5.14.1 Tailings Geochemical Characterisation Geochemical characterisation of the processed tailings to be discharged has been extensively studied by Hydrobiology in the report titled “Mt Morgan Open Cut Pit Water Quality Modelling Stage 2 (March 2025)”, provided in Appendix G. This report was commissioned to model the geochemical properties of the processed tailings as the mineral processing requires use of highly degraded OCP water. The resulting reprocessed tailings (referred to as ‘new tailings in the Hydrobiology report)” will be ultimately discharged into the Sandstone Gully TSF, the OCP and the Upper Mundic Gully TSF (subject to this application being approved). Through tailings reprocessing activities, as outlined in Section 15.5, the acidic properties and elevated metal concentration of latent tailings will be removed. As that the process plant is currently undergoing construction, processed tailings have not been generated or discharged as part of Heritage Minerals mining activities to date. The Hydrobiology report provides a predicted water quality of the new tailings assuming a range of OCP water qualities based on extensive bench testing. The geochemical mixing model predicted an overall improvement in OCP water quality following the addition of new tailings slurry, based on a pH of 9.6 and a 0.61:1 volume ratio (tailings to OCP water). Changes in other anion and cation concentrations were minimal. The 6Mt of tailings authorised for discharge into OCP as per condition W13 of the EA have the same tailings properties for the proposed discharge into the Upper Mundic Gully TSF. Therefore, there will be an improvement in water quality that may report to the Open Cut Pit if a loss of containment event from upper Mundic Gully TSF were to migrate to the OCP. The bench testing indicates that following processing there is a reduction in metals in solids leached by the cyanide. Although these metals were leached from the solids, they were absorbed onto the resin to be subsequently recovered, resulting in a lower final solution of metals in the tailings stream which is referred to as “new tailings” in the reporting. As the bench testing involved leaching of solids, most results are reported in parts per million or mg/L which limits comparison to some soil guidelines when determining potential rehabilitation strategies. The geochemical properties of the new tailings are reported in Table 33 and have been compared to the National Environment Protection (Assessment of Site Contamination) Measure (NEPM) Health Impact Levels (HIL) for Commercial/Industrial settings. The determination of impact to ecological values can not be conducted at this stage of the project as under the NEPM framework determining Ecological Investigation Levels (EIL) depends on specific soil physio chemical properties and land use scenarios and generally apply to the top 2m of soil. The NEPM framework only provides management levels for metals and doesn’t provide analytical for physical parameters that have no known human and ecological toxicity including pH, electrical conductivity, major ions and cations. The results of the geochemical modelling provided in Table 33 indicate that the final tailings solution analytes do not exceed any of the HILs and it is concluded that any leaching from the tailings material is benign to human health. When tailings are appropriately stored in a well maintained TSF subject to regular inspections and monitoring, the tailings materials are generally considered benign to ecological receptors. This proposed amendment provides commitments for the installation of a groundwater monitoring array around the Upper Mundic Gully TSF to assess potential groundwater impacts. As the project progresses and deposition into the relevant TSFs occur, ongoing monitoring of the tailings quality will be undertaken to better inform rehabilitation and closure strategies. It is only following cessation of tailings deposition that more detailed soil studies can be conducted to obtain both physical and chemical soil parameters to confirm rehabilitation and closure strategies.
Project number: 25B061
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