Summary:
The Turquoise Ridge Complex deposits are considered examples of Carlin-type or carbonate-hosted disseminated gold–silver deposits. The Carlin deposit forms the type locality.
The Turquoise Ridge Complex is situated within the Basin and Range province, near the northeast end of the Osgood Mountains. The Osgood Range is underlain by Cambrian Osgood Mountain Quartzite, Cambrian Preble Formation, Ordovician “Comus” Formation and the “upper plate” Valmy Formation. These units are unconformably overlain by the Permian Etchart Formation (Antler Peak Equivalent) of the Roberts Mountains overlap assemblage, and by the Triassic Golconda allochthon. These uppermost units form a belt of outcrops flanking the western and northern sides of the Osgood Range. All of these units are intruded upon by two generations of felsic intrusive rocks – a set of 114 Ma dacite dikes and sills at Turquoise Ridge Underground and Turquoise Ridge Surface and the 92 Ma Osgood Stock and temporally related dikes and sills. To date, no Eocene intrusive rocks have been identified at the Getchell, Turquoise Ridge Surface or Pinson camps.
Geologically, Turquoise Ridge Surface consists of two separate pits:
- Mega Pit; and
- Vista Pit.
The Turquoise Ridge Surface pits lie at the northwest edge of the Kelly Creek drainage and the southeast flank of the Dry Hills, an outlier of the Osgood Mountains. The Dry Hills and Osgood Mountains consist of a complexly folded and faulted sequence of early and late Paleozoic sedimentary and basaltic rocks that have been deformed through multiple orogenic events.
The Paleozoic rocks are subdivided into five groups based on age, lithology, and tectonic history:
- Cambrian phyllite, argillite, and limestone of the Preble Formation;
- A Cambro-Ordovician sequence characterized by black shale, siltstone, dirty limestone, and basaltic rocks that comprise the Comus Formation;
- The Ordovician Valmy Formation, comprising highly-deformed basalt, chert and argillite in the upper plate of the Roberts Mountains allochthon;
- A relatively un-deformed and autochthonous group of Pennsylvanian and Permian age limestone and lesser conglomerate, sandstone, and siltstone of the Antler overlap sequence consisting of Etchart Formation limestone and Battle Formation conglomerate; and
- Highly deformed Permian sandstone, siltstone, basalt, and lesser chert of the Havallah Formation in the upper plate of the Golconda allochthon.
Tilted Miocene basalts overlie the sequence.
Mineralization at Turquoise Ridge Surface is localized in decalcified carbonates, but can occur less frequently in argillized and sulphidized basalt. Silicification is common in Comus Formation sediments immediately adjacent to basaltic contacts with generally lower gold grades. At Vista Underground, mineralization is largely confined to the Trench Fault shear zone within a basalt host.
Mega Pit
The mineralization remaining in the Mega Pit is approximately 2,740 m long and 1,690 m wide, covering an area of 4.1 km2 , and extends at depth to about 1,103 masl. Mineralization thicknesses range from 6-30 m, with the thickness range a function of structural and stratigraphic controls.
A broad zone of low-angle, west dipping, and northerly striking faults characterize a major Paleozoic thrust in the northern part of the Mega Pit. This fault contact between the Valmy and Comus Formations is within the Roberts Mountains thrust zone. Intense shearing with compressional features adjacent to the fault contact affect at least 18 m of the Comus Formation in the north of the Mega Pit.
General stratigraphic location within the Mega Pit is tracked by modeling 13 different mafic units that are described as basaltic sills but cut bedding at a low-angle orientation. These mafic units control mineralization as they are less permeable and have acted as fluid traps within the folded Comus Formation. The principal structural element and the most important ore-controlling structure in the north Mega deposit is the Conelea anticline, which trends, and plunges to the north–northwest. The fold likely developed during the Antler Orogeny when the Valmy Formation was placed above the Comus Formation along the Roberts Mountains thrust.
Gold occurs in arsenic-rich rims or bands on pyrite grains associated with orpiment, realgar, stibnite, cinnabar, and quartz. Four discrete pulses of alteration and related mineralization are recognized at Turquoise Ridge Surface. Higher gold grades in the north Mega Pit occur in Comus Formation carbonate sediments above the Upper Sill. Carbonate-dominant stratigraphy transitions upward into a more tuffaceous and siliciclastic sequence. Mineralization is localized in decalcified carbonates but can occur less often in argillically altered and sulfidized basalt. Silicification is common in Comus Formation sediments immediately adjacent to basaltic contacts, with generally lower gold grades.
Vista Pit
The mineralization within the Vista Pit Resource shell (Vista 8) is approximately 250 m long, and 120 m wide, covering an approximate area of 0.03 km2 , and extends at depth approximately 40 m below the current bottom of the open pit. Mineralization thicknesses range from 15–40 m as a function of structural and stratigraphic controls.
Strata-bound high-grade oxide mineralization occurs primarily within pervasively decalcified sandy carbonate rocks of the lower Etchart Formation limestone near the unconformity between the Etchart and the Valmy Formations. The Etchart Formation host is generally a calcareous sandstone to sandy limestone. The underlying Valmy Formation consists of pillow basalts, massive basalt flows, hyaloclastites, siliceous mudstones, and debris flow breccias. The unconformity is disrupted by northeast-striking faults, forming a horst block in the central part of the pit. These and other structures contain high-grade gold in narrow zones in the Valmy Formation and likely serve as feeders to the much larger strata-bound orebody in the basal Etchart Formation.
Production
The Nevada Gold Mines joint venture was formed on July 1, 2019. Barrick's Turquoise Ridge and Newmont's Twin Creeks operation were contributed to the joint venture and are now collectively referred to as Turquoise Ridge.
| Commodity | Units | 2018 | 2017 | 2016 | 2015 |
All production numbers are expressed as metal in doré.