Summary:
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 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. The Project geology descriptions are provided by general geographic area, from south to north:
- Turquoise Ridge Underground (TRUG);
- Turquoise Ridge Surface;
- Vista Underground (VUG).
Turquoise Ridge Underground (TRUG)
The Getchell Fault, one of the most prominent structural features of the region, generally strikes north-south to north-northwest, and dips approximately 50° eastward in the vicinity of the mine site. The Turquoise Ridge north zone mineralization largely mimics the orientation of the Getchell Fault, with complications from northeast and north–south-striking structures.
The Turquoise Ridge deposit is a Carlin-type deposit and is structurally and stratigraphically controlled and sediment-hosted, containing disseminated micrometer-sized gold occurring on arsenic-rich pyrite rims, primarily within decalcified, carbonaceous rocks. All gold bearing zones at Turquoise Ridge are in proximity to granodiorite dikes that are associated with the Osgood stock.
Lithology and structure strongly influence the geometry of the mineralization. To the north, stratiform mineralized domains exhibit strike lengths exceeding 300 m with typical thicknesses in the 60–150 m range. Down dip lengths of over 300 m are common. In areas where mineralization is stratigraphically controlled, the mineralized domains and bedding have a general north–northwest-trending strike, and dip to the east at 25–45°. Structural controls are dominantly related to high angle (75–85°) northeast- striking faults (Cricket Corridor, Turquoise Ridge (TR) Corridor, Ace Fault) and the intersections of those zones with throughgoing north–south-striking faults. Host rocks are correlated with the Comus Formation and are locally sub-divided into a mid-slope facies (siliciclastic dominant mudstone and siltstone) and a basal slope facies (carbonaceous and calcareous silty limestone, and calcarenite). In addition, some ore grade intervals are present along mineralized faults that cut interbedded pillow basalt. Dacite and dacite porphyry dikes often control the distribution of high-grade gold, particularly where they are cut by high-angle mineralized faults.
Mineralization of the TRUG deposit generally consists of disseminated, micron-sized gold occurring in arsenic-rich rims forming on pyrite, chiefly within decalcified, carbonaceous rocks. All gold bearing zones at Turquoise Ridge Underground are located in proximity to granodiorite dykes that splay from the Osgood stock. Mining and exploration activities at TRUG are centered on limestone and mudstone horizons adjacent to these dykes.
Turquoise Ridge Surface
Geologically, Turquoise Ridge Surface consists of two separate pits: Mega Pit and Vista Pit. 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.
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.
Vista Underground (VUG)
The VVUG mineralization has a strike length of approximately 1,400 m and a dip length of 300 m. Mineralization extends from about 1070–1460 masl. Thicknesses range from 0.1–12 m, with average thickness of 1 m. In the Vista Underground, mineralization is largely confined to the Trench Fault shear zone within the basalts of the Valmy Formation. Mineralization consists of disseminated gold in arsenian pyrite and sooty sulfides. The Trench fault shear zone is defined by three anastomosing structural zones, or ore zones; OZ1, OZ2, and OZ3. Mineralization dominantly resides along the hanging wall and footwall of the defined ore zone (OZ) structures. Locally higher grades are encountered where the brecciated quartz- base “vein”, historically known as the Galena Vein, is crossed.