Gold mineralization at Sukari is hosted exclusively by porphyry. The lack of significant gold grades in chemically reactive serpentinitic wall rocks can be explained in one of two ways:

• The porphyry represented a favourable host either because of its composition, relative to mineralising fluids, or its mechanical properties or both.

• The Sukari Porphyry was relocated, relative to wall rock sequences, by faulting after gold mineralization.

While significant post-mineralization faulting has possibly occurred, the second scenario is unlikely. Porphyry dykes in the hangingwall of the main porphyry body show gold mineralization of essentially the same character as that in the main porphyry and wall rocks immediately adjacent to those dykes are also barren. Those dykes range in thickness from a few centimetres to several metres. It is not reasonable to postulate that they, along with the main porphyry body, were all relocated by faulting after mineralization.

Certainly it is evident that the Sukari Porphyry has acted as a rigid body surrounded by weaker rocks. Footwall and hangingwall rocks have taken up strain by development of strong schistosity, almost certainly accompanied by large decreases in volume. The porphyry has taken up strain by development of predominantly brittle fault structures.

The porphyry host for the mineralization has a strike length of approximately 2,300 m, and ranges in thickness from 100 m to approximately 600 m (Cavaney 2005). Gold mineralization within this is not continuous and its deposition has been influenced by major long-lived structures, the most important of which are tabular sheets of crackle breccia.

Ore and waste are mined using conventional open pit mining methods. The operation utilises selective mining techniques to separate ore and waste. Provision has been made for drilling and blasting all primary and oxide materials.

The Sukari underground mine is a trackless diesel mine, with all equipment rubber tyred and self- powered. The workings are accessed via a ramp system declining at a gradient of 1 in 7. Levels are typically developed at 15 metre vertical intervals, although this is dependent on orebody geometry and mining method.

Mining method: Long-hole stoping, room and pillar, avoca modified.

Primary Crushing and Stockpiling
ROM ore is dumped directly into the 500 t capacity dual-tip crusher pocket by haul trucks or stockpiled on the ROM pad. The tipping station is designed to handle Caterpillar 789C, or equivalent open-pit haul trucks. Ore from the dump pocket is crushed in a 1,400 x 2,100 mm gyratory crusher, fitted with a 600 kW motor and operating at a nominal 165 mm open side setting. Oversize is broken using a hydraulic rock breaker.

Crushed rock (with a P100 of 229 mm and a P80 of 140 mm), which falls into the primary crusher discharge pocket, is withdrawn by a variable speed apron feeder which, in turn, feeds a short sacrificial conveyor. Chute work under the apron feeder directs fines spillage from the apron feeder onto the sacrificial conveyor.

This conveyor discharges onto a transfer conveyor that discharges onto the screen feed conveyor which feeds two double deck vibrating screens, operating in parallel, fitted with 100 mm and 50 mm screen panels. Screen undersize material (-50 mm) is discharged to the secondary crusher product conveyor. Screen oversize material (+50 mm) is discharged to the secondary crusher feed conveyor and fed into the crusher feed bin, situated above the three secondary crushers. The oversize material passes through two of three CH870C cone crushers (two on duty, one on standby). The cone crushers operate in open circuit and reduce the oversize material to a P80 of 35 mm, prior to discharging on to the secondary crusher product conveyor.

The secondary crusher product conveyor discharges to the stockpile splitter chute diverter gate that splits the crushed product proportionally, according to operator input, to each mill feed stockpile conveyor.

Crushed ore is stored on a 15,000 t live capacity mill feed stockpile for each grinding circuit. If the crushing circuit is off-line for an extended period, the dead portion of each pile can be reclaimed by bulldozer, excavator or front end loader.

For the Stage 1/2 grinding circuit, crushed ore is drawn from the Stage 1/2 mill feed stockpile at a controlled rate by a combination of three 1.2 m wide x 6.0 m long variable speed apron feeders discharging onto the SAG 1 mill feed conveyor. The design capacity of each of the apron feeders is 580 tph and any one or more feeders may be operated at the same time.

For the Stage 4 grinding circuit, crushed ore is drawn from the Stage 4 mill feed stockpile at a controlled rate by a combination of three 1.3 m wide x 5.5 m long variable speed apron feeders discharging onto the SAG 2 mill feed conveyor. The design capacity of each of the apron feeders is 750 tph and any one or more feeders may be operated at the same time.

