The alkalic porphyry gold–copper deposits of the Cadia district are located in the eastern Lachlan Fold Belt of New South Wales. The district comprises four porphyry deposits, Ridgeway, Cadia Extended (Cadia Quarry), Cadia Hill and Cadia East, and two related iron-skarn deposits, Big Cadia and Little Cadia.

Six mineralisation styles are known in the district:
• Cadia Hill: Intrusion and volcanic-hosted, sheeted quartz vein mineralisation;
• Cadia East: Volcanic-hosted, disseminated and sheeted quartz vein mineralisation;
• Cadia Far East: Volcanic and intrusion hosted mainly sheeted quartz vein mineralisation;
• Cadia Quarry: intrusion hosted mainly sheeted quartz vein mineralisation;
• Ridgeway: intrusion and volcanic hosted quartz stockwork vein mineralisation;
• Big Cadia, Little Cadia: iron-rich skarns.

Mineralisation in the porphyry deposits occurs as sheeted and stockwork quartz–sulphide veins, and locally as broadly stratabound disseminated mineralisation (Cadia East) and skarn (Big Cadia and Little Cadia).

The Cadia district porphyry deposits have recorded a sequence of alteration and mineralisation events that evolved from early-stage magnetite-stable sodic, potassic and calc-potassic alteration with locally significant gold–copper mineralisation, through a period of transitional stage potassic alteration that introduced most of the gold–copper mineralisation. Propylitic and calc-silicate alteration were developed in the deposit peripheries at this time and a late stage of feldspathic alteration developed irregularly around the deposit margins and locally destroyed mineralisation.

All of the porphyry deposits show a close spatial association with shoshonitic monzodiorite to quartz monzonite dykes and stocks of the CIC. Gold–copper mineralisation is hosted by these intrusions and also by the enclosing FRV wall rocks. Field evidence (e.g., cross-cutting intrusive and vein relationships, vein dykes, intermineral comb quartz layers) strongly supports the hypothesis of deposit formation at the same time as the emplacement of the intrusive rocks that host mineralisation.

Wilson (2003) divided the Cadia porphyry deposits into two types:

• Intrusive wall rock deposits. Monzonitic intrusions in these deposits were interpreted to be country rock, upon which porphyry-style mineralisation was superimposed (e.g., Cadia Quarry and Cadia Hill). These deposits display no field evidence for a temporal relationship between intrusion and mineralisation;
• Intrusive-centred deposits. The intrusions in this deposit class display textural evidence to indicate the existence of a temporal and genetic link between the monzonitic intrusive complexes and hydrothermal alteration and mineralisation (e.g., Ridgeway, Cadia Far East).

The two types have distinctive alteration and mineralisation characteristics, but share a number of paragenetic features.

Cadia East is accessed via two declines, the main access decline, and the conveyor decline.

Panel Cave mining from Cadia East (Panel Cave 1 and 2), with underground crushing and conveyor to surface.

The mining method involves inducing caving of the rock mass by undercutting a block of ore. Mining proceeds by progressively advancing an “undercut” level beneath the block of ore. Above the undercut level, the overlying host rocks are pre-conditioned using blasting and/or hydraulic fracturing, resulting in controlled fracturing of the ore block.

Following pre-conditioning of the overlying host rocks, broken ore is removed through an extraction level developed below the undercut level. The extraction level is connected to the undercut level by drawbells, through which the ore gravitates to drawpoints on the extraction level. The ore is removed by a load–haul–dump (LHD) fleet to underground crushing stations.

At each crushing station, ore is tipped into a coarse ore bin, which then feeds the crusher itself which passes material to a surge bin used to regulate the feed from the crushing station onto the collection conveyors. The collection conveyors are in turn used to regulate feed onto the main trunk belt system and to allow for the automated removal of tramp metals.

The main trunk belt is used to transport ore to the surface at a rate of approximately 4,600 t/h (with work underway to upgrade this to 5,150 t/h). The incline conveyor commences at 4,400 mRL (i.e. the base of Lift 2), extends approximately 7,500 m to the surface and is deposited onto the concentrator coarse ore stockpile where it is gravity fed into the ore processing system.

Waste rock is removed from the underground workings via the decline and is hauled to the South Waste Rock Facility. Fresh air enters the underground workings via the main and conveyor declines and six ventilation intake shafts (VR4, VR6, VR10, VR12 with plans to construct a further system, VR14) A total flow intake of approximately 1,500 m3/s is installed with plans during expansion to raise this to 2,200 m3/s of fresh air to maintain underground air quality.

Air is expelled from the workings via four vertical shafts and exhaust fan installations (VR3A, VR5, VR7, VR8, with plans to construct VR11 and VR15 during the Cadia expansion project).

Blasting consists of development blasting and production blasting to precondition the ore. Emulsion explosives are typically used for blasting purposes. Ammonium nitrate fuel oil (ANFO) may be used on occasions if emulsion charging is not available. Hydraulic fracturing is used to augment the caving process, and is conducted from the 5050 mRL level.

Groundwater that accumulates in the underground mine workings is collected, and then pumped to the surface at a maximum rate of about 160 L/s.

Underground facilities include workshops, wash bays, fuel bays, offices, and crib rooms. Underground workshops are used to maintain the development and production fleet. The Cadia East mine is supplied by a dedicated 132 kV transmission line feed which in turn feeds into the site switchyard. Two 33 kV feeders run from the surface substation to provide a ring main to the underground workings.

