The Cortez deposits are Carlin-type sedimentary rock-hosted deposits. Carlin deposits form as structurally and/or stratigraphically controlled replacement bodies consisting of stratabound, 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.

Host rocks are most commonly thinly bedded, silty or argillaceous carbonaceous limestone or dolomite, commonly with carbonaceous shale. Although less mineralized, non-carbonate siliciclastic and rare metavolcanic rocks can locally host gold that reaches economic grades. Felsic plutons and dikes may also be mineralized at some deposits.

The deposits are hydrothermal in origin and are usually structurally controlled. The carbonate host rocks are part of an autochthonous miogeoclinal carbonate sequence (Lower Plate) exposed as tectonic windows beneath the Roberts Mountain allochthon. The lower Paleozoic allochthonous (Upper Plate) rocks are siliciclastic eugeoclinal rocks that were displaced eastward along the Roberts Mountain Thrust over younger units during the upper Paleozoic Antler Orogeny. Carlin deposits are localized along the thrust. Current models attribute the genesis of the deposits to:

• epizonal plutons that contributed heat and possibly fluids and metals;
• meteoric fluid circulation resulting from crustal extension and widespread magmatism;
• metamorphic fluids, possibly with a magmatic contribution, from deep or mid-crustal levels;
• upper crustal orogenic-gold processes within an extensional tectonic regime.

The past-producing Buckhorn gold-silver mine and the Hilltop gold project are examples of different styles of mineralization in the Cortez district. Buckhorn is a typical example of a lowsulphidation epithermal system while Hilltop is an intrusive-related deposit. Hilltop is an Eocene-age system clearly associated with 38 Ma to 39 Ma felsic gold-copper porphyries, while Buckhorn is a Miocene occurrence, deposited during the initiation of 15 Ma to 17 Ma Northern Nevada Rift extension and associated bimodal volcanism.

Mineralization consists primarily of submicron to micrometre sized gold particles, very fine sulphide grains, and gold in solid solution in pyrite. Mineralization occurs 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.

Argillization is characterized by removal of kaolinite and growth of illite in proximity to controlling faults. Arsenic, antimony, iron, and copper accompany gold in north–northwest oriented fault structures and silver, arsenic, manganese, and lead in northeast trending faults.

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.

Mineralization is predominantly characterized by oxides, and sulphidic and carbonaceous refractory material. Carbon content in the deposits is highly variable and occurs generally in the Devonian Wenban Limestone and Roberts Mountain Formation.

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 ........