Summary:
The deposits that comprise the Nevada Operations are considered to be examples of Carlin-style carbonate-hosted disseminated gold–silver deposits and intrusion-related gold–copper–silver skarn deposits.
Carlin Complex
The mineral deposits along the Carlin Trend form a suite of deposits known as Carlin type deposits and includes the archetype Carlin deposit that was discovered in 1961. The Carlin Complex can be divided into 6 main geologic zones or windows which host the active and prospective mining areas. These mining areas are extracted via open pit methods, underground methods or both.
• South Arturo is a regionally distal Carfin-type deposit characterized by prolate mineralization geometries that are dominantly structurally controlled. Stratabound mineralization is apparent but far less continuous than similar deposits on the Carlin Trend. This likely reflects relatively lower hydrothermal fluid-rock interaction coupled with less favorable host rocks where planar faulting and fold features were the most permeable traps. Thus, mineralization orientations are highly variable due to the locally complex structural controls.
• The Goldstrike deposit, is the largest gold deposit on the Carlin Trend divided into three sub-deposits; Goldstrike OP, Goldstrike UG, and the Ren Project. Mineralization is dominantly hosted within the Popovich, and lesser so within the Rodeo Creek and Roberts Mountain formations.
• Mineralization at Goldstar is preferentially hosted in the Wispy sub-unit of the Popovich Formation and in breccia along upper Roberts Mountain sub-unit contacts.
• Mineralization at Exodus is preferentially hosted in Popovich, Rodeo Creek and Roberts Mountains units, with locally discrete zones of mineralization associated with pre-mineral dikes of various composition and orientations. Both deposits exhibit strong structural controls on mineralization with emphasis on the high-angle Castle Reef Fault.
• Deposits of the Greater Leeville area include Leeville (West Leeville, Turf, Four Corners orebodys), Fallon, Pete Bajo and Rita K. Leeville, Pete Bajo, Rita K and Fallon.
• Gold mineralization at the Gold Quarry is principally hosted within Rodeo Creek, Popovich and Roberts Mountain formations. High-grade mineralization is structurally controlled along complex intersections.
Cortez Complex
The Cortez deposits are Carlin-type sedimentary rock-hosted deposits. At Cortez, mineralization consists of submicron to micrometer-sized gold particles and gold in solid solution in pyrite.
Robertson
The deposit is about 2285 m long, 910 m wide, and approximately 400 m thick. Robertson is an igneous related gold system. Gold mineralization is found in Upper Plate siliciclastics of the Devonian Slaven and Silurian Elder formations, as well as inside Eocene intermediate composition igneous rocks, primarily diorite and granodiorite. Mineralization is primarily concentrated around the Tenabo Stock in three main areas: Gold Pan in the northwest, Porphyry in the east to northeast, and Altenburg Hill in the southeast. Gold mineralization overprints an initial contact metamorphic hornfels event and a subsequent chalcopyrite-pyrrhotite-pyrite-chlorite-actinolite skarn event.
Cortez Hills
The Cortez Hills consists of two in-situ and connected Carlin-type orebodies with differing geometries and an exotic satellite deposit (Pediment deposit) that was eroded and redeposited adjacent to the sub-cropping Cortez Hills orebody.
The underground portion of the Cortez Hills deposit consists of the Breccia Zone, the Middle Zone and the Lower Zone (subdivided into Lower Zone A, B, C and D).
The conical-shaped Breccia Zone mineralization extends from a near-surface elevation of 1,783-1,240 m, terminating just east of the Middle Zone. It is approximately 300 m wide with a northwest trend, and varies in width from 75–580 m.
The Middle Zone occurs between elevations 1,318-1,166 m, is approximately 550 m wide northwest-southeast by 400 m long northeast-southwest, and ranges in thickness from 3-80 m.
The Lower Zone lies at an elevation of 1,298 m to the northwest and 933 m to the southeast, extends 1,310 m northwest-southeast, varies in width from 440 m in the north to 150 m in the south, and ranges in thickness from 20-80 m. An emerging area below Lower Zones C and D has mineralization extending to 759 m elevation along the Hanson Fault.
The Pediment deposit covered an area of 900 m x 180 m, ranged in elevation from 1,785– 1,615 m, and was about 75 m thick.
Breccia gold mineralization is hosted in hydrothermally brecciated and fractured rocks that are spatially associated with the Voodoo Fault and its attendant structures. Mineralization within the Middle and Lower Zones lies at depth to the west and southwest of the Breccia Zone, occurring as tabular, sub-horizontal to shallow dipping zones. Mineralization in the Lower Zone is typically refractory in the north, transitioning to dominantly oxide as the zone plunges deeper to the south.
Long Canyon
Long Canyon is a sediment-hosted disseminated gold deposit. The gold mineralization at Long Canyon is best described as sediment-hosted, Carlin-type gold mineralization.
The Long Canyon project also displays some characteristics that are unlike typical Carlin-type gold deposits. The prevalent association of hematite with gold mineralization at Long Canyon is not a common characteristic among all Carlin-type deposits, although this phenomenon is associated with weathered/oxidized portions of some of the deposits.
Phoenix
Phoenix is a skarn-hosted polymetallic massive sulfide replacement deposit. Phoenix Complex deposits include Fortitude and Bonanza and Greater Phoenix. Combined deposit lengths are 16,000 ft long, deposit width is 3,900 ft, and the deposits have a thickness range from 200-550 ft.
Hydrothermal alteration is centered on the Copper Canyon stock, which has produced about 4,200 acres of hornfels and skarn. Skarn alteration is hosted by all sedimentary rock units adjacent to the Copper Canyon granodiorite, with the reactive calcareous protoliths of the Edna Mountain Formation, Antler Peak Limestone and Battle Formation hosting the bulk of the skarn alteration. Alteration of the Copper Canyon stock consists of quartz–sericite–pyrite argillic, or propylitic, alteration.
Turquoise Ridge Complex
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 combined mining operation is comprised of the Turquoise Ridge Underground, Vista Underground, and Turquoise Ridge Surface (comprised of the Mega and Vista open pits).
Mineralization of the Turquoise Ridge Underground 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 Turquoise Ridge Underground are centered on limestone and mudstone horizons adjacent to these dykes.
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.