Gold and copper mineralization at Telfer consists of stratiform reefs and stockworks hosted by sedimentary rocks of the Malu Formation of the Lamil Group. The Lamil Group comprises relatively weakly deformed and metamorphosed Proterozoic sediment units northeast of the Camel-Tabletop Fault. The important attributes of the Lamil Group are the presence of abundant carbonate units, and weakly developed penetrative deformation.

The topography at the Telfer mine site is dominated by two large scale asymmetric dome structures with steep west dipping axial planes. Main Dome is located in the southeast portion of the mine and is exposed over a strike distance of 3km north-south and 2km east-west before plunging under transported cover. West Dome forms the topographical high in the northwest quadrant of the mine and has similar dimensions to Main Dome. Both fold structures have shallow to moderately dipping western limbs and moderate to steep dipping eastern limbs.

Mineral Resources reported for the Telfer mining centre consist of:
• open pit stockwork and reef mineralization in Telfer Main Dome and West Dome;
• stockwork and reef mineralization mined underground in the Telfer Underground SLC mining operation;
• stockwork mineralization in the VSC below the SLC.

Mineralization within the Telfer deposit is controlled by structure and lithology. Several styles of mineralization were recognized, namely narrow high-grade reefs, pod-like mineralized bodies, sheeted vein-sets and large areas of low grade stockwork mineralization, with the latter forming the majority of the sulphide resource. The primary mineralization was overprinted by surface weathering processes.

The sulphide mineralization is characterized by fresh sulphides, predominantly pyrite and chalcopyrite. The main copper minerals listed in order of occurrence are chalcopyrite, chalcocite and bornite with minor cobaltite and nickel-sulphide. Primary gold generally occurs as free grains, on sulphide boundaries and to a minor degree with silica grains.

Primary gold mineralization is typically associated with pyrite/chalcopyrite sulphides and quartz/dolomite gangue. There is a correlation between vein frequency and gold grade.

Weathering locally modified the mineralization, to depths typically up to 200m, with the boundary between oxide and primary mineralization being irregular. The weathering profile produced local areas of supergene enrichment where the copper to sulphur ratio is characteristically higher. Supergene minerals include gold with limonite/goethite, malachite and chrysocolla in the depleted oxide zone, giving way to chalcocite, pyrite, digenite, covellite, tenorite and cuprite at depth in primary mineralization. The copper to sulphur ratio decreases with depth below the supergene zones. Copper is, in general, more mobile in these supergene area zones than gold. The depth of oxidation is closely related to the structural framework and typically exists to approximately 200m below surface, although local areas show oxidation down faults in excess of 500m below surface.

The highest concentration of gold and copper grades occurs within bedding sub-parallel reef systems. In Main Dome a total of 21 reef structures were identified from drill hole data or mapping of surface and underground exposures within the Open Pit Mineral Resource, and include 10 E-Reefs within the Outer Siltstones, the MVR within the Middle Vale Siltstone and the M10 to M50 series of reefs within the Malu Formation. The primary characteristics of the reef systems are:
• broadly concordant to lithological boundaries;
• laterally extensive (greater than 1km) both along strike and dip;
• true thickness of 0.1m to 1.2m, averaging between 0.3m and 0.7m;
• high relative nugget effect;
• variable dip varying from flat at the crest of Main Dome to about 40º on the eastern flank of Main Dome;
• gold grade is typically high but variable: 5 g/t Au to 50 g/t Au;
• copper grade ranges between 0.2% Cu and 1.5% Cu.

Stockwork mineralization is characterized by narrow, often discontinuous veins that crosscut stratigraphy. Large domains of stockwork mineralization were defined in the open pits and also within the Telfer Deeps and Vertical Stockwork Corridor Mineral Resources. Stockwork mineralization is best developed in the axial zones of Main Dome and West Dome and is discordant to lithological boundaries, although some stratigraphic units have more abundant stockworks than others and vein intensity within stockwork can be greater adjacent to reefs. Stockworks are laterally extensive, between 0.1km to 1.5km scale and the geometry of the stockwork zones is related to structure and stratigraphy.

Stockwork mineralization can also include areas of breccia dominated by quartz, carbonate and sulphides.

The Telfer operation is comprised of open pit mining at both Main Dome and West Dome and underground mining at Main Dome. Open pit mining is a conventional truck and hydraulic excavator operation. Underground selective and bulk long hole open stope mining methods are used for excavation of the high-grade reefs and Western Flanks respectively, while stockwork ore and waste are mined using sub level cave bulk mining methods. Underground sub level cave mining ore and Western Flanks bulk open stope ore is hoisted to the surface via a shaft.

Open pit mining has continued at both Main Dome and West Dome open pits (including stockpile reclaim). Since 31 December 2019, the Mineral Resources and Ore Reserves were depleted by 0.2 million ounces of gold and 0.01 million tonnes of copper. Mining of Ore Reserves in the Main Dome open pit was completed in the fourth quarter of FY20. Further studies are currently evaluating potential for additional Ore Reserves in Main Dome open pit.

The Telfer Underground comprises the sub level cave mine and selective high-grade reef mining and Western Flanks reef and stockwork mining. Since 31 December 2019, both the Mineral Resources and Ore Reserves have been depleted by 0.05 million ounces of gold and <0.01 million tonnes of copper. Underground Ore Reserves for the sub level cave were completed. Further studies are currently evaluating the potential for additional Ore Reserves at the sub level cave and other underground areas to extend the underground mine life.

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The Telfer concentrator comprises a dual train comminution circuit followed by flotation and a carbon-in-leach (CIL) circuit. Both streams contain a gravity gold recovery circuit. Approximately 40% of the gold at Telfer is produced as doré which is smelted on site. Following the gravity recovery circuit ore with: gravity recovery circuit ore with:

-A relatively low pyrite content is treated in a conventional single stage flotation circuit to produce gold copper concentrate;
-A higher pyrite content is treated via a sequential flotation process. The first stage is a conventional copper flotation with depression of pyrite to produce a gold-copper concentrate. Tails from the first stage are refloated to produce a pyrite-gold concentrate which is leached with cyanide in a conventional CIL circuit to recover the remaining gold as doré.

The gold-copper concentrate is trucked to Port Hedland for shipping to smelters, primarily in the East Asia region.