The Cortez deposits are “Carlin” style sedimentary rock-hosted and porphyry/epithermal deposits. Carlin deposits form as structurally and/or stratigraphically controlled replacement bodies consisting of strata-bound, tabular, disseminated gold mineralization occurring in Silurian-Devonian carbonate rocks. Deposits are localized at contacts between contrasting lithologies, metamorphosed to varying extents. They can also be discordant or breccia-related. The deposits are hydrothermal in origin, are usually structurally controlled and at Cortez are hosted in Lower Plate carbonate strata exposed by two erosional windows through allochthonous Upper Plate siliclastic units; the windows are on either side of Crescent Valley.

Mineralization consists primarily of submicron to micrometre-sized gold particles, very fine sulphide grains, and gold in solid solution in pyrite. Gold is disseminated throughout the host rock matrix in zones of silicified and decarbonatized, argillized, silty calcareous rocks and associated jasperoids. Gold may occur around limonite pseudomorphs of authigenic pyrite and arsenopyrite. Major ore minerals include native gold, pyrite, arsenopyrite, stibnite, realgar, orpiment, cinnabar, fluorite, barite, and rare thallium minerals. Gangue minerals typically comprise fine-grained quartz, barite, clay minerals, carbonaceous matter, and late-stage calcite veins.

In the Cortez district, the favoured host rocks for gold mineralization are the Wenban Limestone, followed by the Horse Canyon and Roberts Mountain formations. Mineralization reflects an interplay between structural and lithological ore controls in which hydrothermal solutions from intrusives moved to favourable porous decalcified limestone. Mineralized host rocks are predominantly characterized as oxides, along with sulphidic and carbonaceous refractory material. Carbon content in the deposits is highly variable and occurs generally in the Wenban and Roberts Mountain Formations.

Supergene alteration extends up to 656 ft depth resulting in oxide ores which overlie the refractory sulphides. Alteration has liberated gold by the destruction of pyrite and resulted in the formation of oxide and secondary sulphate minerals which include goethite, hematite, jarosite, scorodite, alunite, and gypsum.

Open pit ore is mined by a conventional shovel-truck process at the rate of approximately 20 million tons per year with a LOM stripping ratio of 5.1:1 (waste to ore ratio). Mining occurs in two separate areas approximately 12 mi apart; Cortez Hills open pit (CHOP) and Pipeline, composed of the Pipeline and Crossroads open pits.

CHOP operation is currently limited to the final benches of the CHOP. The Pediment open pit was completely mined and waste from the CHOP was dumped to backfill this pit with a ramp connecting to access the CHOP.

The Pipeline Complex open pit operations include the Pipeline Pit and Crossroads phases. Pipeline ore and waste haul horizontal distances are 10,000 ft and 7,000 ft, respectively.

The open pit designs include consideration of the dewatering requirements and the different types of waste that may be encountered during stripping. At the CHOP, a protected cultural area was declared on the northeast side of the pit, which has limited the pit expansion on that side.

The following design criteria are used for the Pipeline and Crossroads open pits:

• Pipeline 50 ft bench height;
• Crossroads 40 ft bench height;
• Alluvium inter-ramp wall slope angle varying from 35° to 42°;
• Fresh rock inter-ramp wall slope angles ranging from 37° to 45° depending on material type and wall orientation;
• Bench face angles ranging from 55° to 70° depending on material type;
• 10% maximum haul road grade; and
• 120 ft wide haul roads.

The underground mining is all mechanized, with large scale equipment using a combination of cut and fill mining with cemented backfill and primary and secondary longhole stoping with cemented and uncemented backfill.

In the cut and fill stopes, headings are cycled using conventional drill/blast/muck/support on a round by round basis. Material is loaded into haul trucks and hauled to surface. Top-cut headings are typically 15 ft by 15 ft and undercut widths vary from 18 ft to 30 ft (depending on ground conditions and ore geometry) with 15 ft heights. Wider ore cuts are being implemented to improve productivity and mine production.

Longhole stopes are planned to be mined using 60 ft sublevels with a series of primary and secondary longhole stopes. Development will typically be 15 ft high and the primary and secondary stopes are planned to be 25 ft wide. Primary stopes will be mined in short sections to maintain stability and each section will be filled with CRF after mining. Secondary stopes will be mined after primary stopes have been completed. Where the ore zone is more than 60 ft high, the stopes will be stacked vertically, as needed, to extract the ore. Long holes are planned to be vertical down holes from the upper sublevel.

