Deposit Types
Two main PGE deposit types occur within the Bushveld Complex:
• Relatively narrow (maximum 1 m wide) stratiform layers (reefs) that occur towards the top of the Upper Critical Zone typically 2 km above the base of the intrusion (Merensky reef-style), mainly found in the Western and Eastern Limbs. These narrow zones have been the principal targets for mining in the past; however, more recently wider zones with more irregular footwall contacts have been mined (termed potholes).
• Contact-style mineralisation at the base of the intrusion (Platreef-type) occurs mainly in the Northern Limb.

In general, within the Northern Limb, the Platreef comprises a variably layered, composite norite–pyroxenite–harzburgite intrusion that lies at the base of the Bushveld Complex, in contact with metasedimentary and granitic floor rocks. McDonald and Holwell (2011) reviewed the major literature on the Platreef and Northern Limb, and have concluded:
• The Platreef remains a complex and enigmatic deposit.
• Stratigraphic relationships with other stratiform deposits such as the Merensky and UG2 reefs have been suggested.
• The extent to which the Northern Limb was connected to the rest of the complex across the Thabazimbi–Murchison Lineament (refer to Figure 7.1 where this is shown as the TML fault) remains to be established.
• The Platreef represents a complex of sills intruded into basement granite-gneiss, Transvaal Supergroup sediments or pre-Platreef Lower Zone intrusions.
• Intrusive relationships of the Main Zone gabbronorites, into solidified and deformed Platreef, removes the Main Zone as a source of metals for the Platreef.
• Mineral chemistry, bulk geochemistry, and Sr, Nd, and Os isotope geochemistry of the Platreef are most consistent with an ultramafic (Critical or Lower zone) component.
• Platreef Nd values and 187Os/188Os initial isotope ratios overlap clearly with the Merensky Reef but not the UCZ.
• Conventional and mass-independent S isotopes suggest a primary mantle source of S that was overprinted by the addition of local crustal S where Platreef intruded pyrite-rich shales. Assimilation of S is viewed as a modifying process, not as the primary trigger for mineralisation.

Mineralisation
The Platreef Project is hosted within the Palaeoproterozoic (2.06 Ga) Bushveld Igneous Complex (BIC), which is the largest of the known layered igneous intrusions, covering an area > 65,000 km². The BIC hosts up to 75% of the world’s platinum resources (Naldrett et al, 2009).

The BIC includes an early bimodal volcanic sequence (the Rooiberg Group) that is followed by an intrusive layered series of ultramafic and mafic units known as the Rustenburg Layered Suite (RLS) and the Lebowa Granite and Rashoop Granophyre Suites. The RLS is 7 to 8 km thick and ranges in composition from dunite to diorite.

Hall (1932) divided the RLS into 5 zones in descending order:
• Upper Zone (UZ) — Gabbroic succession.
• Main Zone (MZ) — A succession of gabbronorites with occasional anorthosite and pyroxenite bands.
• Critical Zone (CZ) — The Lower Critical Zone (LCZ) consists of orthopyroxenitic cumulates, and the Upper Critical Zone (UCZ) comprises packages of chromitite, harzburgite, pyroxenite, norite, and anorthosite. The CZ hosts PGE–Au–Ni–Cu and chromite deposits in several different chromitite layers known as reefs. The most significant are the Merensky Reef and the Upper Group 2 (UG2) Reef of the Eastern and Western Limbs. These range on average from 0.4–1.5 m in thickness and the contained PGE (Pt, Pd, Rh, Au) content typically ranges from 4–10 g/t (Cawthorn, 2005).
• Lower Zone (LZ) — Upper and lower peridotites separated by a central harzburgite.
• Marginal Zone (MZN) — Norites with variable proportions of accessory clinopyroxene, quartz, biotite and hornblende, indicating magma contamination from the underlying metasediments. This unit is not always present.

