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.
At Cortez, mineralization is sedimentary rock-hosted and consists of submicron to micrometer-sized gold particles and gold in solid solution in pyrite. Phoenix is a skarn-hosted polymetallic massive sulfide replacement deposit. Carlin, Turquoise Ridge, and Long Canyon are a sediment-hosted disseminated gold deposit.
Host rocks for Carlin-style deposits 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. Deposits typically have a tabular shape, are stratabound, localized at contacts between contrasting lithologies, but can also be discordant or breccia related. Mineralization consists primarily of micrometer-sized gold and sulfide grains disseminated in zones of siliciclastic and decarbonated calcareous rocks and are commonly associated with jasperoids. 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.
Host rocks for intrusion-related gold–copper–silver skarn deposits include sedimentary carbonates, calcareous clastic rocks, volcaniclastic rocks or (rarely) volcanic flows. They are commonly related to high to intermediate-level stocks, sills, and dykes of gabbro, diorite, quartz diorite, or granodiorite composition. Mineralization frequently displays strong stratigraphic and structural controls. Deposits can form along sill–dike intersections, sill–fault contacts, bedding–fault intersections, fold axes, and permeable faults or tension zones. Pyroxene-rich Au skarns typically contain a sulfide mineral assemblage comprising native gold ± pyrrhotite ± arsenopyrite ± chalcopyrite ± tellurides ± bismuthinite ± cobaltite ± native bismuth ± pyrite ± sphalerite ± maldonite. Garnet-rich Au skarns can contain native gold ± chalcopyrite ± pyrite ± arsenopyrite ± sphalerite ± magnetite ± hematite ± pyrrhotite ± galena ± tellurides ± bismuthinite.
Mineralization
Carlin Complex
Mineralization within the Carlin Complex typically consists of micrometer-sized gold and sulfides disseminated in zones of siliciclastic and decarbonated calcareous rocks and commonly associated with jasperoids. Mineralization is predominantly in the form of oxides, sulfides, or sulfide minerals in carbonaceous rocks, and the ore type determines how and where it is processed.
Cortez Complex
Mineralization consists primarily of submicrometer- to micrometer-sized gold particles, very fine sulfide grains, and gold in solid solution in pyrite. Gold 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 pyrite and arsenopyrite.
Phoenix Complex
Gold mineralization occurs freely at gangue-gangue or at sulfide-gangue grain boundaries, and only rarely as inclusions within gangue minerals. Some inclusions were noted in quartz, pyroxene, epidote, and orthoclase. The remaining gold occurred as inclusions totally encapsulated by sulfide minerals including pyrite, pyrrhotite, and to a lesser extent arsenopyrite, chalcopyrite, and sphalerite. Silver minerals are dominantly electrum, hessite, and lesser argentite.
Copper oxide mineralization locally contains minor amounts of chalcanthite, malachite, chrysocolla, azurite, and lesser cuprite. Enriched copper mineralization typically has chalcopyrite + covellite present. Covellite locally rims chalcocite grains where the effects of oxidation are more advanced. In hypogene mineralization, chalcopyrite occurs as disseminations and bedded replacements with skarn and silicate minerals, and in conjunction with pyrite.
Turquoise Ridge Complex
Preferred host lithologies for gold mineralization are the Comus and Prebble Formations, followed by the Valmy and Etchart Formations. Sub-microscopic gold mineralization is associated with pyrite, arsenopyrite, quartz, calcite, realgar and orpiment. Gold-bearing zones can be located close to granodiorite dikes and beneath basaltic intrusions.
Deposit Descriptions
Carlin Complex
- South Arturo open pit: deposit lengths range from 2,000-4,900 ft, deposit widths are 1,000-1,800 ft, and deposit thicknesses range from 85-150 ft;
- South Arturo El Niño underground: deposit lengths range from 1,700-3,500 ft;
deposit widths are 200-700 ft, and deposit thicknesses range from 400-550 ft;
- Ren underground: deposit lengths range from 2,000-4,200 ft, deposit widths are 350-1,600 ft, and deposit thicknesses range from 50-200 ft;
- Goldstrike open pit: deposit lengths range from 1,000-2,200 ft, deposit widths are 1,000-1,500 ft, and deposit thicknesses range from 100-300 ft;
- Goldstrike underground: deposit lengths range from 1,500-14,800 ft, deposit widths are 100-750 ft, and deposit thicknesses range from 650-1,950 ft;
- Goldstar open pit: deposit lengths range from 2,700-4,700 ft. deposit widths are 800-1,700 ft, and deposit thicknesses range from 200-1,300 ft;
- Exodus underground: deposit lengths range from 2,700-4,700 ft, deposit widths are 850-1,700 ft, and deposit thicknesses range from 200-1,300 ft;
- Green Lantern open pit: deposit lengths range from 3,000-4,000 ft, deposit widths are 800-1,100 ft, and deposit thicknesses range from 300-1,500 ft;
- Fallon (formerly North Leeville) underground: deposit lengths range from 150-2,000 ft, deposit widths are 200-1,100 ft, and deposit thicknesses range from 20-60 ft;
- Leeville/West Leeville underground: deposit lengths range from 6,400-13,100 ft, deposit widths are 450-3,200 ft, and deposit thicknesses range from 15-100 ft;
- Rita K underground: deposit lengths range from 400-4,300 ft, deposit widths are 100-1.500 ft, and deposit thicknesses range from 20-50 ft;
- Pete Bajo underground: deposit lengths range from 1,300-7,000 ft, deposit widths are 1,200-4,000 ft, and deposit thicknesses range from 150-1.875 ft;
- Gold Quarry open pit: deposit lengths range from 1,600-4,900 ft, deposit widths are 1,200-3,800 ft, and deposit thicknesses range from 150-650 ft.
Cortez Complex
- Cortez Hills open pit and underground: combined deposit lengths are 6,800 ft, the deposit is 4,000 ft wide, and deposit thicknesses range from 10-350 ft;
- Crossroads and Pipeline: deposit length is 11,000 ft, deposit width is 3,400 ft, and deposit thicknesses range from 50-1,400 ft;
- Gold Acres: deposit length is 7,000 ft, deposit is 2.600 ft wide, and deposit thicknesses range from 25-600 ft;
- Goldrush: deposit length is 17.300 ft, deposit width is 1,400 ft, and deposit thicknesses range from 10-350 ft;
- Robertson, combined deposit lengths are 7,500 ft, deposit is 3,000 ft wide, and deposit thicknesses range from 150-1,400 ft.
Long Canyon Complex
-The Long Canyon complex consists of the Long Canyon deposit, which has a 13,000 ft strike length, is about 2,500 ft. wide, with mineralized zones varying in thickness from 20–250 ft.
Phoenix Complex
- 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.
Turquoise Ridge Complex
- Turquoise Ridge Surface (Mega, Vista), Turquoise Ridge Underground (North and South). Vista Underground: deposit lengths range from 2.600-4,600 ft, deposit widths range from 980-2,600 ft, and deposit thicknesses range from 10-100 ft.