Cortez Gold Mine

Site Name

Primary: Cortez Gold Mine
Secondary: Cortez Joint Venture Gold Mine
Secondary: Cortez Leach Operation
Secondary: Cortez Mine
Secondary: Garrison Mine

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Commodity

Primary: Gold
Primary: Silver
Tertiary: Mercury
Tertiary: Arsenic
Tertiary: Antimony

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Location

State: Nevada
County: Lander
District: Bullion District

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.

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Holdings

Not available

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Workings

Not available

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Ownership

Owner Name: Taylor, Jr., Vernon F.
Years: 1983 -

Owner Name: Idaho Mining Co.
Years: 1983 -

Owner Name: Placer Amax
Years: 1983 -

Owner Name: Cortez Gold Mines, Art Walsh, General Manager
Years: 1983 -

Owner Name: Barrick Gold

Owner Name: Kennecott Explorations (Australia) Ltd. (Rio Tinto)
Percent: 40.00
Info Year: 2006

Owner Name: Barrick Gold
Percent: 60.00
Info Year: 2006

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Producer
Deposit Type: DISSEMINATED; SEDIMENT-HOSTED
Operation Type: Surface
Year First Production: 1969
Year Last Production: 1993
Discovery Year: 1966
Discovery Method: Geophysical Anomaly
Years of Production:
Organization:
Significant: Y
Deposit Size: L

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Physiography

Not available

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Mineral Deposit Model

Model Name: Sediment-hosted Au

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Orebody

Form: Irregular; gold is localized where limestone was faulted, brecciated and folded along margin of Tertiary intrusive, gold is micron-to sub-micron-size.

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Structure

Type: L
Description: The deposit occurs within the Cortez Window in the Roberts Mountain Thrust.

Type: R
Description: The deposit lies within the Cortez Window of the Roberts Mountain Thrust (Cortez Range).

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Alterations

Alteration Type: L
Alteration Text: Host rocks have been decalcified, silicified, leached, bleached, argillized, and oxidized. This phase of low grade hydrothermal alteration occurred after intrusion of the porphyry sill, and removed most of the carbon. A decalcification halo extends into the host rock surrounding the mineralized area. Mineralization is associated with silicification.

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Rocks

Name: Rhyolite
Role: Associated
Description: PLUG, dikes, and sills
Age Type: Associated Rock
Age Young: Tertiary

Name: Andesite
Role: Associated
Description: FLOWS
Age Type: Associated Rock
Age Young: Tertiary

Name: Quartz Monzonite
Role: Associated
Description: STOCK
Age Type: Associated Rock
Age Young: Oligocene

Name: Porphyry
Role: Associated
Description: BIOTITE-QUARTZ PORPHYRY DIKES and sills
Age Type: Associated Rock
Age Young: Oligocene

Name: Limestone
Role: Host
Age Type: Host Rock
Age Young: Devonian

Name: Sandstone
Role: Host
Age Type: Host Rock
Age Young: Early Devonian
Age Old: Wenlock

Name: Siltstone
Role: Host
Age Type: Host Rock
Age Young: Early Devonian
Age Old: Wenlock

Name: Limestone
Role: Host
Age Type: Host Rock
Age Young: Early Devonian
Age Old: Wenlock

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Pyrite
Gangue: Hematite
Gangue: Goethite
Gangue: Quartz
Gangue: Clay

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Comments

Comment (Economic Factors): Between 1969 and 1984, the Cortez mine produced 7700 kilotonnes of ore containing 24.6 metric tonnes of gold. 1968-82 production was estimated to be over 1,000,000 ounces of gold. Production is in part estimated since the Cortez Joint Venture production for Cortez, Gold Acres, and Horse Canyon was not always reported separately. During 2003, Cortez Joint Venture Gold Mines produced 1,065,402 ounces of gold (61% mill, 31% heap leach and 8% carbonaceous ore sale). Proven and probable mineral reserves for all the Cortez JV mines as of December 31, 2003 are estimated at 12.7 million ounces of gold with a projected mine life of 11 years. Included in the mineral reserves is the Cortez Hills deposit currently estimated at 5.2 million ounces, consisting of 3.7 million ounces proven (25.1 million tonnes at average grade of 4.59 g/t) and 1.5 million ounces probable (12.3 million tonnes at average grade of 3.91 g/t), as well as the Pediment deposit currently estimated at 1.2 million ounces consisting of 1.1 million ounces proven (35.4 tonnes at average grade of 0.99 g/t) and 0.1 million ounces probable (3.5 million tonnes at average grade of 0.74 g/t). The Pipeline/Cortez Hills/Pediment deposits are expected to produce more than 15.7 million ounces of gold over 17 years (1997-2014). In their 2005 annual report, Placer Dome Inc. (60%) announced that reserves at the Cortez Complex aggregate 257,940,800 tons grading 0.041 ounces of gold per ton proven+probable.

Comment (Location): The mine area is located at the edge of Crescent Valley and is part of the Cortez Joint Venture which operates Cortez, Pipeline and South Pipeline mines.

Comment (Workings): open pit mine; the original Cortez deposit pit is occupied in part now by Cortez Bass Pond, a pit lake that meets drinking water standards and hosts bass and sunfish. The ADA 52 pit was developed on a satellite to the main Cortez deposit.