In the East and West Limbs of the BIC, the RLS was intruded into the Magaliesberg Formation of the Proterozoic Transvaal Supergroup. In the North Limb, the RLS intrudes progressively older country rocks northward (Magaliesberg Formation, Malmani Subgroup and Duitschland Formation). In the East and West Limbs of the BIC, the CZ includes the Merensky Reef and UG2 chromitite that are exploited for PGE mineralisation. In the North Limb of the BIC, the mineralised horizons have been referred to as the Platreef. The North Limb hosts the Platreef Project.

Structure
Structurally, the Northern Limb is separated from the rest of the Bushveld Complex by the Thabazimbi-Murchison Lineament (TML). The TML is a pre-Bushveld, major, compressional tectonic boundary (suture zone) that formed as a result of the collision of the Pietersburg terrane and Kaapvaal shield around 2.97 Ga during the Murchison Orogeny (Friese, 2003, 2004). The Ysterberg-Planknek and Zebediela Faults play a significant role in the regional geology of the Northern Limb.

The tectono-thermal evolution can broadly be subdivided into pre- and syn-emplacement folding and multiple faulting events. Folding in the Northern Limb has been controlled by two principal transpressional events caused by movements along the TML in the south and the Palala Shear Zone.

According to Nex, (2005), this led to the formation of two main open-fold geometries within the Transvaal sediments. The first and most dominant folding event was caused by NE-SW sinistral transpression. This resulted in regional NNW trending low amplitude, sub horizontal open folding. These F1 folds developed within Archaean basement and Transvaal Supergroup and represent the earliest developed structures which formed contemporaneously as a result of mild ENE-WSW compression during the Limpopo-Murchison Orogeny at 2.78–2.64 Ga. Subsequent NW-SW transpressive inversion refolded the earlier F1 fold axis resulting in basin and dome fold interference patterns (Friese, 2012).

Significant brittle faults and ductile shear zones are known throughout the Northern Limb, and the major, widely-spaced, ENE-trending shear zones dominate the regional map pattern. These combine to form large strike-slip duplex systems, which host a complex array of riedel shears, normal faults, thrusts and dilational tension fractures that have been invaded in part by igneous dykes and quartz-feldspar veins. These faults are reactivated during a major E-W crustal extension event associated with major brittle fracturing.

Sublevel Blasthole (Longhole) Stoping with cemented backfill / rock fill, supplemented with development waste rock, where possible, to fill open stopes in the thicker ore zones. This method will minimise mining costs and achieve the highest productivity. The remainder of production may come from thinner high-grade zones. Drift-and-Fill methods will be used in these zones. Two longhole mining scenarios were studied and evaluated for the Platreef 2020 FS: Transverse Longhole Stoping and Longitudinal Longhole Stoping. Longitudinal Longhole Stoping was considered specifically where deposit thickness could not support stope lengths of 15 m or more, normal to the strike, where Transverse Longhole Stoping would not be a suitable option. From the Vertical Miner software results, three major areas (Zone 1 north section, Zone 4, and Zone 2 central locations), were identified as the ideal candidate for this mining method. Stope shapes were created accordingly. After reviewing the longitudinal stope shapes created by MSO, the entire methodology had to be dismissed due to geotechnical considerations, such as hydraulic radius and mining direction. The ribs of the stope were parallel to the fault planes and the hydraulic radius provided would not support such stope shapes with meaningful sizes.

For the thinner and high-grade areas, two mining methods were evaluated: Cut and-Fill and Drift-and-Fill. After reviewing the created mining shapes (pancakes), it was concluded that Drift-and-Fill mining would be possible and is the best solution due to higher productivities and performance rate.

ROM is crushed underground to a top size (F100) of 270 mm. The pre-production ROM material is conveyed by a ROM handling system and can be routed to either one of the two 5,200 t ROM silos or the ROM stockpile.

Upon commencement of the concentrator plant operations, this stockpiled material will be reclaimed via a ROM reclaim system consisting of a static grizzly, surge bin, vibrating screen and reclaim conveyor fitted with a magnet for tramp metal removal. The reclaim conveyor feeds the secondary screens.

