Cortez gold mine, Cortez Mining District, Lander County, Nevada, USAi

Regional Level Types
Cortez gold mine	Mine
Cortez Mining District	Mining District
Lander County	County
Nevada	State
USA	Country

Sec 19, T27N R48E Sec 24 T27N R47E. Large Carlin trend open pit gold mine.
Structure: The deposit occurs within the Cortez Window in the Roberts Mountain Thrust. The deposit lies within the Cortez Window of the Roberts Mountain Thrust (Cortez Range).

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.

Tectonics: Basin and Range

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: 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.

Deposit type: Sediment-hosted Au

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.

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.

Other Databases

Link to USGS MRDS:	10310410

Other Regions, Features and Areas containing this locality

References