4.3.12. 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).
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 29 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 29 indicate that the final tailings solution
analytes do no 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.
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Wulguru Technical Services Pty Ltd – Supporting Information to Amend an Environmental Authority
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