Comment (Identification): This record describes the current (2006) Cortez Joint Venture gold mine in Lander county, just west across the county line from the historic Cortez underground silver mine in Eureka County, which is described in a separate record, M232243. This record supersedes records W016364 and M232235 and includes all information from those records. MRDS records # W016364 and M232235 should be deleted from the database and replaced by this record.

Comment (Deposit): Gold mineralization at Cortez is dominantly fault-controlled by low-angle thrust-related features (Duplex Zone) and high-angle faults. Mineralization was further modified by post-mineral faulting. Ore is hosted mainly bySilurian Roberts Mountains Formation and to a lesser extent by the Wenban Limestone. Crosscutting Tertiary dikes have been dated at 34.5 Ma and are barren of mineralization. Gold is localized where limestone was faulted, brecciated and folded along margin of Tertiary intrusive, gold is micron-to sub-micron-size. Anomalous amounts of Hg, As, Sb, and W are associated with ore in veinlets, and as individual grains in hematite-goethite pseudomorphs after pyrite.

Comment (Geology): The deposit is also associated with the Horse Canyon volcanics (andesite flows, rhyolite plug) and the Roberts Mountain Limestone (silty, argillaceous, carbonaceous, pyrite-bearing limestone).

Comment (Development): Gold was discovered in 1966 following geochemical studies of the Cortez Window by the U.S.G.S. Subsequent drilling by Placer Dome U.S. (formerly AMEX) led to the outlining of the Cortez gold deposit. In 1968 a joint venture among American Exploration and Mining Co., Bunker Hill Co., Vernon Taylor and Webb Resources announced reserves of 3.4 million tons ore at 0.29 oz/ton gold. Open pit mining began in 1969 with processing in a 1500 t.p.d. mill. Reserves were exhausted and mining ceased in 1976. In 1981-1983, Cortez Gold mines reworked low-grade stockpile ore from the open pit directly above the millsite. This ore was further separated into low and high-grade piles, crushed to -3/4 and then decarbonized and heapleached with cyanide. In 1983 the entire operation employed 104 persons. A new phase of mining by Cortez Gold Mine commenced in 1988 and continued until 1993. Total production to that time was about 873,000 ounce of gold from about 6.4 million tons of ore. Estimated additional unmined mineralized material remaining in 1999 totaled about 362,000 ounces. The original Cortez concentrator was placed on care-and-maintenance in late 1999. Cortez is now generally described together with the other mine properties of the Cortez Joint Venture, (CJV), Pipeline and South Pipeline, located across Crescent Valley to the west of Cortez. Mining is now centered on the Pipeline deposits. CJV Production will swing back to the Cortez Hills with the impending development of the recently disovered Pediment and Cortez Hills deposits. In their 2005 annual report, Placer Dome Inc. (60%) announced that reserves at the Cortez Complex aggregate 257,940,800 tons grading 0.041 ounces of gold per ton proven+probable.

Comment (Commodity): Commodity Info: Gold is submicroscopic (micron to submicron-sized particles of native gold), occurring in minute specks of arsenian pyrite.

Comment (Commodity): Ore Materials: native gold, pyrite

Comment (Commodity): Gangue Materials: hematite, goethite; gangue: quartz, carbonate, clays

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References

Reference (Deposit): Roberts, R.J. et al, (1967) Geology and Mineral Deposits of Eureka Co. Nev., N.B.M. Bull. 64

Reference (Deposit): Wells J.D., Stoiser, L.R., and Elliot, J.E., (1969) Geology and Geochemistry of the Cortez Gold Deposit, Economic Geology v. 64, p. 526

Reference (Deposit): Stager, H.K., (1977) Geology and Mineral Deposits of Lander Co., N.B. of M. Bull. 88

Reference (Deposit): Division of Mine Inspection, Dec. 1983, Directory of Nevada Mine Operations Active in Calendar Year 1983.

Reference (Deposit): NBMG, 1983, Map 84, Active Mines and Oil Fields.

Reference (Deposit): NBMG, 1983, Active Mines and Oil Fields, Map 84.

Reference (Deposit): Bentz, J. and Smith, P., 1982, Field Examination Report, July 12, 1982.

Reference (Deposit): Metal Market Consultants, Inc., 1987, Gold Situation Map

Reference (Deposit): Nevada Dept. of Minerals, 1993.95-2

Reference (Deposit): .Geological Society of Nevada 1997 Spring Field Trip: Geology and Ore Deposits of Northeastern Nevada, Special Publication No. 25

Reference (Deposit): Long, K.R., DeYoung, J.H., Jr., and Ludington, S.D., 1998, Database of significant deposits of gold, silver, copper, lead, and zinc in the United States; Part A, Database description and analysis; part B, Digital database: U.S. Geological Survey Open-File Report 98-206, 33 p., one 3.5 inch diskette.

Reference (Deposit): Geology and Ore Deposits of the American Cordillera, Field Trip Guidebook Compendium of the April 1995 Geological Society of NevadaSymposium; 501 p.

Reference (Deposit): Gulf Resources 1971-1974, Placer Development 1975-1984 Annual Reports.

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Nevada Gold

Nevada has a total of 368 distinct gold districts. Of the of those, just 36 are major producers with production and/or reserves of over 1,000,000 ounces, 49 have production and/or reserves of over 100,000 ounces, with the rest having less than 100,000 ounces. Read more: Gold Districts of Nevada.