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.

The Cadia East–Far East deposit occupies a mineralised zone 2.5 km in strike length, 600 m in width and over 1,900 m in vertical extent. It is located below and to the east of the Cadia Hill deposit.

Mineralisation is developed in the Forest Reef Volcanics (FRV), and in a series of subvertical to steeply northdipping monzodioritic to quartz monzonitic dykes, that are termed the Cadia Far East intrusive complex (CFEIC). The syn-mineral nature of at least some of the intrusions is indicated by the presence of mineralised xenoliths within monzonite porphyry dykes that also host porphyry- style veining and alteration.

The Weemalla Formation has been intersected at depth, and consists of finely-bedded siltstone interbedded with basaltic volcanic rocks. Overlying this unit are five lithofacies of the FRV:
• Upper bedded unit: about 80 m thickness of finely planar-laminated feldspathic siltstone;
• Volcaniclastic unit: approximately 200 m thickness of sandy matrix polymictic conglomerate and volcaniclastic sandstones and locally volcanic breccia;
• Lower bedded unit: around 60 m thickness of bedded calcareous sandstone typically altered to skarn mineral assemblages;
• Massive volcanic rocks: about 150 m thickness of massive pyroxene phyric basalt to andesite lavas;
• Lower sequence: at least 1,100 m thickness of polymictic conglomerates and volcaniclastic sandstones.

Intrusive porphyry dykes and sills are interpreted to be co-eval with the FRV volcanic units. In the Cadia East area, the 5–30 m thick porphyry dykes appear to be stratigraphically controlled by the bedded units, and acted as feeders to overlying sills. The largest dyke has been traced for 1,500 m along strike, are coincident with a change in shape of the orebody on section 15570E, and are cross-cut by mineralised veins. Two large porphyry sills located above the lower bedded unit can be traced along the upper portion of Cadia East. Numerous smaller sills and dykes also exist in this area. The uppermost of the units termed the capping porphyry and is thickest (~70 m) in the middle of the deposit.

Mineralisation at Cadia East is divided into two broad overlapping zones: an upper, copper-rich disseminated zone and a deeper gold-rich zone associated with sheeted veins. The upper zone forms a relatively small cap to the overall mineralised envelope and has a core of disseminated chalcopyrite (and rare bornite), capped by chalcopyrite–pyrite mineralisation (Fox et al., 2009).

The deeper zone is localised around a core of steeply-dipping, sheeted, quartz–calcite–bornite– chalcopyrite–molybdenite veins, with the highest gold grades associated with the bornite-bearing veins. Copper and molybdenite form a mineralised blanket above and to the east of the higher- grade gold envelope.

Au:Cu values are vertically zoned. The upper, disseminated zone of volcanic-hosted mineralisation typically has low Au:Cu values (<1), whereas the envelopes of sheeted quartz-calcite-sulphide veins have higher Au:Cu values (typically >2).

The current operations are planned as a series of three lifts (Lifts 1, 2, and 3). The relative elevation of these lifts and all underground infrastructure is expressed in mine height datum which is 5,000 m above AHD (i.e. 5,900 m mine Relative Level (RL) is equivalent to 900 m AHD). Lifts 1 and 2 are approximately 1,200–1,400 m high with their bases located at approximately 4650 mRL and 4450 mRL. Lift 3 sits below Lift 2 with a block height of 275 m and a base at the 4,175 mRL. Lift 1 refers to the following panel caves: PC1–1, PC1–2, PC1–4, PC5001. Lift 2 refers to the following panel caves: PC2, PC2–3, PC2–4 and PC2–5. Lift 3 refers to the following panel cave: PC 3–1.

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

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 transports 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.

Cave initiation will commence adjacent to existing caves for operations on the Lift 1 and Lift 2 levels. Cave 3 will be initiated under the existing Lift 2 caves. Hydraulic fracturing activities will be conducted in two main functional areas, the orebody and infrastructure areas.

Hydrogeological reviews indicated that estimated groundwater inflows to the Cadia East mine show a rising trend in the inflow over time from 0.3 ML/day to about 1.2 ML/day. Discharge of groundwater in the field will occur in two main areas, baseflow into creeks and into mining voids. The mine is currently pump dewatered. There is no discharge of mine dewatering to the environment, with water reused in processing facilities or recycled into the underground operations.

An El Teniente layout will be used for the extraction level. A number of undercutting processes are planned for Cadia East, including post undercut, and W-cut advanced undercut with apex drive. A monitoring and cave engineering horizon was designed for the 5050 mRL. Additional intake and exhaust ventilation requirements are incorporated in the mine design to accommodate an annual production rate of 33–35 Mt/a.

Infrastructure required to support each cave will include primary crushers, four-way tipple arrangements, ROM and crushed ore bins, and conveyor systems.

Equipment requirements include primary development, cave development, and production equipment. A secondary production fleet will support this equipment. These equipment types are conventional to panel cave mining operations.

Maintenance workshop facilities, refuelling station, crib rooms, and offices will support the underground operations. The existing 33 kV and 11 kV electrical distribution systems will be extended to supply power to the operating caves.

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.

The key unit operations in the processing plant are:

• 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.

Processing of the Cadia East underground ore stream will be through Cadia Valley Operations Ore Treatment Plant concentrators 1 & 2. Metal recovery is through gravity and conventional flotation to a Copper/Gold concentrate. This circuit currently processes Cadia East Material with similarly styled material to future ore sources. Cadia East is the sole source of feed for both Concentrator 1 and Concentrator 2. Production of up to 33mtpa is anticipated to be produced through the concentrators. While the scale of processing will position the operation among the world’s largest gold mines, the technology associated with the ore processing is industry standard for this style of deposit and is already custom and practice at Cadia Valley Operations.

The process plant has already been progressively debottlenecked from 26mtpa, when Cadia East was commissioned, to the current rate of 30mtpa. The Study has identified further debottlenecking opportunities to increase the rate to 33m ........