Ann Mason is hosted by several phases of the Jurassic-age Yerington batholith, and younger quartz monzonite porphyry dykes (Jqmp-a, Jqmp-b and Jqmp-c). Copper mineralization primarily occurs within a broad zone of main-stage potassic alteration containing chalcopyrite and bornite. An assemblage of chalcopyrite-epidote or chalcopyrite-epidotequartz mineralization locally overprints main-stage potassic alteration and copper mineralization.

Within the Yerington district, Tertiary volcanic rocks, Mesozoic host rocks and coppermolybdenum porphyry deposits have been rotated 60 degrees to 90 degrees westward by Miocene normal faulting and extension. As a result, mineralized intercepts in vertical drill holes through Ann Mason represent approximately horizontal intervals across the original pre-tilt geometry of the deposit.

The Ann Mason deposit has the characteristics of a typical, large copper-molybdenum porphyry system. Projected to the surface, the 0.15% Cu envelope covers an area approximately 2.8 km northwest and up to 1.3 km northeast. At depth, this envelope extends more than a kilometre below surface. The mineralization remains open in most directions.

Within the 0.15% Cu envelope the highest grades occur within a 200 m to 800 m thick, westplunging zone that surrounds the intrusive contact between granodiorite (Jgd) and porphyritic quartz monzonite (Jpqm). Within this zone, copper grade is dependent on vein density, sulphide species, frequency and relative age of quartz monzonite porphyry dykes and the mafic content of the granodiorite. Mineralization is closely associated with quartz monzonite porphyry dykes (Jqmp-a, -b and -c). The top of the mineralized envelope is truncated by the Singatse Fault and much of the southwest edge is truncated by the northwest-trending Fault 1A.

Sulphide zoning is that of a typical porphyry copper with an outer pyritic shell, and concentric zones of increasing chalcopyrite and decreasing pyrite progressing inward to a central zone of chalcopyrite-bornite.

Within the northeast, southeast, and southwest quadrants of the deposit chalcopyrite and chalcopyrite-bornite are the primary sulphide domains and are the most dominant in terms of overall deposit tonnage. Little or no overlap occurs between pyrite and bornite or between pyrite and molybdenite. In the northwest quadrant the primary sulphide domain is chalcopyrite = pyrite; a domain that forms thick intervals of >0.3% Cu, with only minor bornite present at depth, near the granodiorite-porphyritic quartz monzonite contact.

Chalcopyrite occurs as individual grains in veins and disseminated in rock, as fillings in brecciated pyrite grains, attached to or included in pyrite grains, and attached to or included in bornite. Bornite occurs as separate grains in veins, and disseminated in rock and attached to chalcopyrite. Sparse chalcocite occurs as replacement rims on chalcopyrite, but more commonly as replacement rims or exsolution replacement of bornite.

Molybdenum occurs as molybdenite in quartz and quartz-chalcopyrite veins and on fracture or shear surfaces as molybdenum paint. Within quartz veins, molybdenite occurs as disseminations, centerline segregations and discontinuous selvages. Molybdenum within a 0.005% Mo grade shell occurs largely within the 0.15% Cu grade shell. Where late albite alteration has reduced copper grade, molybdenum mineralization is mobilized into fractures and shear zones and extends to greater depth than copper.

Silver =0.6 g/t and gold =0.06 g/t are closely associated with the occurrence of bornite within the chalcopyrite-bornite sulphide domain. Hydrothermal alteration associated with porphyry copper and molybdenum mineralization at Ann Mason is similar to alteration described in many porphyry copper deposits. Voluminous sodic-calcic alteration zones on the flanks of the Yerington district deposits may have been leached of copper and iron, possibly providing those components to mineralizing fluids (Dilles and Proffett, 1995).

Alteration assemblages include an outer propylitic zone (chlorite±epidote±pyrite), widespread potassic alteration (secondary biotite, secondary biotite+K-feldspar or Kfeldspar) associated with main-stage copper-molybdenum mineralization, and more restricted late-stage zones of chlorite±epidote±albite, sodic (albite±chlorite), and sericitic alteration. Molybdenum mineralization is not significantly affected by the late sodic alteration, beyond partial remobilization from veins into nearby fractures and shears.