Once the concentrator plant is in production, ROM ore is conveyed from Shaft 2 headgear, at a peak flow rate of 1,350 dry tph and a top size of 270 mm, into either one of the two 5,200 t ROM silos. Each ROM silo is equipped with apron feeders for extraction onto the ROM Screen Feed Conveyor that feeds the secondary screens in the crushing circuit. Tramp metal is removed prior to crushing by means of a tramp metal magnet situated on the conveyor.

Crushing and Screening
The concentrator crushing and screening circuit consists of secondary and tertiary crushing stages in closed circuit with secondary and tertiary screening stages.

Ore from the ROM Screen Feed Conveyor, ROM Reclaim Conveyor, and Secondary Crusher Product Conveyor are transferred to the Secondary Screen Feed Bin. Ore from the Secondary Screen Feed Bin is transferred via vibratory feeders to the secondary screening circuit which is comprised of two double-deck screens. The top deck (+30 mm) of both the secondary screens reports to the Secondary Cone Crusher Feed Conveyor which is equipped with a magnet for the removal of tramp metal. The bottom screen deck (+12 mm–30 mm) material of both the screens reports to the Tertiary Cone Crusher Feed Conveyor, which is also equipped with a magnet for the removal of tramp metal. The secondary screen undersize (–12 mm) from both secondary screens reports to the Screen Product Conveyor. The Secondary Cone Crusher circuit is comprised of two cone crushers operated in parallel. Each cone crusher is fed via a feed bin and vibrating feeder.

The secondary crusher product (–35 mm) is removed by the Secondary Cone Crusher Product Conveyor and fed back to the Secondary Screen Feed Bin for re-screening.

The Tertiary Cone Crusher circuit is comprised of two cone crushers operated in parallel. Each cone crusher is fed via a feed bin and belt feeder.

Milling
The crushing and screening circuit product (P100 =13 mm) is stored in two off 8,000 t silos. Each silo is dedicated to a 2.2 Mtpa milling module. The crushed ore is transferred via belt feeders onto the associated Mill Feed Conveyor. Belt feeders are specified to minimise spillage and dust of the relatively fine material.

Each milling circuit consists of a single 22½’ft diameter x 34½’ft EGL ball mill, with a grate discharge liner arrangement and 12 MW (2 x 6 MW) variable speed geared pinion drives. Each ball mill operates in closed circuit with a classification cyclone cluster. The mill feed material (F100 of 13 mm) is fed to the mill feed hopper where process water is added for in-mill density control. Copper collector is also added into the mill feed hopper to assist in maximising copper recovery in the high-grade flotation circuit.

The milled material discharges onto the vibrating mill discharge screen for scats removal. Mill scats are deposited onto a scats stockpile via a scats removal conveyor. Scats from the stockpile are removed via front end loader and taken to the waste handling area, or alternatively scats can be re-loaded onto the mill feed conveyor.

The screened material is collected in the mill discharge sump and pumped to the mill classification cyclone cluster, which produces an overflow product of P80 = 75 µm. The cyclone underflow is recycled to the mill feed hopper for further regrinding. The cyclone overflow gravitates to the Rougher Flotation Feed Tank via a linear trash screen and two-stage sampling system. The oversize from the linear screen is removed as trash.

The process plant is designed to process 4.4 Mtpa Platreef ore and includes all ore processing requirements from the ROM storage silos through to final concentrate load out and tailings disposal. The concentrator consists of the following:
• 1 x 4.4 Mtpa combined crushing and screening circuit.
• 2 x 2.2 Mtpa modules for the milling and flotation circuits.
• 1 x 4.4 Mtpa combined concentrate handling circuit.
• 1 x 4.4 Mtpa combined tailings disposal circuit.
• 1 x 4.4 Mtpa combined reagent building.
• 1 x 4.4 Mtpa combined utilities. The modular approach adopted allows for increased processing flexibility and plant redundancy while also allowing for phasing of capital expenditure.

Flotation Circuit
Two separate 2.2 Mtpa flotation circuits were selected based on the production ramp-up and comminution trade-off studies conducted.

The optimised flotation circuit flow sheet as developed during testing was used as the basis of d ........