Overview
Stage | Production |
Mine Type | Underground |
Commodities |
|
Mining Method |
- Sub-level caving
- Longhole open stoping
|
Processing |
- Carbon re-activation kiln
- INCO sulfur dioxide/air process
- Smelting
- Flotation
- Concentrate leach
- Agitated tank (VAT) leaching
- Counter current decantation (CCD)
- Pressure oxidation
- Carbon in leach (CIL)
- AARL elution
- Carbon adsorption-desorption-recovery (ADR)
- Solvent Extraction & Electrowinning
- Cyanide (reagent)
|
Golden Point Underground (GPUG) will initially supplement, then eventually fully replace the existing Frasers Underground gold production. |
Source:
p. 10,27
Oceana Gold Limited holds 100% interests in the Frasers mine and is a wholly owned subsidiary of OceanaGold Corp.
Deposit Type
- Orogenic
- Vein / narrow vein
- Breccia pipe / Stockwork
Summary:
At the Frasers open pit and Frasers underground mine (FRUG), deposits are centred on mining the hangingwall shear. In outcrop, the shear typically dips at 15 to 20 degrees to the east and is approximately 5 metres thick. At depth, the dip of the shear flattens to approximately 5 to 10 degrees and develops into an approximately 20 to 30 metres thick mineralised high-grade zone of quartz cataclasite, and mineralised schist. Within the open pit, gold mineralisation comprises mineralised schist and cataclasite, shear-parallel quartz veins and arrays of sub-vertical quartz veins. hangingwall shear and arrays of subvertical quartz veins account for most of the mineralisation within the open pit, although there are a few shear-parallel quartz veins. These veins typically splay off the base of the hangingwall Shear and dip at between 5 and 10 degrees to the west.
A large amount of erratic mineralisation occurs between the base of the hangingwall shear and the footwall fault. At the resource drilling stage, this mineralisation manifests as poorly developed clusters of elevated gold grades, which often appear discontinuous. During mining however, these typically present as extensive zones of quartz vein arrays and mineralised shears. The footwall fault lies between 80 metres and 120 metres below the hangingwall shear and is identified as a cataclastic zone up to 10 metres thick. To date, no economic mineralisation has been located below the footwall fault.
FRUG encompasses the down-dip continuation of the hangingwall shear mined in the Frasers open pit, which is known to extend approximately 600 metres beyond the limit of the open pit design. The thickest, most mineralised part trends approximately northeast and tapers in length from approximately 350 metres at its western end to approximately 150 metres at the eastern limit of drilling, where it abuts the Macraes Fault zone. Mineralisation is contained within the intrashear schist which is generally 80 metres to 100 metres thick, with the higher gold grades confined to the upper part, which is dominated by cataclasite, lode schist and local stockwork pelite lithologies. Numerous drill holes have penetrated through the intrashear schist into the Footwall Psammite, particularly at the western end where the Footwall Fault is relatively shallow, at depths of less than 500 metres. Mineralisation is consistent with the ore delineated in the Frasers open pit. The highest gold grades are contained within the strongly developed and visually distinguishable zone within the upper hangingwall, characterised by quartz cataclasite and silicified breccias. This typically forms a well mineralised, continuous zone up to 15 metres thick, with a grade of approximately 3 g/t Au. Less intensely mineralised lode schist is typically developed lower in the hangingwall package.
GPUG encompasses the down-dip continuation of the hangingwall shear mined in the Round Hill and Golden Point open pits. Current drilling has shown this to extend more than 700 metres beyond the limit of the open pit design. The thickest, most mineralised part is a series of stacked lodes proximal to the Golden Point pit. Mineralisation continues as a single higher-grade lode down-dip to the north-northeast. Mineralisation is contained within the intrashear schist, which is generally 80 metres to 100 metres thick, with the higher gold grades confined to the upper part, which is dominated by cataclasite, lode schist and local stockwork pelite lithologies. Numerous drill holes have penetrated through the intrashear schist into the footwall psammite. Mineralisation is consistent with the ore delineated in the Golden Point and Round hill open pits, however down-dip of Golden Point this is constrained to a single lode. The highest gold grades are contained within the strongly developed and visually distinguishable zone within the upper hangingwall, characterised by quartz cataclasite, and mineralised schist. This typically forms a well mineralised, continuous zone up to 5-10 metres thick, with a grade of approximately 3 g/t Au.