Ridgeway is a vertical porphyry copper/gold deposit located within the Cadia Valley and approximately 5 km from the ore treatment facility and adjacent to the Cadia Hill deposit. The upper portion of the deposit down to 5040 Level (approximately 800 m below surface) has been mined using SLC methods, resulting in a column of caved material that extends to the surface to form a subsidence zone. An underground crusher was installed at the base of the SLC area and crushed ore was conveyed out of the mine via an inclined conveyor system. SLC mining is now complete.

The Ridgeway Deeps Lift 1 block cave operation was mined between 2007 and 2015 and was Newcrest’s first block cave operation. The change to block caving was introduced after the identification that the grade profile for Ridgeway was declining to the point where subsequent SLC levels below the 5040 mRL were uneconomic. It was also recognised that experience with techniques and methods of cave establishment were required for the then future Cadia East operations which were significantly larger in scale. As a result of extensive reviews and study it was proposed that a 5.6 Mt/a block cave mine be established 250 m downdip of the base of the SLC at the 4786 mRL. Subsequent to establishment and ramp-up, the mine was debottlenecked to the point of achieving a total of 9.6 Mt/a. A total of 17 Mt grading 0.57g/t Au and 0.29% Cu remains in the Lift 1 level.

The majority of the Mineral Reserves for Ridgeway are located in the Ridgeway Deeps Lift 2 block cave. This cave is similar in nature to the original Lift 1 cave with a similar downdip extension depth, similar layout and establishment method, and operational targets for 8 Mt/a.

Concentrator 1:

• Primary crushing in a 60-inch x 113-inch gyratory crusher. This was the main primary crusher for the original Cadia concentrator and now primarily used for surface stockpile reclaim;
• Coarse ore stockpile (COS) to a live capacity of 40,000 t and subsequent reclaim facilities;
• Distribution bin and double-deck vibrating screens for feed size preparation;
• Secondary crushing, treating screen oversize, using MP1000 cone crushers;
• High pressure grinding rolls (HPGR) unit treating screen undersize prior to the SAG mill;
• Primary open circuit milling in a single 40-ft SAG mill fitted with a 20 MW motor. The motor will be upgraded to 22 MW. Oversize pebbles from the SAG mill are returned to the distribution bin;
• Secondary milling in three ball mills, with the two original mills fitted with 8.7 MW motors, and a more recently installed third ball mill with a 16 MW motor. The mills are in closed circuit with cyclone classification for a target grind size of P80 of 150–190 µm.

Concentrator 2:

• Reclaim and conveying system to transfer ore from Concentrator 1 COS to Concentrator 2 facilities;
• Secondary and tertiary crushing using MP800 cone crushers and open circuit prescreening;
• Dedicated coarse (crushed) ore stockpile (COS) and apron feeder reclaim facilities;
• Primary open circuit milling in a single SAG mill fitted with a 7.5 MW motor;
• Secondary milling using a 7.5 MW ball mill and parallel Vertimill VTM1250, in closed circuit with classifying cyclones.
• Tertiary milling using a Vertimill VTM 3000.

Concentrator 1 was commissioned in 1998, designed for Cadia Hill ore and had a design capacity of 17 Mt/a. In 2012, Concentrator 1 was upgraded for the processing of harder Cadia East ore which included the addition of a HPGR circuit, ahead of the SAG Mill, and a third ball mill and third flotation train. Concentrator 1 has a current throughput of about 23 Mt/a.

Concentrator 2 was commissioned in 2002, and had a target rate of 4 Mt/a, and currently treats approximately 7 Mt/a. In mid-2008, the facilities were upgraded to suit predictions of harder and fines-deficient ore from Ridgeway Deeps block cave mine. The upgrade included installation of a secondary crushing circuit and additional regrind mill power. A 2.24 MW Vertimill was installed in 2011 to reduce flotation feed size and improve metal recoveries

Concentrator 1 has historically been referred to as the low-grade process facility, or the Cadia concentrator. Concentrator 2 has historically been referred to as the high-grade process facility, or the Ridgeway concentrator.

Concentrator 1 Design The key unit operations in the processing plant are:

-Flash flotation and gravity processing of flash flotation concentrate to produce a gravity gold concentrate for smelting;
-Rougher and scavenger flotation of the slurry from the milling circuit with concentrate reporting to regrind circuits utilising Vertimills;
-Cleaner and cleaner-scavenger flotation;
-Recleaner flotation utilising both conventional and Jameson cell technology. The Jameson cells assist in fluorine rejection;
-Two HydroFloat cells processing train 3 scavenger tailings for coarse composite mineral recovery; -Thickening of rougher/scavenger tailings before pumping to a TSF.

Concentrator 2 includes the following processes:

-Gravity concentration of ground slurry in a centrifugal concentrator to produce a gravity gold concentrate for further upgrade and smelting;
-Flash flotation in the milling circuit to recover an additional concentrate suitable for further gravity gold recovery, with the remaining copper concentrate reporting to the flotation cleaning circuit;
-Rougher and scavenger flotation in tank flotation cells. Rougher and scavenger concentrates report to cleaning flotation via a regrind circuit using a Vertimill VTM 1250;
-Thickening of rougher/scavenger tailings before pumping to a TSF;
-Cleaner and cleaner–scavenger flotation;
-Recleaner flotation using both conventional and Jameson cells and subsequent thickening of final gold–copper concentrate.

The final copper concentrate slurry with a grade of around 25% copper is pumped to the town of Blayney where it is filtered and exported using the same filtration facilities as used by Concentrator 1.

Approximately 15% of the gold in feed ore is recovered as gravity concentrate and then smelted on site to produce gold doré for sale.