Cortez mine (Cortez Hills deposit; Cortez Hills underground mine; Garrison mine), Cortez Mining District, Eureka County, Nevada, USAi

Regional Level Types
Cortez mine (Cortez Hills deposit; Cortez Hills underground mine; Garrison mine)	Mine
Cortez Mining District	Mining District
Eureka County	County
Nevada	State
USA	Country

Now a large opencast gold operation and underground mine using underhand cut-and-fill method. Extends into Lander county.

Historically included the Cage shaft, Artic level, Fitzgerald drift.
Structure: The deposit lies within the Cortez Window of the Roberts Mountain Thrust (Cortez Range). The deposit occurs within the Cortez Window in the Roberts Mountain Thrust.

Alteration: 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. Pervasive oxidation and marble development are key alteration types associated with gold deposition

Tectonics: The upper portion of the Devonian carbonate sequence beneath the Roberts Mountains thrust hosts the Cortez Hills deposit.

Commodity: Commodity Info: Gold is submicroscopic (micron to submicron-sized particles of native gold), occurring in minute specks of arsenian pyrite. Ore Materials: native gold, pyrite Gangue Materials: hematite, goethite; gangue: quartz, carbonate, clays

Deposit: The recent Cortez Hills discovery is the result of focused exploration efforts along an extensive high-angle fault corridor that localizes the distribution of gold in the Cortez area. Projection of this structural corridor under cover, coupled with gravity data, were the key exploration techniques that led to the discovery of Cortez Hills. Gold is localized where limestone was faulted, brecciated and folded along margin of Tertiary intrusive, gold is micron-to sub-micron-size. The upper portion of the Devonian carbonate sequence beneath the Roberts Mountains thrust hosts the Cortez Hills deposit. Pervasive oxidation and marble development are key alteration types associated with gold deposit. The Cortez Hills deposit is hosted in the upper portion of the Devonian carbonate sequence. It has a strike length of more than 300 m and is approximately 200 m wide. The mineralized zone starts approximately 120 m below surface and continues up to 460 m. The deposit remains open with expansion potential evident to the west and at depth.

Deposit type: Sediment-hosted Au

Development: Placer Dome is the operator and owner of a 60% joint venture interest in the Cortez Joint Venture (CJV). The remaining 40% is held by Kennecott Explorations (Australia) Ltd. The recent Cortez Hills discovery is the result of focused exploration efforts by CJV staff along an extensive high-angle fault corridor that localizes the distribution of gold in the Cortez area. Projection of this structural corridor under cover, coupled with gravity data, were the key exploration techniques that led to the discovery of Cortez Hills. June 2004, Placer announced that they had drilled a hole in the area between the Cortez Hills and the Pediment deposits that had returned 1.5 ounces per ton over more than 400 feet.

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).

Ore(s): Gold is localized where limestone was faulted, brecciated and folded along the margin of a Tertiary intrusive.


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.
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.

Other Databases

Link to USGS MRDS:	10310435
Link to USGS MRDS:	10310410

Other Regions, Features and Areas containing this locality

References