The Jundee-Nimary gold is hosted within a 4km (long) and 2.5km (wide) sequence of Achaean basalts and weakly differentiated dolerite sills separated by sedimentary rocks (carbonaceous shales, cherts, and siltstone) and dacitic porphyries. Younger felsic and intermediate porphyries and dolerite dykes intrude the sequence. The mine sequence is bounded to the west by a thick succession of felsic volcanics and volcaniclastics, and mineralisation diminishes eastwards as the mafic package becomes more magnesium–rich to ultramafic in composition.

The age of the gold mineralisation is approximately 2,700 million years (Archaean), and is controlled by a series of complex brittle fractures. Over 70% of the gold is hosted within the more brittle dolerite units with the remaining 30% associated with the basalts. The field has undergone multiphase deformation – folds, thrusts, strike and oblique slip faults. The gold is hosted within narrow, discontinuous quartz +/- carbonate veins (typically <0.5m wide) which contain abundant visible gold.

Low grade mineralisation occurred with the first phase of faulting. This was followed by the emplacement of large dacitic intrusions which locked up the initial fault lines causing some abutment pressure – during the second faulting phase this abutment pressure prevented the faults from slipping as easily as before causing a “ripping” between the two fault lines fracturing the rock and causing a relatively large low pressure zone. Gold-rich fluids then flowed into these low pressure zones (main mineralization event). There is minor post mineralisation faulting which causes some minor offsets of the loads.

Currently all ore is sourced from Jundee underground operations which yield ~1.0M tonnes ore each year. There are three underground portals which provide access to the various underground lode structures – Barton, Gateway and Invicta portals. The Barton portal contains three actively working ore bodies (Nim3, Lyons and Westside) with Invicta consisting of the Invicta (remnants), Cardassian and Deakin ore bodies, while the Gateway and Gringotts ore bodies are mined from the Gateway portal.

Currently all ore is sourced from Jundee underground operations which yield ~1.0M tonnes ore each year. There are three underground portals which provide access to the various underground lode structures – Barton, Gateway and Invicta portals. The Barton portal contains three actively working ore bodies (Nim3, Lyons and Westside) with Invicta consisting of the Invicta (remnants), Cardassian and Deakin ore bodies, while the Gateway and Gringotts ore bodies are mined from the Gateway portal. The underground mining is being carried out by an up-hole long-hole open stoping sequence as its primary method. No backfill or paste is used.

The Jundee Processing circuit is a conventional CIL plant with a hard rock processing capacity of approximately 1.35M tonnes per year. On the current blend ratio of 1M tonnes of hard underground ore, the annual mill throughput is 1.7M tonnes. The make-up feed is sourced from some 1.2M tonnes of accumulated LG oxide material.

The grinding circuit is preceded by a single toggle overhead eccentric swing jaw crusher (150kw) with a 350 tonne per hour crushing capacity. The jaw crusher feeds a stockpile that holds 25,000 tonnes of crushed ore to ensure steady and controlled grinding circuit feed.

Pre-leach classification is achieved using a hydrocyclone classifier circuit consisting of 22 cyclones that are fed in parallel from a distributor. Only 8 to 12 of the cyclones are operated at any one time depending volume of slurry in the milling circuit. The cyclones are operated at a feed pressure of 110-130kpa.

The Jundee mill feed contains sufficient coarse gold to justify the inclusion of a gravity concentration process in the milling circuit. The cyclone underflow (spigot) product of two to three cyclones is diverted to a gravity circuit that comprises 2 x 30” Knelson concentrators. The concentrate produced by the Knelsons is fed to an In-line Leach Reactor (ILR) that uses a high concentration cyanide/caustic solution to rapidly leach the gold into a solution that is known as a pregnant eluate.

The cyclone overflow reports to the leaching and adsorption circuit where the non-gravity recoverable gold is recovered. The circuit comprises a Carbon in Leach (CIL) circuit with two leach tanks and six adsorption tanks.

The gold bearing eluate produced from the desorption process and the pregnant solution from the ILR is then circulated through electrowinning cells for approximately 20 hours. During this stage gold is electroplated onto cathodes that have been rapped with steel wool. The barren solution remaining at the end of the electrowinning cycle is then returned to the first leach tank to fully utilise the excess cyanide associated with the solution.