The Turquoise Ridge Underground deposit is a typical Carlin-type deposit and is characterized by structurally and stratigraphically controlled, sediment-hosted, replacement deposits containing micron-sized gold.

The mineralization at Turquoise Ridge Underground is located in two main zones, North and South. In the North Zone, the strongest mineralization appears to be associated with BPH Limestone, Black Fragmental rocks, and carbonaceous rocks within the Proximal Slope unit. Stratigraphic contacts between these units and contacts with the Cretaceous dikes are important primary controls on mineralization. Powder Hill rocks, Distal Slope rocks, basalts, and dikes are insignificant hosts for mineralization in the north.

The North Zone of Turquoise Ridge Underground extends over an area of 800 m x 800 m and at present consists of over 33 mineralized domains. The domains are divided into the following groups based on mode/control of mineralization:
• Stratigraphic/stratabound
• Cross-stratigraphic, structurally controlled
• East-west structural
• Other

North Zone stratigraphic mineralized domains exhibit strike lengths in excess of 305 m with typical strike dimensions in the 61 m to 152 m range. Down dip lengths of over 305 m are common. Being stratigraphically controlled, the mineralized domains and bedding have a general north-northwest trending strike, of approximately 330° to 340°, and dip shallowly to the east (between 25° and 45°). Mineralization is typically in the order of six metres to nine metres thick, but in some instances, may pinch down to less than three metres or swell out to 30 m in thickness. North Zone cross-stratigraphic domains have been modelled at near perpendicular angles to the bedded domains. This mineralization cuts across bedding and is grouped as structural, as mineralization may be driven by low angle shearing, faulting, or joint sets. General trends of these domains are a north-northwest strike (340° to 360°) and a west dip of 10° to 20°. Widths of these domains may be in excess of 30 m.

Most of the North Zone domains are grouped as east-west stratigraphic and are economically important or currently being mined. Two of the domains are large, steeply dipping, and sheetlike with strike lengths in the order of 152 m and 305 m. Dip dimensions are approximately 48 m and 110 m, respectively. Thickness is variable but averages six metres to 12 m.

The remaining five domains in the “Other” category are irregular sheet-like bodies with various orientations. Dimensions range from 152 m to 198 m in strike length, 100 m to 122 m in dip length, and thicknesses from three metres to over 30 m.

Mineralization in the South Zone is largely restricted to the Powder Hill unit, with only minor amounts present in the upper parts of the Proximal Slope unit. Fragmental rocks of the Powder Hill unit are the most important host for mineralization, followed by tuffaceous intercalated limestones and mudstones, then by carbonaceous intercalated mudstones and limestones of the Proximal Slope unit. Mineralizing fluids appear to have interacted with favourable host rocks via discrete fault and fracture zones, although specific pathways are usually not readily identifiable.

The South Zone extends over an area of 610 m by 305 m and at present consists of over 15 mineralized domains. The domains are divided into the following groups based on mode/control of mineralization:

• Cross-stratigraphic, structurally controlled
• Steeply-dipping Strataform
• Moderately-dipping Strataform

The six cross-stratigraphic bodies strike north and are generally steeply dipping to the west or nearly vertical. Strike lengths vary from 30 m to 198 m with an average length of 122 m. Dip lengths vary from 61 m to 152 m with an average dip length of 99 m. Thicknesses average 20 m.

The South Zone Strataform domains tend to strike north and dip to the east at approximately 30° to 60°. This orientation is significantly different from the North Zone. The two zones are separated by the northeast trending Turquoise Ridge shear zone, which could have contributed to the difference in alignment of the two zones. The three steeper dipping bodies (-58°E to -80°E) have an average strike length of 70 m , average dip length of 49 m and average thickness of 12 m. The six moderately dipping (-30°E to -50°E) range from 90 m to 520 m. The average dip length is approximately 260 m and thicknesses average 30 m.

The Turquoise Ridge Complex consists of Turquoise Ridge Underground, Vista Underground, and Turquoise Ridge Surface (consisting of the Mega and Vista Pits).

TURQUOISE RIDGE UNDERGROUND
Turquoise Ridge Underground is accessed via two shafts and a system of internal ramps and utilizes underhand drift and fill mining methods with cemented rockfill. Turquoise Ridge Underground also employs mechanical mining as well as sill benching as mining methods. Turquoise Ridge Underground currently has two roadheaders in operation. Ground conditions at Turquoise Ridge Underground are poor, and the Rock Mass Rating (RMR) may be less than 20, or very poor, in ore headings. The mine is currently producing 2,700 tonnes of ore per day.

