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.

Alteration and sulphide mineral assemblage suggest the composition of the ore fluid was similar to other orogenic gold deposits.

The Jundee operations currently consists of underground and open pit feed sources and a processing plant.

Three underground portals are in use which provide access to the various underground lode structures, namely the Barton, Gateway and Invicta portals. The Barton portal contains three active ore bodies (Nim3, Lyons and Westside) with development access well advanced in the Armada lode, whilst Invicta consists of the Invicta (remnants), Cardassian, Wilson and Deakin ore bodies. Gateway and Gringotts ore bodies are both mined via the Gateway portal, again with development access well advanced in the Revelation and Nexus lodes. Recent exploration has uncovered a new prospective mining area at depth (Zodiac).

The mining process is a geological driven model and production intensive due to the narrow vein nature and lithology of the ore bodies. There is a necessity for controlled mining practices to minimise the amount of waste dilution incurred during the ore extraction process. In general, the ore is mined using top-down long-hole (uphole) open stope bench retreat mining. The benches are nominally 20m apart (vertically). The lode structures have a dip of between 35-70 degrees and have multiple orientations. The mining method typically extracts the ore in 30-50m panels separated by 5-10m rib pillars which remain in-situ to provide geo-mechanical stability and support. For ore structures that dip between 25.0 degrees and 35.0 degrees the ore is extracted using manual mining techniques (air-leg mining) to optimise recovery and reduce costs. Ore is hauled to the surface from the underground stope and development areas by 55 tonne articulated dump trucks.

The lode domains across the operation are geographically expansive, resulting in a complex ventilation system that includes capacity limiting cascading ventilation in lower reaches of the mines. Each of the three mines has distinct characteristics that provide challenges for ventilation. The Barton mine has four primary return air rises (RAR) each equipped with a 350kW axial fan. The Invicta mine presently has two RAR, each with a 350kW fan drawing air directly from the Invicta, Deakin and Cardassian ore bodies. At Gateway, two 450kW exhaust fans are installed underground and two large internal rises, one for return air (exhaust) and one for intake air (fresh) have been completed to extend the primary ventilation. During the warmer periods of the year, the mine utilises 3 refrigeration units to provide cooled air to the deeper extents of the mine. These units can be moved to alternative locations to maximise its impact on active work areas. Current active ventilation projects include the addition of 3 primary fan installations, with variable speed drives, across the Barton, Deakin and Gateway mines.

The underground mining and fleet maintenance is carried out by the contract underground mining company Byrnecut Australia (BAPL). Daily mine management is shared by BAPL and Northern Star personnel who form an integrated management structure to ensure that functional duplication is eliminated. BAPL are also responsible for the supply of surface mining equipment which facilitates secondary haulage, road maintenance, mill crusher feed and waste rock management operations.

All mine design, blast design, production geology, survey, geotechnical and mine planning is undertaken by Northern Star personnel.

The crushing circuit is two-stage consisting of a C125 jaw crusher and HP6 secondary crusher closed by a double deck banana screen with a nominal 400 tonne per hour crushing capacity. The crushing circuit can operate in either full secondary crush or partial secondary crush modes to suit milling requirements. The crushing circuit feeds a reclaim stockpile with a 25,000 tonne capacity. Three variable speed belt feeders below the reclaim feeders ensure steady and controlled grinding circuit feed.

The grinding circuit comprises of two mills. The 5.5m diameter x 8.2m EGL ANI SAG mill is low aspect with a 3,850 kilowatt motor. The SAG mill normally operates in open circuit, but has the capability to be operated in closed circuit SAG only configuration. The 4.3m diameter x 6.7m EGL Nordberg ball mill fitted with a 2,20 0kw motor operates in closed circuit with hydrocyclones. The grinding circuit throughput is greater than 220 dry tonnes per operating hour on 100% blend of underground ore. Oversize material from the SAG mill trommel screen reports to a pebble crusher for size reduction. The HP100 pebble crusher is driven by a 90kW motor. The crusher typically operates with a closed side setting of 14mm. The crushed product from the pebble crusher is fed back into the SAG mill feed.

Classification is achieved using a cyclone cluster consisting of 22 cyclones. Eight to 15 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 to achieve a leach feed sizing target of 80% passing 106 microns.

4.5MW ball mill installation commissioned in May 2020.

The Jundee processing circuit is a conventional CIL plant with a hard rock processing capacity of approximately 3.0M tonnes per year.

The Jundee mill feed contains sufficient coarse gold to support the inclusion of a gravity concentration process in the milling circuit. The cyclone underflow product of up to four cyclones is diverted to a gravity circuit that comprises 2 x 30” Knelson concentrators. Approximately 40% of gold in the mill feed is recovered via the gravity circuit. The concentrate produced by the Knelson’s is fed to an in-line leach reactor (ILR) that uses a high concentration cyanide-caustic solution to rapidly leach the gold. Gold is recovered from this solution by means of an electro-winning process in the gold room.

The cyclone overflow slurry reports to the carbon-in-leach circuit for the processing of non-gravity recoverable gold. The circuit comprises two leach tanks and six adsorption tanks where the gold is leached into a solution and adsor ........