Two prominent structures form structural boundaries to the Ann Mason Mineral Resource. The relatively flat Singatse Fault truncates the upper surface of the 0.15% Cu envelope over a portion of the deposit and juxtaposes sterile Tertiary volcanic rocks on top of the mineralized intrusives. The high-angle, northwest-trending, southwest dipping Fault 1A marks the current southwest margin of >0.15% Cu mineralization in the deposit, juxtaposing propylitically altered rocks with pyrite mineralization in the hanging wall against potassically-altered rocks with copper-molybdenum mineralization in the footwall. Fault 1A and other northwest- trending structures offset the intrusive contact between granodiorite (Jgd) and porphyritic quartz monzonite (Jpqm) to successively deeper levels towards the west and southwest. Copper-molybdenum mineralization in the footwall of the fault remains open at depth along the entire strike length of the fault.

Ann Mason is envisioned as a large-scale conventional open pit mine, involving the development of a single pit with five pit phases.

Mining will use conventional rotary drilling, blasting, and loading with large 56 m3 cable shovels and 360-tonne trucks working on 15 m benches.

Pit slopes are variable depending on the geotechnical parameters of the rock types and range from 50 degrees in the overlying volcanic rocks, to 37 degrees in rocks that host the porphyry mineralization.

At the peak of material movement in Years 1 to 7, the major equipment fleet is expected to consist of seven 311 mm drills, two 41 m3 front-end loaders, four 58 m3 electric cable shovels and forty 360-tonne trucks. A typical fleet of support equipment (track dozers, rubber tired dozers, graders) are utilized to assist development and maintenance of the mining operation.

Waste material will be placed to the southwest of the Ann Mason pit in a waste rock management facility (WRMF). For this study, waste materials have been assumed to be nonacid generating based upon a review of sulphur present in the deposit.

Reclamation of the WRMF will be concurrent with mining. The final height of this facility will be at elevation 1680 for an overall maximum height of 210 m.

The proposed method of copper and molybdenum recovery from the Ann Mason deposit consists of conventional crushing and milling, followed by rougher and cleaner froth flotation.

The Ann Mason concentrator is designed as a nominal 3,000,000 tonne per month plant. Mine haul trucks will tip into twin primary gyratory crusher stations that are designed for 86% availability. Surge capacity between the mill and crusher stations is handled by two ~40,000 tonne stockpiles.

Material is drawn off the stockpiles using apron feeders. SAG mill feed control would consist of variable speed feeders plus mill feed size distribution measurement. The SAG mill discharge classification is achieved as follows:
• Trommel screen (40 mm) directs oversize to a series of recycle conveyors and allows undersize to gravitate to the SAG mill discharge sump.
• Trommel screen undersize material is further classified by a vibrating, multi-angle scalping screen which cuts at 1.5 mm, oversize recycling back to the SAG mill, undersize feeding forward to the ball mill circuit.

Scalping screen undersize flows by gravity to the ball milling circuit. The ball mill operates in closed circuit with cyclones. The cyclone pack cuts at a P80 of ~155 µm, providing the required liberation for good flotation.

The cyclone overflow reports to the feed box of the rougher flotation circuit. The rougher/scavenger flotation plant consists of nine tank cells in series. Each cell would have independent air flow control.

Rougher flotation concentrate is reground in a ball mill to an 80% passing size of 45 µm. The ball mill operates in closed circuit with a cyclone cluster. Cyclone overflow reports to the 1st Cleaner feed box.

Detailed Process Description
Crushing
Mill feed will be delivered to one of two primary tip locations by 360-tonne haul trucks at a frequency averaging seven trucks per hour. Peak delivery rate is assumed to be 5,500 dmt/h. Mill feed is discharged directly into the primary crusher throat. This area is served by a hydraulic rock breaker to handle oversize rocks.

The primary crusher can accept 1,000 mm top size and will run with a 200 mm open side setting. Grizzly oversize enters the crusher and discharges by gravity after crushing into a 30-tonne rail lined surge pocket. An apron feeder is used to withdraw crushed mill feed from the surge pocket onto a short sacrificial conveyor. This conveyor discharges onto the main stockpile feed conveyor, which in turn feeds up to one of two crushed mill feed stockpiles.

