The Macraes deposit is an example of an orogenic style gold deposit. This style of deposit is recognized to be broadly synchronous with deformation, metamorphism, and magmatism during lithospheric-scale continental-margin orogeny (Groves et al., 1998). Most orogenic gold deposits like Macraes occur in greenschist facies rocks. The Macraes deposit falls into the mesozonal category with mineralization having occurred near to the brittle-ductile transition at about 300°C.

The mineralization at Macraes is principally developed within the gently northeast dipping HMSZ, though anomalous grades are also recorded in narrow, steeply dipping quartz veins locally occurring in the hangingwall schists, collectively known as the Eastern Lodes. Mining to date has occurred along a continuous strike length of 6km in numerous staged open pits, three smaller discrete satellite pits immediately to the north, and at Golden Bar, approximately 7km to the south.

Within the shear zone, mineralization is constrained between the Hangingwall Shear and the Footwall Fault. Schists above the Hangingwall Shear and below the Footwall Fault are barren. Economic mineralization is typically restricted to the upper part of the HMSZ. The Hangingwall Shear, which varies from 1m to >30m in thickness contains the most continuous and consistent mineralization. This zone is locally underlain by extensive but low grade stockwork zones which may be developed over a width of up to 100m.

Higher grade zones of mineralization within the shear zone form tabular shoots that may have strike lengths of >300m and extend up to 800m down-dip (i.e. Frasers and Round Hill). These zones are observed to trend towards the north, oblique to the shear zone dip direction. This orientation is interpreted to be due to the interaction of the HMSZ with folds within the host schist units, creating a preferred lineation direction for mineralization.

Mineralization distribution is broadly consistent along the HMSZ but shows considerable variability in grade, width, continuity and geometry at mine-scale. This variability is attributed to the local development of the HMSZ structure during mineralization and the influence of host rock lithology, particularly with respect to competency contrasts.

The following four types of mineralization occur within the HMSZ at Macraes (Mitchell et al., 2006):
• Mineralized schist. This style of mineralizati on involved hydrothermal replacement of schist minerals with disseminated sulphides and microcrystalline quartz. Mineralization was accompanied by only minor deformation.
• Black sheared schist. This type of schist is pervaded by cm to mm scale anastamosing fine graphite and sulphide bearing microshears. This type of mineralization is typically proximal to the Hangingwall Shear. Scheelite mineralization occurs in the silicified cataclastic shears.
• Shear-parallel quartz veins. These veins lie within and/or adjacent to the black sheared schist, and have generally been deformed with the associated shears. The veins locally cross cut the foliation in the host schist at low to moderate angles. Veins are mainly massive quartz, with some internal lamination and localized brecciation. Sulphide minerals are scattered through the quartz, aligned along laminae and stylolitic seams. These veins range from 1cm to >2m. . Scheelite mineralization is associated with quartz veining in some areas.
• Stockworks. These veins occur in localized swarms that are confined to the Intrashear Schist. Individual swarms range from c. 100 to 2000 m2 in area and consist of numerous (10 - 100) subparallel veins. Most of these veins formed subperpendicular to the shallow east dipping shear fabric of the Intrashear Schist. Stockwork veins are typically traceable for 1 5m vertically with most filling fractures that are 5 – 10cm thick, but can be up to 1m thick. Swarms of stockwork veins within the Intrashear Schist were lithologically controlled by the dimensions and locations of more competent pods of Intrashear Schist.

Gold is closely associated with pyrite and arsenopyrite in all of the above styles of mineralization. Rarely free gold up to 300µm occurs in quartz veins, but most gold occurs as 1-10µm scale blebs hosted in and near sulphide grains (Angus, 1993).

Scheelite mineralization is associated with gold mineralization and quartz veining and displays complex crosscutting relationships. McKeag (1987) documented its occurrence in at least three veining generations.

The Frasers underground orebody encompasses the down dip continuation of the Hangingwall shear mined in the Frasers open pit. The orebody is relatively shallow dipping (15 – 20 degrees) to the east. The orebody is tabular with undulations and has a thickness varying between 5 to 30 metres.

The mining method used underground involves 15-metre-wide open stopes with 6 metre yielding pillars between stopes. Mining areas are separated by 20 metres to 60 metres wide regional pillars. The mining areas are generally restricted to about 120 metres width and 160 metres length. Mine production targets the high-grade ore at the top of the 30 metres thick mineralisation. Stope heights vary between drive height (5 metres) and up to 25 metres.

The most utilised mining method at FRUG is retreat long hole open stoping. In addition, sub-level caving is used to recover some of the regional pillars after they are no longer required. Declines and access drives are mined to a 5.0mW x 5.5mH arched profile. Ore drives are mined to a 5.0mWx5.0mH profile. This allows enough space for services, secondary fans, vent ducts and mobile equipment. Stoping panels were designed based on the main considerations below:

- Ore drives placed at 21m centres to allow for 15m wide stopes with 6m yielding pillars between them. There are no secondary stopes designed to be extracted following the mining of the primaries;
- Ore drives positioned such that they will have a gentle uphill gradient for water drainage but not orientated directly north-south, parallel to the strike of many of the faults present, to maintain drive stability;
- Regional pillars maintained between panels of 30m width if they contained no development and 60m wide if they contained development drives. In some cases, pillars containing access drives were reduced to a width of 40m if the contained drives were no longer needed following stoping of the surrounding areas;
- The stoping panels should be retreated towards a solid abutment rather than towards an internal regional pillar; and
- Panel accesses designed in conjunction with primary ventilation return loops and secondary egress routes and positioned such that they will stay intact following stoping of nearby areas.

The plant's crushing and grinding comprises the following components and stages:

- Two single stage jaw crushing circuits, which reduce the ore to a top size of approximately 200mm; the products from these two circuits are directly fed to the two SAG mills and an emergency feeder on the conveyor system feeding the higher capacity circuit provides continuity of feed to the grinding circuit if the jaw crusher feed is interrupted;

- A complex grinding circuit to reduce the particle size of the ore to 80% passing at 130 µm; the original, higher capacity crushing circuit feeds a 2,300kW SAG mill and the new crushing circuit feeds a 1,500kW SAG mill; discharge from the two SAG mills is combined with the discharge from one of the two ball mills (2,300kW) and directed to the primary cyclone cluster. Discharge from the higher capacity ball mill (2,500kW) is fed to the secondary cyclone cluster. The underflows from both cyclone clusters are combined and fed in parallel to the flash flotation circuit and two ball mills (2,300kW and 2,500kW);

- Regrind of the flotation concentrate is performed in a 900kW ball mill to 80% passing of 15 µm to improve pressure oxidation kinetics.

The Macraes Process Plant recovers gold by concentrating the metal into a relatively small fraction of flotation concentrate, oxidising the reground concentrate in a pressure oxidation autoclave, washing the oxidised residue and then utilising a carbon-in-leach process to recover gold from the residue.

In detail the plant comprises:
- A flash flotation circuit made up of roughing and cleaning stages. The circuit is fed from the circulating load of the grinding circuit via cyclone underflows to recover the bulk of fast floating sulphide minerals containing high gold content in the coarser size fraction;

- The main flotation circuit made up of roughers, scavengers, cleaners, recleaners and cleaner scavenger flotation cell trains to produce a gold bearing sulphide concentrate at optimum sulphur grade for the downstream pressure oxidation circuit;

- Limestone is added to the regrind circuit discharge to control net acid generation in the pressure o ........