The SOT deposit includes the main Southwest, South, Wedge, and Central zones and a number of smaller, fault-bounded zones. The planned open pit will incorporate the majority of these zones.

The zones form contiguous sectors of mineralisation representing multiple mineralising centres, each with distinct styles of mineralisation, alteration, and host rock lithology. The boundaries between the individual deposits and zones coincide with major faults. Faulting has resulted in different erosional histories for the zones, depending on the depth to which a zone has been down-faulted or uplifted relative to neighbouring zones.

The Southwest zone is a gold-rich porphyry system characterised by a south-west plunging, pipe-like geometry that has a vertical extent of up to 700 m. The high-grade core of the zone is about 250 m diameter; the low-grade shell (0.3% Cu) surrounding the core may extend for distances as much as 600 m x 2 km.

Quartz-dominant veins with variable amounts of sulfide (pyrite, chalcopyrite, and bornite), K feldspar, chlorite, and carbonate are ubiquitous in the Southwest zone, and there is a general correlation between vein density and copper and gold grades. Most veins are several millimetres to several centimetres wide, although veins within the core of the zone can be up to a metre thick or more. Vein contacts can be either planar or variably deformed, and folded and/or faulted veins are common. Veins within the high-grade core display subparallel to sheeted forms with a preferred south-west dipping orientation. These pass into more irregularly oriented stockwork veins in peripheral mineralised zones, where sub vertical north to north-west striking orientations are most common.

The South zone is developed mainly in basaltic volcanics and related to small, strongly-sericite altered quartz-monzodiorite dykes. Zone dimensions are about 400 m x 300 m in area, and mineralisation extends to depths of more than 500m.

Copper mineralisation in the South zone is associated with stockworks of thin (usually <10 cm) quartz–sulfide veins. In surface exploration pits and trenches, veins occur as steep, north west striking, strongly sheeted sets. However, veins intersected in drillholes have a stockwork style and lack the strong preferred orientation visible in surface exposures. In the South zone, mineralisation is hosted dominantly in quartz monzodiorite in the southwestern part of the zone, in basalt throughout the central part, and in a minor zone of basaltic tuff and breccia on the northern margin. Contorted quartz veins are present, but there is no clearly defined zone of high quartz vein density such as at the Southwest zone. As a result, fracture-controlled sulfide veins are minor, and sulfides are present dominantly as disseminated chalcopyrite, bornite, and molybdenite. Chalcopyrite is the principal copper sulfide, but in higher grade areas bornite locally exceeds chalcopyrite in abundance. Magnetite occurs as disseminations and as veins; small zones with elevated gold values are found locally.

The Wedge zone has an irregular, crescent-like shape. The zone is about 500–600 m at the widest point in the north, tapering to a width of about 200 m in the south, and approximately 1,400 m long. Mineralisation extends to depths of over 500 m.

The Wedge zone contains a core of high sulfidation mineralisation hosted principally in basaltic tuff and breccia, grading downward and southward into chalcopyrite mineralisation in basalt and quartz-monzodiorite host rocks. High sulfidation mineralisation consists of pyrite, chalcopyrite, bornite, enargite, covellite, and primary chalcocite in advanced argillic-altered host rocks. Higher grades of copper (>0.8% Cu) occur in a shallowly east dipping zone in the upper hundred metres of basaltic tuff/breccia unit. Gold is absent except locally in drillholes adjacent to the South Fault. Mineralisation is open to the north.

The Central zone is about 2,300 m wide and tapers from about 200 m in length in the east to more than 600 m to the west. Mineralisation extends to depths of over 500 m.

Mineralisation in the Central zone is characterised by an upward-flaring, high- sulfidation zone that overprints and overlies porphyry-style chalcopyrite–gold mineralisation. A secondaryenriched supergene chalcocite blanket tens of metres in thickness overlies the highsulfidation covellite–pyrite zone. Chalcopyrite–gold mineralisation is dominant on the southern and western margins of the Central zone within either basalt or quartz?monzodiorite adjacent to intrusive contacts with basalt. Higher grades are associated with zones of intensely contorted quartz stockwork veins, where the gold (ppm) to copper (%) ratios reach 2:1. Peripheral, lower grade mineralisation has gold to copper ratios of less than 1:1. Hematite, pyrite, chalcopyrite, bornite, magnetite, and gold are disseminated in the zone and are also found as fracture fillings. Hematite is pervasive and overprints magnetite.