Secondary crushed ore with a P80 of 50 mm is transferred to a second crushed ore stockpile prior to grinding through the new milling circuit. The new milling circuit is a two-stage circuit, consisting of a SAG mill and ball mill, with hydrocyclone classification and pebble crushing facility.

Grinding
Two grinding circuits are installed and designed to be operated in parallel. Each grinding circuit is a two-stage milling circuit with hydrocyclone classification and pebble crushing facility.

Stage 1/2 grinding circuit
The SAG mill is an 8.53 m diameter by 4.27 m long (3.81 m effective grinding length) grate discharge mill with a 5,595 kW synchronous motor installed and an air clutch for starting the mill. The mill operates at 76% critical speed with a ball charge of 15% to draw 4,540 kW at the pinion. The mill is fitted with a steel lining system with 40 mm grate apertures and 60 mm pebble ports. The make-up ball size is 125 mm.

Pebbles from the SAG mill are conveyed to the HP500 pebble crusher (cone crusher), operating with a closed side setting of 12 mm. The design crusher feed rate is 150 tph or 30% of new feed to the mill. Crushed pebbles are returned to the SAG mill via the SAG mill feed conveyor.

Cyclone underflow reports to two 5.03 m diameter by 9.29 m long overflow ball mills with rubber linings and fitted with 4,100 kW synchronous motors and air clutches for starting. The ball mills are operated with a 34% ball charge and a make-up ball size of 60 mm. Each mill is fitted with a 2.2 m diameter by 2.4 m steel discharge trommel, manufactured with 20 mm by 100 mm slots. Trommel oversize reports to a scats bay for later collection.

Stage 4 grinding circuit
The SAG mill is an 8.53 m diameter by 5.50 m long (4.65 m effective grinding length) grate discharge mill with a 7,000 kW variable speed induction motor incorporating slip energy recovery and a resistance chopper drive for starting the mill. The mill is designed to operate at 75% critical speed with a ball charge of 10% to draw 5,335 kW at the pinion. The mill is fitted with a steel lining system with 25 mm grate apertures and 60 mm pebble ports. The design make up ball size is 125 mm.

Pebbles from the SAG mill are conveyed to the pebble crusher feed bin and are withdrawn at a controlled rate using a belt feeder to one of two (one on duty, one on standby) XL300 pebble crushers (cone crushers), operating with a closed side setting of 12 mm. The belt feeder speed is controlled by a level indicator situated above each pebble crusher and a PID feedback loop controller to maintain a choke feeding arrangement. Crushed pebbles are conveyed to the pebble crushing circuit surge bin and are withdrawn at a controlled rate using a belt feeder for return to the SAG mill via the SAG mill feed conveyor.

Cyclone underflow reports to the 6.10 m diameter by 9.76 m long overflow ball mill with rubber linings and fitted with 7,000 kW induction motor and resistance chopper drive for starting. The ball mill is designed to be operated with a 33% ball charge and a make-up ball size of 60 mm to 80 mm. The mill is fitted with a 2.2 m diameter by 2.4 m steel discharge trommel, manufactured with 20 mm by 100 mm slots. Trommel oversize reports to a scats bay for later collection and trommel undersize reports to the mill discharge pump box.

The process route encompasses:
• Crushing
• Stockpiling crushed ore
• Grinding
• Flotation of a bulk sulphide concentrate
• Thickening of the concentrate and the flotation tailings
• Fine milling of the concentrate
• Leaching the precious metals from the concentrate and/or flotation tailings in a dilute cyanide solution
• Adsorbing the precious metals onto activated carbon
• Eluting (or desorbing) the precious metals from the carbon
• Recovering the precious metals as gold doré
• Combining the flotation tailings and leached concentrate and pumping to the TSF.

Flotation
Two flotation circuits are installed and designed to be operated in parallel. The flotation circuits are designed to recover a bulk sulphide concentrate for further comminution and subsequent leaching. The circuits are designed to operate 24 hours a day, seven days a week with a utilization of 91.3%.

The Stage 1/2 flotation circuit comprise ........