The Prominent Hill iron-oxide copper gold (IOCG) deposit is located in the north-eastern portion of the Archaean to Mesoproterozoic Gawler Craton, South Australia. Most of these deposits are thought to be genetically related to the Gawler Range Volcanic (GRV) – Hiltaba magmatic event which affected the central and eastern portions of the Gawler Craton around 1600-1580 million years ago. Copper-gold-silver mineralisation at Prominent Hill is mostly hosted within hematite-matrix breccia containing fragments of sandstone, siltstone, dolostone, and mafic to intermediate volcanic rocks. Copper mineralisation occurs as disseminations of chalcocite, bornite and chalcopyrite in the matrix of the breccia.

The current maximum extent of the reported Mineral Resource is 2,700m (east-west) by 1,300m (vertical). Multiple lenses exist within a mineralised zone having a plan width (across strike) of approximately 300-400m. Only a subset of this mineralised zone has sufficient continuity of grade to have been reported as a Mineral Resource. The upper and lower limits of the reported Mineral Resource are 123m and 1,399m respectively below the pre-mining topographic surface.

Underground Mineral Resources are constrained within the limits of copper and gold mineralisation domain wireframes. Final definition also ensures that reported mineralisation demonstrates adequate size and continuity to support the selected mining method. This process does result in some internal dilutionary material below the specified cut-off grade being included within the reported Mineral Resources.

The assumed mining method for the estimated Mineral Resource is sub-level open stoping (SLOS) with cemented fill and a minimum mining width of five metres. The Prominent Hill Underground Mineral Resource is being mined successfully using SLOS. Some remnant skins and pillars near mined-out stopes have been excluded from the reported Mineral Resource where economic extraction is considered unlikely, such as where the remaining mineralised material is thinner than the minimum mining width.

Geotechnical assumptions are based on work completed by Beck Engineering (BE), and confirmatory work by OZ Minerals engineering and geotechnical personnel based on observations made during mining.

Only stopes containing more than 60 percent combined Measured and Indicated Resources were included in the Ore Reserve estimate. A minor amount of Inferred Resource existing within predominantly Measured and Indicated stopes has been included within the Ore Reserve estimate. The minimal value contributed by Inferred Resource is not material to the Ore Reserve Estimate and comprises approximately 0.31% and 0.39% of the copper and gold metal respectively estimated in the Ore Reserve.

Unclassified material within stope shapes is treated as waste of zero grade in the Ore Reserve estimate.

To validate modifying factors, on completion of each stope and a routine part of the site reconciliation process, the mill production from the stope is compared to the estimate from mining and to the Ore Reserve estimate.

Twenty six stopes were completed in the 2020 Ore Reserve reporting period in the Ankata orebody. These stopes performed generally in line with expectations, a minor increase in ore mined from secondary stopes during the period saw a small decrease in fill dilution however total dilution remain unchanged.

The Prominent Hill processing plant has been operating since February 2009 and comprises a conventional crushing, grinding and flotation circuit to recover copper, gold and silver to produce a high-quality concentrate. The plant can process approximately ten million tonnes per annum subject to the ore blend.

The current life of mine schedule has the plant running at that capacity approximately eight to nine million tonnes per annum until the end of quarter one 2025 with a high proportion of stockpiled open pit gold and lower grade ore. On exhaustion of these gold and marginal ore stocks the plant will be fed with underground ore alone.

Optimisation studies indicate that the plant can be configured to run at four to six million tonnes per annum dependent on the blend of copper and gold ores for minimal capital expenditure. If necessary, lower throughputs can be processed in batches providing the ability to process ore at a range of underground production rates.