The Oyu Tolgoi copper-gold porphyry deposits are situated in a poorly exposed inlier of Devonian mafic to intermediate volcanic, volcaniclastic, and sedimentary rocks that have been intruded by Devonian to Permian felsic plutons. These rocks are unconformably overlain by poorly consolidated Cretaceous sedimentary rocks and younger unconsolidated sedimentary deposits.

Oyu Tolgoi consists of Oyut, Hugo Dummett (Hugo South and Hugo North) and Heruga deposits. The
Oyu Tolgoi copper-gold deposits currently comprise, from north to south:
• Hugo North – The Hugo Dummett Deposit north 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.
• 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.

Hugo Dummett deposits
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.

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.

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.

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 (Figure 7.6). 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°).

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.

Porphyry copper mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated forms in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some breccias containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry copper and lithocap levels, mainly because many such phreatomagmatic breccias formed late in the evolution of systems, and the explosive nature of their emplacement fails to trap mineralizing solutions.

Alteration zones in porphyry copper deposits are typically classified based on mineral assemblages. In silicate-rich rocks, the most common alteration minerals are potassium (K) - feldspar, biotite, muscovite (sericite), albite, anhydrite, chlorite, calcite, epidote, and kaolinite. In silicate-rich rocks that have been altered to advanced argillic assemblages, the most common minerals are quartz, alunite, pyrophyllite, dickite, diaspore, and zunyite. In carbonate rocks, the most common minerals are garnet, pyroxene, epidote, quartz, actinolite, chlorite, biotite, calcite, dolomite, K-feldspar, and wollastonite. Other alteration minerals commonly found in porphyry copper deposits are tourmaline, andalusite, and actinolite. Figure 8.2 shows the typical alteration assemblage of a porphyry copper system.

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

Hugo North and Hugo South are also known as the Hugo Dummett deposits. The Oyut deposit, formerly known as Southern Oyu Tolgoi, is currently mined as an open pit using a conventional drill, blast, load, and haul method. The Hugo North deposit is currently being developed as an underground mine.

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.

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.

Underground mining
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.

The new Lift 1 footprint design relocates the ore passes and loading chutes away from the poorer ground associated with the lower splay fault and locates them in the north and south pillars. The north and south rim drives are also located in the pillars.

When ore passes in the north and south pillars are complete, each extraction drive will have access to two tipping points. Average LHD unit (scoop) tramming distances are slightly longer than in the 2016 design. However, the arrangement provides easier access to alternative tipping points when a pass and loading chute is unavailable for maintenance or repair. The undercutting direction in Panels 1 and 2 remains the same as the 2016 Feasibility Study, but in the 2020 design, the undercut advances away from the stable northern and southern pillars.

Increasing drawbell spacing improves the stability of the extraction level in zones of poorer rock mass quality. Flow modelling, using the latest modelling techniques available, was carried out to assess the impact of the spacing change on ore recovery. The modelling indicated that there will be minimal change in ore recovery resulting from the wider spacing. Despite significant advances in flow modelling and calibration of models against actual performance in other mines, the effect of drawpoint spacing on ore recovery is not well understood. When caving operations commence, the impact of drawbell spacing on material flow can be calibrated against actual performance in Panel 0. This will be facilitated by data from the extensive flow marker network being installed prior to the start of caving.

If, after caving commences, it is determined that rock mass conditions and abutment stresses are more favorable than currently envisaged, it may be possible to recover part of the mineral resource in the pillars from the current extraction level by extending Panel 0 or by modifying the start of Panels 1 and 2. Alternatively, a recovery level below the current extraction level could be constructed to recover some ore from the pillars and from areas in Panel 0 where recovery was incomplete because of poor rock mass conditions.

The mine design consists of 211 km of lateral development, five shafts, and two decline tunnels from surface. The primary life-of-mine ore-handling system will transport ore to surface by a series of conveyors to surface.

The Lift 1 mining levels are approximately 1,300 m below surface. Six distinct levels will be developed to mine Lift 1. The apex level is 17 m above the undercut level, which in turn is 17 m above the extraction level (floor to floor). The haulage level is 44 m below the extraction level. In the footprint area, the exhaust ventilation level lies between the extraction and haulage levels.

Over 64 km of lateral development and over 5 km of vertical development have been completed since the Project commenced.

The planned conveyor to surface system comprises three identical 2,202 m long 1.6 m wide steel cord conveyors operating at 6 m/s and capable of conveying the full planned production rate of 32 Mtpa. The conveyors will be installed in a 6.0 m wide by 5.4 m high conveyor decline. The conveyor system has a vertical lift of 1,180 m, and each belt is driven by two 6 MW motors directly coupled to the drive. When operational, the conveyor will deliver all ore from Hugo North to surface, and the Shaft 2 rock hoisting system will operate as a standby system.

A service decline, driven parallel to the conveyor decline, will provide regular access points to the conveyor decline, mine access for materials and maintenance, and act as an alternative means of egress from the mine.

Prior to commissioning Shaft 2, all personnel, equipment, and materials for mine development were delivered to the underground workings via Shaft 1. Shaft 2 now provides the main access to the mine for personnel and materials. Two main access drives connect the Shaft 2 station to the main workshops, offices, and extraction level. Traffic travels in a clockwise direction to and from the shaft. Ramps from the main access drives provide access up to the undercut and apex levels, and down to the haulage and ventilation levels. The conveyor decline from surface will provide alternative access for materials.

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. Crushed ore from Hugo North will initially be trucked from the Shafts (Shafts 1 and 2) to the open pit crusher and then to the coarse ore stockpile. As underground production increases, a conveyor will be commissioned from the Shaft 2 headframe ore bin to the existing overland conveyor which feeds the existing coarse ore stockpile. When the underground incline conveyor to surface is commissioned, ore from the incline conveyor will discharge to a new conveyor running parallel to the open pit overland conveyor. Both overland conveyors will discharge to the coarse ore stockpile. Ore from the open pit will continue to be trucked to the surface crusher and transferred to the coarse ore stockpile via the existing overland conveyor.

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 Phase 2 modifications include the installation of a fifth ball mill and associated equipment. The parameters used to determine the design parameters of the Phase 2 comminution circuit have been determined by various testwork programs.

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.

As part of the concentrator modifications, a new duty and standby pump will be installed at each SAG mill pump box to transfer slurry to the new ball mill discharge pump box. The new ball mill will operate in parallel with the four existing ball mills. The mills are charged with 65 mm forged steel balls, and typically operate with 33% ball charge volumes.

A variable-speed pump, installed at each ball mill discharge box, feeds a cluster of eight by 800 mm diameter Cavex cyclones. The projected cyclone overflow will contain 33% solids w/w at a target P80 of 140 µm to 160 µm.

The Phase 1 concentrator commenced operations in January 2013 and reached its nameplate throughput of 32 Mtpa in September 2013.

As ore from Hugo North Lift 1 becomes available, concentrator modifications (the Phase 2 modifications) will be required to process the blend of higher-grade ore from Hugo North with lower grade ore from the Oyut open pit. Significant study and design work have been carried to optimize the modifications. On depletion of Hugo North ore, the concentrator will be able to revert to processing the remaining mineral reserves in the Oyut open pit. Minimal allowance is included in the modification program for further expansion of the concentrator, whilst ensuring that the planned modifications do not interfere with any future possible expansion.

The Phase 2 modification of the Oyu Tolgoi concentrator include:
• A fifth ball mill to achieve a finer primary grind P80 of 140 µm to 160 µm for a blend of Hugo North and open pit ores.
• Addi ........