The mineral deposits at Oyu Tolgoi lie in a structural corridor where mineralization has been discovered over a 26 km strike length. Four deposits hosting Mineral Resources have been identified: Oyut, Hugo North, Hugo South, and Heruga.

Tolgoi copper-gold deposits currently comprise, from north to south:
• Hugo North – The Hugo Dummett Deposit north of approximately 4766300 N (or the 110 Fault).
• Hugo South - The Hugo Dummett Deposit south the 110 Fault.
• Oyut - The Oyut Deposit includes the Southwest Oyu, South Oyu, Wedge, Central Oyu, Bridge, Western, and Far South zones within mining licence MV-006709.
• Heruga – is within the area governed by the arrangements between Oyu Tolgoi LLC and Entrée LLC except for a small northern portion that lies within the Oyu Tolgoi mining licence MV-006709.

The Oyut deposit is currently being mined as an open pit using conventional drill, blast, load, and haul methods. The Hugo North deposit is currently being developed as an underground mine using the block caving mining method. A staged approach is envisaged for developing the Hugo North deposit, involving mining two block cave lifts (Lift 1 and potentially Lift 2). Mineral Reserves have been estimated for Lift 1, which comprises three panels (Panel 0, Panel 1, and Panel 2).

Hugo South and Hugo North
The Hugo Dummett deposits, Hugo North and Hugo South, contain porphyry-style mineralization 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 northeast and extend over 3 km.

The Hugo North zone is virtually contiguous with the Hugo South zone and lie within a similar geological setting. The two deposits are separated by a 110°-striking, 45° to 55° north-dipping fault that displaces Hugo North vertically down a modest distance from Hugo South. The dividing line between the two deposits is approximately 4766300 N, a location marked by the thinning and locally discontinuous nature of the high-grade copper mineralization (defined by greater than 2% copper). The east-striking 110° Fault for the projections of the major faults in the area of the Hugo Dummett deposits), delineates the gold- and copper-rich zone hosted in augite basalt and quartz?monzodiorite of the Hugo North deposit from the more southerly, goldpoor, ignimbrite- and augite basalt-hosted mineralization at Hugo South.

The quartz monzodiorite bodies are contemporaneous with alteration and mineralization. The quartz monzodiorite is considered to be the progenitor porphyry, and two zones are distinguished on the basis of alteration characteristics and position within the deposit.

The highest-grade copper mineralization 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 to 1.

Oyut deposit
The Oyut deposit includes the most mineralized domain called Southwest Oyu (Southwest), but also includes South Oyu (South), Wedge, and Central Oyu (Central) domain and several smaller, fault- bounded zones. The open pit incorporates most of these domains. They form contiguous sectors of mineralization representing multiple mineralizing centres, each with distinct styles of mineralization, alteration, and host rock lithology. The boundaries between the individual 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 downfaulted or uplifted relative to neighbouring zones.

Southwest Oyu zone
The Southwest Oyu zone is a gold-rich porphyry system characterized by a south-west–plunging, pipe-like geometry that has a vertical extent of as much as 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 by 2 km.

Over 80% of the deposit is hosted by massive to fragmental porphyritic augite basalt of the Upper Devonian Alagbayan Group, with the remainder hosted by intra-mineral, Late Devonian quartz- monzodiorite intrusions. The quartz-monzodiorite intrusions form irregular plugs and dykes related to several distinct phases:
• Early, strongly altered quartz-veined dykes mainly limited to the high-grade central Central deposit core.
• Superimposed younger fragmental dykes entraining early quartz vein clasts but lacking strong sulfide mineralization.
• Voluminous massive quartz-monzodiorite (informally referred to as QMD) containing weaker mineralization, flanking and underlying the high-grade deposit core.

Several phases of post-mineral dykes cut the Southwest zone. Most of the dykes belong to the rhyolite, hornblende-biotite andesite, or biotite granodiorite intrusive phases. Dykes commonly have steep dips, and many are localized along faults. The rhyolite dykes tend to strike west to west-northwest in the deposit core and north-east when emplaced along major faults. Hornblende- biotite andesite dykes strike east-north-east except where they intrude along the major north- east trending faults.

Central zone
The Central zone is about 2,300 m wide and tapers from about 200 m long in the east to more than 600 m to the west. Mineralization extends to depths of over 500 m. The Central zone is hosted within a swarm of feldspar-phyric quartz-monzodiorite intrusions, emplaced into porphyritic augite basalt and overlying basaltic tuff of the Alagbayan Group. The basaltic tuff is in turn overlain by unmineralized sedimentary and mafic volcanic rocks of the Alagbayan Group unit DA4, which dip moderately to the east (30–60°).

South Oyu zone
The South Oyu zone is developed mainly in basaltic volcanics and related to small, stronglysericite altered quartz–monzodiorite dykes. Zone dimensions are about 400 m by 300 m in area, and mineralization extends to depths of more than 500 m.

Heruga
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 molybdenum-rich carapace at higher elevations overlying gold-rich mineralization at depth. The top of the mineralization starts 500–600 m below the present ground surface. The deposit has been drilled over a 2.3 km length, is elongated in a north–northeast direction and terminates to the north on an east– northeast-trending regional fault with 500 m of apparent dextral displacement. The deposit is transected by a series of north–northeast trending vertical fault structures that step down 200 m to 300 m at a time to the west and have divided the deposit into at least two structural blocks.

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.

Rougher concentrate discharges into the combined concentrate launder, t ........