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 Southwest zone is now being mined by an open pit mine. Central zone ores will also be mined by open pit mining methods. Hugo North will be mined by underground block caving.

The Hugo Dummett underground deposit will be mined by block caving; a safe, highly productive, cost-effective method. The deposit is comparable in dimension and tonnage to other deposits currently operating by block cave mining elsewhere in the world. The mine planning work has been prepared using industry standard mining software, assumed metal prices as noted in the tables.

Oyu Tolgoi employs a conventional SAG mill / ball mill / grinding circuit (SABC) followed by flotation, as shown in the basic flowsheet. Phase 1 (OT LLC’s open pit and concentrator), which commenced production in 2013, uses two grinding lines, each consisting of a SAG mill, two parallel ball mills, and associated downstream equipment to treat up to 100 ktpd of plant feed from the Southwest Zone pit.

Combined with Hugo North underground production, concentrator feed rates will be as high as 121 ktpd, which represents the tailings handling capacity of the plant. The Phase 2 (Lift 1) concentrator development programme optimises the concentrator circuit to enable it to maximise recovery from the higher grade Lift 1 plant feed. The Phase 2 concentrator expansion will include:

• The addition of a fifth ball mill to achieve a finer primary grind P80 of 150–160 µm for a blend of Hugo North and open pit feeds, compared to 180 µm for Southwest Zone.
• Additional roughing and cleaner column flotation capacity to process the higher level of concentrate production when processing the higher grade Hugo North plant feed.
• Additional concentrate dewatering and bagging capacity.