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 23 AECOM 6.1.1 Rock Management Strategy Philosophy for PAF Material The rock management strategy for PAF material is based on current rock management practices adopted on site. The overriding philosophy is: • The rock will likely be geochemically consistent with the materials produced by the current mining operations; • Addition of limestone to PAF material may be required to create a lag in acid generation until rock is encapsulated within a permanent repository; • The refinements to the management practices over the past decades are based on maintaining a pH of above 5.5 until final capping is complete to control the rate of sulphate release; • Testing of the material prior to removal allows the limestone addition to be adjusted as necessary; • Regular (monthly) PAF slurry testing and regular surface limestone application after placement have proven to be effective at managing the material prior to placement of subsequent layers or permanent cover; and, • The current rock management practices are effective at controlling the mine rock materials and preventing acid rock drainage. Gladstone Open Pit Rock Removal The management strategy for PAF and PTEL rock sourced from the GOP component of the project is based on the following philosophy: • Gladstone PAF rock will, where possible, be placed in permanent repositories immediately following removal from the pit (ca. within a 10 week period). This is of primary importance for high sulphur (>3%), low ANC (<2 kg H2SO4 per tonne) Gladstone PAF rock; • PTEL rock has been characterised based on mercury concentrations as other elevated trace elements (arsenic and selenium) are released as a function of sulphide oxidation and are therefore addressed by PAF rock management practices. A block model has been developed to enable management of waste based on mercury concentrations. • Addition of limestone to Gladstone PAF material will be required to create a lag in acid generation until rock is encapsulated within a permanent repository and this will be introduced by layering limestone over compacted lifts of high PAF rock; • Alternatively, if the high sulphur (>3%), low ANC (<2 kg H2SO4 per tonne) component of this Gladstone PAF material can practically be delineated and separated during mining, then immediate placement and compaction of this material with limestone amendment would enable the remaining Gladstone PAF to be managed in the same manner as the PAF from the wider Waihi North Project; • Low capacity PAF (NAG pH<4.5, NAPP<10 kg H2SO4/t) and PTEL (Hg>3.5 mg/kg) material from the GOP can be managed in the same manner as PAF from the wider Waihi North Project; • Regular (monthly) PAF slurry testing and regular surface limestone application after placement have proven to be effective at managing material prior to placement of a permanent cover. This frequency of testing is considered adequate for the more reactive Gladstone PAF rock based kinetic testing. Gladstone Open Pit PAF Rock Backfill For PAF rock being deposited into the GOP, the following philosophy applies: • PAF rock will be placed in lifts within the GOP. Limestone addition will be required to introduce a buffer to acidity from the exposed pit walls; • PAF rock placed in the base of the pit will be placed and compacted in order to limit oxidation. The base of the pit refers to material placed below the Gladstone TSF liner; and,
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