Key challenges in attainment of production levels and costs are the development of sufficient stoping areas and the transition from top-cut development to undercut stoping. Completion of the infrastructure development in the North Zone is necessary to achieve and optimize higher production levels. The Third Shaft project is well underway and is expected to be commissioned in late 2022. The shaft will drastically decrease the haulage requirements in the North Zone.

VISTA UNDERGROUND
Vista Underground is built on the existing infrastructure of Turquoise Ridge Surface. All Vista Underground mining activities are contained within the Turquoise Ridge Surface mining footprint and claim areas.

Vista Underground is located in a portion of the historic Vista Phase 6 pit. The mine is a longhole stoping operation that targets a narrow, steeply dipping structure with an average
horizontal thickness of approximately 3.3 m. Vista Underground is currently producing 1,000 tonnes of ore per day.

Vista Underground consists of two portals and a system of underground ramps accessing a steeply dipping (60° to near vertical) mineralized zone where narrow-vein longitudinal stoping is used. Vista Underground has been developed to access the vein in multiple horizons with two main barrier pillars to be mined on retreat. Stope production began near the centre of the orebody and progressed upward and outward in a triangular retreat sequence utilizing cemented rock fill for void replacement.

Mining is currently conducted using a contractor workforce. Initially, the mine proposed using Avoca stoping methods however poor rock mass conditions were encountered in the hanging wall which limited the maximum allowable strike length of the stopes. To minimize overbreak and improve operating conditions and production, the mining method was changed to retreat long hole stoping and more active stoping faces were opened to meet the production requirements.

Backfill is completed using CRF and run of mine (ROM) waste material where applicable. CRF is comprised of waste material from Turquoise Ridge Surface and a cement slurry and is mixed using pit-mix processes.

Vista Underground is operated by a mining contractor. The contractor supplies all underground production/development equipment and skilled labor to execute all mining activities.

TURQUOISE RIDGE SURFACE
Turquoise Ridge Surface mine operates the Vista and Mega pits, as well as providing ore rehandle and surface project work at Turquoise Ridge Underground. Turquoise Ridge Surface uses conventional open pit mining methods including drilling, blasting, loading, and hauling.

Surface mining operations occur in two developed open pit areas: Vista Pit to the north and Mega Pit to the south. Vista Pit measures approximately 1.1 km north to south, 1 km east to west, and the current pit depth is over 213 m. Mining in 2019 occurred in the Vista 8 (V8) phase, which will continue to deepen the pit by another 91 m in the ultimate design. The Mega Pit measures 4 km overall north to south, and approximately 1.6 km east to west. The current pit depth is over 1,200 feet (365m) and will deepen by approximately 50 m in the ultimate design. Mining in 2019 at the Mega Pit occurred in Cut 25.

The Sage/Juniper capacity is 12,000 tpd, however, actual throughput can vary depending on ore type and constituents.

The Sage autoclave processes refractory ore that contains high levels of sulphide minerals.

The Sage mill includes a SAG mill followed by ball milling. The cyclone overflow reports to a thickener. Thickener underflow reports to an acidification circuit where sulphuric acid is added as necessary to ensure adequate autoclave free acid solution levels.

ROM higher grade oxide ore from the Turquoise Ridge Surface sources are blended for Au grade, hardness, and carbonate content and fed to the Juniper Mill. The Juniper mill consists of a variable speed SAG mill operating in closed circuit with a discharge screen. SAG mill product is fed to a ball mill operating in closed circuit with cyclones. 1.5 MW installed power; 18 ft x 6.5 ft (6 m x 2 m) 900HP SAG (Variable speed drive), turns both directions, 11.5 ft x16.5 ft (3.8 m x 5.5 m) 1150 HP ball mill. Final product grind size is 90% - 200 mesh.

The Sage autoclave processes refractory ore that contains high levels of sulphide minerals, which must be oxidized in an autoclave to liberate the gold so that it can be recovered economically.

Autoclaving uses high temperatures, high pressure, oxygen, and steam to oxidize the sulphide minerals. Once the sulphides have been oxidized, the liberated gold is recovered through cyanidation.

The Juniper Mill processes oxide ores by grinding to liberate the gold in the ore, which is then recovered through cyanidation in a combined process stream with the Sage Mill autoclave discharge.

The existing dump leach facilities consist of a standard lined pad. Material is leached in 90-day cycles. Solutions are then routed to a pregnant solution pond and processed through CIC with the resulting barren solution returning to a barren pond and reapplied to the dump leach pad.

SAGE MILL PROCESS DESCRIPTION
The Sage/Juniper capacity is 12,000 tpd, however, ........