Mineralisation
The Macraes deposit is a classic example of an orogenic style gold deposit, with mineralisation broadly synchronous with deformation, metamorphism, and magmatism during lithospheric-scale continental-margin orogeny. Most orogenic gold deposits like Macraes occur in greenschist facies rocks. Orogenic deposits typically formed on retrograde portions of pressure-temperature time paths during the last increments of crustal shortening, and thus postdate regional metamorphism of the host rocks. The following four types of mineralisation occur within the HMSZ at Macraes:
(a) Mineralised schist. This style of mineralisation involves hydrothermal replacement of schist minerals with sulphides and microcrystalline quartz. Mineralisation is accompanied by only minor deformation;
(b) Black sheared schist. This type of schist is pervaded by small scale anastomosing fine graphite, and sulphide bearing microshears. This type of mineralisation is typically proximal to the hangingwall shear;
(c) 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 crosscut the foliation in the host schist at low to moderate angles. Veins are mainly massive quartz, with some internal lamination and localised brecciation. Sulphide minerals are scattered through the quartz, aligned along laminae and stylolitic seams. These veins range from 1 centimetre to more than 2 metres; and
(d) Stockworks. These veins occur in localised swarms that are confined to the intrashear schist. Individual swarms are up to 2,000 square metres in area and consist of numerous subparallel veins. Most of these veins formed sub perpendicular to the shallow east dipping shear fabric of the intrashear schist. Stockwork veins are typically traceable for 1 metre to 5 metres vertically with most filling fractures that are 5 centimetres to 10 centimetres thick but can be up to 1 metre thick.
Mining Methods
- Sub-level caving
- Longhole open stoping
Summary:
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.
Flow Sheet:
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.
Processing
- Carbon re-activation kiln
- INCO sulfur dioxide/air process
- Smelting
- Flotation
- Concentrate leach
- Agitated tank (VAT) leaching
- Counter current decantation (CCD)
- Pressure oxidation
- Carbon in leach (CIL)
- AARL elution
- Carbon adsorption-desorption-recovery (ADR)
- Solvent Extraction & Electrowinning
- Cyanide (reagent)
Flow Sheet:
Summary:
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 ........

Combined production numbers are reported under
Macraes Operation
Operational Metrics:
Metrics | 2021 | 2020 | 2019 |
Ore tonnes mined
| ......  | ......  | 994 kt |
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Reserves at December 31, 2021:
The Mineral Reserves reported a 1.61 g/t Au.
The Mineral Resource reported using a 1.25 g/t Au.
Category | Tonnage | Commodity | Grade | Contained Metal |
Proven
|
0.33 Mt
|
Gold
|
2.23 g/t
|
|
Probable
|
3.21 Mt
|
Gold
|
1.88 g/t
|
|
Proven & Probable
|
3.54 Mt
|
Gold
|
1.91 g/t
|
0.22 M oz
|
Measured
|
0.7 Mt
|
Gold
|
2.98 g/t
|
|
Indicated
|
6.1 Mt
|
Gold
|
2.48 g/t
|
|
Measured & Indicated
|
6.9 Mt
|
Gold
|
2.53 g/t
|
0.56 M oz
|
Inferred
|
0.3 Mt
|
Gold
|
2.1 g/t
|
|
Operating Costs:
| Units | 2021 | 2020 | 2019 | 2018 | 2017 | 2016 |
UG mining costs ($/t mined)
|
USD
| 56.3 | 43.8 | 37.9 | 41.1 | 38.5 | 37.8 |
Corporate Filings & Presentations:
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Aerial view:
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