The Bridge zone has a triangular shape, tapering from about 500 m wide in the north to about 30 m wide in the south, and is approximately 250 m in length. Mineralisation extends to depths of over 500 m.

Low-grade copper mineralisation is characterised by lower vein densities than in surrounding zones and is hosted in chlorite and epidote-altered basalt and lesser sericite- and albitealtered quartz-monzodiorite. Magnetite veinlets post-date the quartz veins but predate the main sulfide event.

The Hugo Dummett deposits, Hugo North and Hugo South, contain porphyry-style mineralisation associated with quartz-monzodiorite intrusions, concealed beneath a sequence of Upper Devonian and Lower Carboniferous sedimentary and volcanic rocks. The deposits are highly elongated to the north–north-east and extend over 3 km. The dividing line between the two deposits is 4,766,300 mN, a location marked by the thinning and locally discontinuous nature of the high grade copper mineralisation (defined by greater than 2.0% copper).

The Hugo North deposit is virtually contiguous with the Hugo South deposit and lies within a similar geological setting. The two deposits are separated by a 110°-striking, 45°–55° north dipping fault that displaces Hugo North vertically down a modest distance from Hugo South. Hugo North extends from +500 masl to depths well below 400 masl, has a strike length in excess of 1,800 m, and is 500 m wide.

The highest-grade copper mineralisation in the Hugo North deposit is related to a zone of intensely stockworked to sheeted quartz veins known as the QV90 zone, so named because >90% of the rock has >15% quartz veining. The high-grade zone is centred on thin, east dipping quartz monzodiorite intrusions or within the apex of the large quartz-monzodiorite body, and extends into the adjacent basalt. In addition, moderate-to high-grade copper and gold values occur within quartz-monzodiorite below and to the west of the intense vein zone, in the Hugo North gold zone. This zone is distinct and has a high Au (ppm) to Cu (%) ratio of 0.5:1.

The Heruga deposit is the most southerly of the currently known deposits at Oyu Tolgoi. The deposit is a copper–gold–molybdenum porphyry deposit and is zoned with a molybdenumrich carapace at higher elevations overlying gold-rich mineralisation at depth. The top of the mineralisation starts 500–600 m below the present ground surface.

Quartz-monzodiorite intrusions intrude the Devonian augite basalts as elsewhere in the district, and again are considered to be the progenitors of mineralisation and alteration. Within Heruga itself, quartz-monzodiorite intrusions are small compared to the stocks present in the Hugo Dummett and SOT areas, perhaps explaining the lower grade of the Heruga deposit. Non-mineralised dykes, which make up about 15% of the volume of the deposit, cut all other rock types. However, the quartz-monzonite body appears to flare to the east and form

The initial investment decision to construct Phase 1 of Oyu Tolgoi was made in 2010. Mining of the Oyut deposit started in 2012 using open pit mining methods. The Oyut open pit mine currently has an ore production rate of about 40 Mtpa. Part of the initial investment decision to develop Phase 1, included continued investment into the development of the Hugo North underground mine as a block caving operation (Phase 2 of the Project). Development of Phase 2 commenced in July 2016.

The current production rate from the Oyut open pit continues until 2023, after which it will be progressively reduced as production builds up from underground. Open pit mining will continue in parallel with Hugo North Lift 1 to keep the Oyu Tolgoi concentrator operating at its design capacity. Following depletion of Lift 1, production from the Oyut open pit will be increased to meet mill capacity.

Oyut open pit mining
Open pit mining is carried out using conventional drill, blast, load, and haul methods and is conducted 24 hours per day, 365 days per year.

The slope design criteria which are based on the Golder 2012 Study are summarized as follows:
- Single, 15 m bench height (or batter) in all rock slopes.
- Maximum bench face angle of 65° in weathered zone.
- Average bench face angle of 70° in bedrock zone.
- Minimum berm width of 7.5 m
- Where structural controls are present, berm widths of 9 m, 12 m, and 15 m are applied to be consistent with recommended inter-ramp angles.
- Geotechnical catch berms at least 15 m wide at 90 m intervals where there are no ramp traverses to break the height of the slope.
- Ramp width of 40 m.

