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.

The Prominent Hill Underground Mineral Resources are being mined successfully using sub-level open stoping (SLOS) with cemented fill (paste or CHF) for most stopes, with rock fill for selected secondary/tertiary stopes and a minimum mining width of five metres.

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 feed blend. The current life of mine schedule has the plant running at that capacity until May 2020 when open pit copper ore stocks are depleted.

From then until 2023, throughput will be approximately nine to ten million tonnes per annum with a high proportion of stockpiled open pit gold ore. On exhaustion of gold ore stocks the plant will be fed with underground ore alone.

Plant turndown studies indicate that the plant can be configured to run at two to four 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.