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 ........

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 planne ........

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.

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