The current open pit design utilized an industry standard Lerchs-Grossmann (LG) pit optimization approach to produce a nested set of pit shells, which represent the best economic sequence of pit phase development. The nested pit shells are used to guide the design of practical pit phases and the sequence of mining. The pit optimization process used Measured and Indicated resource classification blocks only for potential revenue generation. Inferred mineral resources were treated as waste.

The open pit mine is a conventional shovel-truck operation. The primary shovel fleet includes electric rope shovels and diesel hydraulic shovels with 290 t class haul trucks. The primary fleet is supported by front-end loaders and ancillary equipment.

Underground mining
Sinking of a multipurpose shaft (Shaft 1) to access the Hugo North deposit began in February 2005 and reached its final depth in January 2008. A total of 15 km of lateral development was completed from Shaft 1 by August 2013, when the underground project was placed into careand-maintenance.

The Phase 2 commenced with sinking activities in Shaft 2 (a multipurpose shaft) and Shaft 5 (an exhaust ventilation shaft) and development of accesses to the Lift 1 footprint. Shaft 2 sinking and installation of fixed guides and other equipment was completed in October 2019 and became operational in December 2019. Shaft 2 is now the main access for personnel and materials and for rock hoisting. Previously, all personnel, materials, and rock hoisting were carried out through Shaft 1. Sinking of Shaft 5 was completed in early 2019.

The Hugo North mine plan envisages construction of a block cave operation with a nameplate production rate of 33 Mtpa. Lift 1 is planned to be extracted in three panels (Panel 0, Panel 1, and Panel 2). Mining is planned to start in Panel 0 followed by Panel 2 and Panel 1. Hugo North Lift 2 is currently planned as a block cave operation with the footprint approximately 400 m below Lift 1. Development of Lift 2 is at a conceptual stage.

Mine haul trucks currently dump ore from the Oyut open pit directly to the dump pocket of the primary gyratory crusher. Crushed ore is then conveyed approximately 2.7 km to a coarse ore stockpile at the plant site.

Ore reclaimed from the course ore stockpile is currently fed to two comminution lines, each consisting of a SAG mill, two parallel ball mills, and associated downstream equipment. Cyclone overflow from the circuit at a P80 of 140–180 µm reports to the rougher flotation cells.

The typical ball load in the SAG mills is 15% to 18% by volume. The total mill loading is 28% and the rotational speed is 75% to 80% of critical speed. The SAG mill product has a top size of 85 mm, which discharges from the mill through a trommel screen with 9 mm openings. The oversize is screened and washed over a vibrating screen and reports to the pebble crusher circuit. Between 10% to 20% of the feed circulates from the SAG mills to the pebble crushers, depending on ore type and grate condition.

Undersize from the trommel screen and vibrating screen is combined and transferred to the ball mill feed distributors at an expected P80 of 2,400 µm. The washed pebbles are conveyed to a surge bin ahead of three cone crushers. Self-cleaning electromagnets on the conveying system protect the cone crushers from tramp metal and crushed pebbles are transferred to a surge bin before being fed proportionately to the SAG mill feed conveyors via belt feeders.

A variable-speed pump, installed at each ball mill discharge box, feeds a cluster of eight by 800 mm diameter Cavex cyclones.

Plant throughput varies with ore hardness and is also limited by the volumetric capacity of various components of the plant.
The volumetric limit of the existing concentrator is determined by the following:
• Rougher cell tailings valve capacity, which limits hourly throughput to about 5,800 t/h and occurs when the rougher cell tailing valves are completely open but restricting flow to avoid overflowing into the concentrate launders.
• Concentrate handling equipment, which limits concentrate handling to about 3,000 t/d (about 5,800 t/h of ore feed at peak Oyut head grades).
• Tailings thickener settling rate limit, which is not well defined, but is thought to restrict plant throughput to about 5,800 t/h.

Rougher flotation
The design of the existing 160 m3 mechanical tank cells. Retention time in the rougher circuit is 25 minutes at design throughput rates.

Cleaner-scavenger flotation
Concentrate from the rougher flotation cells is re ........