The crushed mill feed stockpiles provide a live capacity equivalent to ~18 hours of plant production. Mill feed is withdrawn from the stockpile via two lined discharge chutes and two apron feeders (one operating, one standby). Each apron feeder is variable speed and capable of providing the entire mill feed rate. Each apron feeder discharges via a discharge chute onto the SAG mill feed conveyor. The two SAG mill feed conveyors feed two separate lines consisting of SAG milling, ball milling, and rougher/scavenger flotation.

Spillage and run-off in both the primary crusher building and the stockpile feeders tunnel is pumped to surface for appropriate handling. The primary crusher is served by an overhead maintenance crane of 20-tonne capacity.

SAG Milling
From the stockpile discharge feeder, mill feed is withdrawn in measured quantities onto the mill feed conveyor. This conveyor discharges via head chute and into the mill feed hopper. The SAG mill feed material size distribution will be monitored and/or controlled using a highspeed camera system.

Each SAG mill (two total) is a 40 ft diameter by 24 ft long, grate discharge, semi-autogenous grinding mill. Slurry and pebbles exit the mill after passing through the mill discharge grate and pebble ports onto a trommel screen fixed to the mill discharge trunnion. Trommel screen oversize material (pebbles) is directed by chute onto the SAG mill pebble conveyor for re-cycling. Trommel screen undersize gravitates into the mill discharge sump where it is further diluted with water. From the discharge sump, coarse slurry is pumped to the SAG mill scalping screen via a distributor box. Screen undersize slurry gravitates via the screen underpan through a sampling plant to the ball mill circuit. Screen oversize material gravitates via the oversize chute back to the SAG mill for re-grinding.

SAG mill slurry spillage is collected in a drive-in sump and then returned to process by a submersible slurry pump.

The milling area is served by an overhead crane. Relining is achieved using the common relining machine.

SAG mill grinding media is stored in a ball bunker located part-way along the mill feed belt. The bunker is served with a small spillage pump and a ball loading crane and magnet. Balls are added to mill feed at timed intervals via a ball loading chute.

Ball Milling
After SAG milling, the particle size is further reduced to 80% -155 µm by conventional, closed circuit milling, in two 27 ft diameter by 48 ft long overflow discharge ball mills.

The scalping screen undersize from each SAG mill is fed by gravity to a ball mill discharge sump. The undersize combines with dilution water and ball mill discharge before being pumped to the cyclone classification cluster. The clusters consist of a total of twelve 840 mm cyclones, 10 operating and 2 standby.

Cyclone underflow gravitates to the feed chute of the ball mill. The cyclone overflow reports to a linear trash screen for removal of woodchips and other tramp material prior to flotation. The screened cyclone overflow stream gravitates to the flotation circuit. The stream of woodchips and tramp plastic from the linear screen is dewatered by a woodchip sieve bend before being dumped in a storage area.

The screened cyclone overflow reports to a sampling station that consists of a sampling launder and an automatic sampler. Spillage contained in the ball mill area is pumped to the mill discharge sump for re-treatment.

Ball mill grinding media is delivered to the plant in bulk and is stored in the ball mill ball bunker. The ball bunker is serviced by a crawl and electric hoist arrangement allowing balls to be lifted into a kibble using the ball loading magnet and tipped into the mill via a ball loading chute.

The proposed method of copper and molybdenum recovery from the Ann Mason deposit consists of conventional crushing and milling, followed by rougher and cleaner froth flotation. A copper- molybdenum separation step generates final product copper and molybdenum concentrate products. This section describes the flowsheet, design criteria, and process description for a 100,000 t/d processing plant.

The Ann Mason concentrator is designed as a nominal 3,000,000 tonne per month plant. Mine haul trucks will tip into twin primary gyratory crusher stations that are designed for 86% availability. Surge capacity between the mill and crusher stations is handled by two ~40,000 tonne stockpiles.

Material is drawn off the stockpiles using apron feeders. SAG mill feed control would consist of variable speed feeders plus mill feed size distribution measurement. The SAG mill discharge classification is achieved as follows:
• Trommel screen (40 mm) directs oversize to a series ........