South Kalgoorlie

The South Kalgoorlie Operations is located within the Eastern Goldfields Superterrane and the 842sqkm tenement package traverses across the Coolgardie, Ora Banda, Kambalda and Boorara Domains.

History

The Hampton-Boulder-Jubilee (HBJ) and New Celebration deposits which form the core of the present day South Kalgoorlie Operations (SKO) have transacted on multiple occasions over the past 30 years.

HBJ History

The HBJ operation was originally discovered and owned by Hampton Australia. In 1994 Normandy Mining acquired the asset and in 1996 Normandy mining divested the asset to New Hampton Goldfields Ltd. In 2001 Harmony Gold acquired the asset via a takeover of New Hampton Goldfields Ltd.

New Celebration and Combined HBJ History

The new celebration deposit was originally discovered be Newmont Australia Ltd. In 1990 Newmont Australia Ltd acquired Australmin Holdings Ltd and merged with BHP Gold Ltd; the merged entity changed its name to Newcrest Mining. In 2001 Newcrest sold the operation to Hill 50 Gold and in 2002 the consolidation of both operations occurred when Harmony Gold acquired Hill 50 Gold.

Harmony Gold sold the South Kalgoorlie assets to Dioro Exploration in 2007. After a bidding war between Avoca Resources and Ramelius Resources; Avoca gained control of Dioro Exploration in 2010.

In 2011 Avoca Resources merged with Anatolia Minerals Development Ltd to form Alacer Gold. In 2013 Alacer sold their Australian gold assets to Metals X; and in 2016 Metals X demerged their gold assets into Westgold Resources.

In 2018 Westgold Resources sold the South Kalgoorlie assets to Northern Star Resources Limited.

Geology and Mineralisation

The SKO is located within the Eastern Goldfields Superterrane and the 842sqkm tenement package traverses across the Coolgardie, Ora Banda, Kambalda and Boorara Domains. Mineralisation is associated with the greenstone sequence that consists of a mafic to ultramafic volcanic succession that is overlain by an intermediate to felsic volcano-sedimentary sequence.

The structural architecture is dominated by NNW-trending crustal scale shear zones including the Boulder-Lefroy, Zuleika, Abattoir, Boorara, Kunanulling, Binduli and Spargoville structural corridors. These structural corridors are the key influencers of mineralisation in the greater Kalgoorlie district and to date have yielded a mineral endowment of over 100Moz.

Mineralisation styles within the district is a combination of Orogenic and Paleoplacer style deposits. Mineralisation is controlled by Brittle-Ductile shear zones, 1st and 2nd order structures that act as Lithostratigraphic contacts (as fluid conduits) through late stage brittle structures. Mineralisation is also associated with fertile sulphidised intrusive porphyries and layered mafic intrusions with chemically reactive lithologies that provide a rheological contrast.

Mining

The HBJ gold deposit is mined by a top down long hole open stoping (LHOS) method. This mining method was selected to minimise the impact of mining induced stress on the underground operations through the use of geotechnical driven mining sequences at the same time as increasing the extraction rates.

The orebody is accessed through the hanging wall by a central access from which north and south drives are developed.

An initial porphyry ore lode is encountered and developed. Further out in the footwall there is an ore unit sitting against an ultra-mafic contact. Development of the 2401 (ultra-mafic) lodes are delayed as late as possible but just in time for stoping extraction. The priority is placed on developing the western lode followed by a step over to the eastern lode for both the northern and southern extents. Once the stoping panels are mined back towards the main access, development from the access to the north and south can be completed thus completing development. The stoping front retreats from both northern and southern extents towards the access following a top down LHOS method leaving rib pillars. Additional sill pillars are also planned on the eastern lodes where the ultra-mafic contact is present to ensure the safe extraction of the ore whilst minimizing the potential for footwall failure. The dimensions of the stopes vary depending on what lode they are extracting.

On the western lodes, the stopes generally between 25 and 35m long and 17m high. On the eastern lodes, the stopes are smaller to minimize footwall exposure, generally between 15 and 20m long and 12m high. The HBJ gold mine has been extracted down to a depth of 580m below surface. Possible extensions are being investigated both to the north (Mutooroo) and the south (South Jubilee) of the current workings.

Processing

The Jubilee processing plant processes up to 1.2 Mtpa of free milling gold ore at an overall gold recovery of approximately ~90%.

The crushing circuit is three-stage consisting of a primary jaw crusher feeding a 6,000t live capacity coarse ore bin, operating at an average rate of 350 t/h, and a secondary and tertiary crusher circuit closed by a vibrating screen operating at an average rate of 200 t/h. The -8 mm crushing circuit product reports to the 3,500t live capacity fine ore bin.

Ore from the fine ore bin is fed to the grinding circuit via conveyor, quicklime is added to the belt for pH adjustment. The grinding circuit consists of a primary and secondary mill in closed circuit with hydrocyclones. The fine particles (80% less than 0.150 mm) report to the cyclone overflow, which is fed to the leaching circuit. The cyclone cluster is made up of Warman Cavex cyclones, with one cyclone diverted to the gravity circuit. The gravity circuit recovers coarse, free gold and consists of a screen, 30” Knelson Concentrator, and CS500 Acacia reactor. The gravity circuit recovers approximately 35% of the gold recovered by the Jubilee plant.

The leaching and adsorption circuit consists of three leach tanks and eight adsorption tanks. The product from the cyclone overflow is passed over a 0.8 mm aperture vibrating screen and the undersize reports to the first leach tank. Cyanide and oxygen are added to leach the gold into solution. The slurry flows through a splitter box, with each half reporting to a separate adsorption train (adsorption tanks are split into two parallel trains consisting of four tanks per train). Carbon is moved counter-current to the slurry flow and adsorbs the gold from solution. The slurry from the last two adsorption tanks flows over a tail screen prior to discharge to the tailing storage facility.

Loaded carbon is pumped from the circuit in daily batches. Gold is recovered from the carbon in the elution circuit. A split Anglo-American Research Laboratories process is utilised where the gold laden carbon is first acid washed, and then eluted in a column using hot caustic cyanide solution. At the completion of the elution step the barren carbon is sent to the kiln for regeneration prior to being returned to the adsorption circuit. The gold bearing solution is circulated through two electrowinning cells, where the gold in solution is electrowon onto stainless steel mesh cathodes. The gold bearing sludge is removed from the cathodes, smelted in a furnace and poured into gold bars for shipment to the mint.

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