOP), • Appendix A, Table 32 (Martha Pit), • Appendix A, Table 33 (Historical Rock samples from Trio, Favona, Martha and Correnso) Appendix B outlines the statistical analysis of rock geochemistry datasets for the proposed WUG. 4.2.1 Distribution of Trace elements in the Rock Material Figure 3 to Figure 7 show the relative distribution of trace elements in rock that have been identified as elevated based on the analysis in Appendix A and B and summarised in Table 1. Gladstone Open Pit 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, cadmium, and mercury, which are elevated. The Geochemical Abundance Index (GAI) as defined in the GARD guide (INAP, 2018), indicates that cadmium has a GAI of 1 and as such does not warrant further examination as it is not significantly elevated. Although antimony, arsenic, cadmium, and mercury 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 Gladstone rock. 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.
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