The CHUG is accessed by twin declines driven to the upper level of the deposit. Access for the Middle and Lower zones is by a spiral ramp adjacent to the orebody. The twin declines are interconnected at regular intervals and these connections contain air doors to separate the intake and exhaust airways. At the portal area, there are permanent offices and maintenance facilities together with a backfill plant and shotcrete batch plant. An additional set of twin declines (RFD), for access to the Lower Zone and the Deep South Zone are being developed. The declines will provide additional means of egress as well as house a conveyor for ore and waste transport from the Lower and Deep South zones. The mining is all mechanized with large scale equipment.

The plan is to expand the production tonnage rate through the mining of the Middle and Lower zones and then to further increase the production rate with the addition of the Deep South Zone as a stand alone mining zone.

Historically, Cortez ore was processed in a 1,800 stpd CIL plant called Mill No. 1, as crushed ore on heap leach pads, and refractory ore was pre-oxidized in a circulating fluid bed roaster. The crushing and leaching circuits were built in 1969; the roasting circuit was completed in 1990. The Mill No. 1 processing facilities were placed on care and maintenance in 1999.

Ore from Cortez can either be processed on site in the oxide processing facilities or transported to the Goldstrike operation for refractory ore treatment.

OXIDE ORE MILLING
Mill No. 2, or the Pipeline mill, was commissioned in 1997 to process oxide ore from the Pipeline open pit mine. The treatment plant currently includes crushing, SAG mill, ball mill, grind thickener, carbon-in-column (CIC) circuit for the grind thickener overflow solution, CIL circuit, tailings counter-current-decantation (CCD) wash thickener circuit, carbon stripping and reactivation circuits, and a refinery to produce gold doré. Plant throughput is up to 16,000 stpd. depending on the hardness of the ore being processed. The plant is permitted for an
annual average of 15,000 stpd.

Electro-winning takes place in six EW cells. After loading with gold, the stainless steel cathodes are washed with high pressure water. The gold sludge from the EW cells is dewatered in a filer press. The stripped cathodes are returned to the EW cells. The filter cake is dried, melted in an induction furnace, and poured into doré bars. The stripped carbon is transferred from the elution column over a screen and into the reactivation furnace feed tanks. Carbon reactivation is accomplished in two propane-fired kiln-type furnaces.

OXIDE ORE HEAP LEACHING
Low-grade oxide material is leached as ROM ore on three prepared double-lined leach pads. Pregnant solution from the leach pads is fed to CIC columns for gold recovery. The loaded carbon from the heap leach operation is transported to the mill for gold recovery in the carbon elution and EW circuits and the refinery. The carbon is also acid washed and regenerated at the mill. Make-up solutions come from the mill or mine dewatering wells to account for evaporative losses and ore saturation requirements.

Area 28 heap leach circuit has a water balance that is interlinked with the Pipeline mill circuit since it uses the tailings pond under-drain solution as leach solution and excess pad effluent is processed in the mill CIC circuit. Area 28 is at maximum capacity for ore stacking and is no longer an operating leach pad. The Area 30 heap leach circuit is independent of the Pipeline mill. Ore delivery to the pad recommenced in 2013. An expansion of seven million square feet is planned for 2019 with ore deliveries extending through 2031. Area 34 heap leach is a third pad that was designed to treat CHOP ore. The first cells were placed under leach during March 2011 and ore deliveries are scheduled to continue through June 2019 based on the current LOM plan.

REFRACTORY ORE TREATMENT
Ores that have an CNAA/FA ratio of less than 50% are transported to Goldstrike for processing in the roaster. Haulage of refractory ore to Goldstrike is currently limited by permit to 1.8 million stpa. At Goldstrike, the ore will be processed in the roaster followed by a CIL circuit. Goldstrike also has the opportunity to process refractory ore using the TCM process, which includes pressure oxidation (POX) followed by resin-in-leach (RIL) with calcium thio-sulphate (CaTS), however, this option is not contemplated for Cortez ore at this time.

Fluid bed roasters were constructed at Goldstrike in 1999 to treat carbonaceous refractory ores that could not be treated effectively by the POX circuit that existed at Goldstrike at that time. The roasters process double refractory (i.e., sulphide refractory and total organic carbon refractory) feed with a moderate carbonate content to help fix the sulphur dioxide that is generated by the oxidation reactions. Minimum sulphide sulphur levels are required to maintain sufficient bed temperatures. When the ore does not contain the minimum required sulphide sulphur content, sulphur prills are used as a supplement to provide the energy necessary to operate the roasters.

The roasters use oxygen to oxidize organic carbon and sulphide sulphur prior to processing the neutralized slurry in a conventional CIL circuit. There are two circuits, each include crushing and dry grinding followed by a two stage roaster, a calcine quenching tank, and dust and gas handling operations. The quenched gas goes to the gas cleaning stage, which is common to both roaster circuits, and the calcine is processed in common neutralization and CIL circuits. The loaded carbon is sent to the main refinery for elution, regeneration, and production of the gold doré.