Gold Acres mine, Bullion Mining District, Lander County, Nevada, USAi

Regional Level Types
Gold Acres mine	Mine
Bullion Mining District	Mining District
Lander County	County
Nevada	State
USA	Country

Sec 36 T28N R46E. Major gold mine in district.
Structure: Gold Acres is exposed at the western margin of the Gold Acres window along the NW-trending Roberts Mountains thrust, Cortez-Uinta axis. It lies along the Eureka-Battle Mountain trend of aligned deposits and mineralization. High angle normal faulting of various orientations has broken the mineralized zone into three parts: the main Gold Acres orebody, the Little Squirt orebody, and the Little Gold Acres orebody.

Alteration: Alteration of the deposit consists of carbonization, silicification, argillization, oxidation, and skarn formation. Carbonization (up to 3% by weight organic carbon) is the dominant alteration type in the deposit and can give a sooty appearance to the rocks. Silicification is strongest at the northern end of the deposit and decreases to the south. Argillization is primarily confined to fractures in lower plate carbonates associated with carbonization. The clays are generally illitic. Supergene oxidation is evident throughout the deposit area as limonitic and hematitic staining along fractures. Skarn development is associated with the emplacement of a Cretaceous granitic stock. Two prograde, skarn zones are developed, both barren of gold. A retrograde skarn alteration mineral assemblage consists of chlorite, tremolite, epidote, calcite, quartz, illite, nontronite, smithsonite(?), pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, and galena. Gold is not associated with the retrograde assemblage.

Tectonics: geosyncine

Commodity: Ore Materials: native gold, molybdenite, and scheelite, pyrite, arsenopyrite, sphalerite, galena, pyrrhotite

Deposit: The deposit is in the west edge of a window in the upper plate of the Roberts Mountains Thrust, and is localized in brecciated chert and limestone in the lower plate (siliceous, volcanic and carbonate rock assembles). The ore is localized in the Roberts Mountains thrust fault breccia zone, in a shear adjacent to a sill. The shear zone is between limestone and sill; sandstone and chert overlie sill. Blocks of tactite within the shear zone are mineralized, the zone is mineralized with native gold, molybdenite, and scheelite. The intrusive contains minor pyrrhotite. The breccia zone contains disseminated iron-oxides and gold as submicroscopic particles. The intrusives exposed in the pit have been dated as 98 my old. The surrounding country rocks have been pyrometasomatized to various degrees, with the introduction of some scheelite. Tactites zones best developed in the lower plate rocks. Epithermal gold mineralization may be overprinted on Cretaceous tactite Cu-Zn mineralization. The orebody strikes N40W, dipping nearly flat to the southwest. The ore limits are determined by assay values. Gold is hosted primarily in the Silurian Roberts Mountains Formation.

Deposit type: Sediment-hosted Au

Development: Gold mining from the North Ore zone (Gold Acres) began in 1935, first by underground methods then by open pit after 1942. From 1942-1961, Gold Acres produced $10 million in gold and silver and was one of the largest gold producers in Nevada in the 1950s. Placer Dome U.S., Inc. began exploring the district in 1959 and consolidated the land position at Gold Acres in 1969 by and conducted a drilling program. By 1973, mineable reserves for what became the South Pit (Little Gold Acres) were estimated at 1.6 million tons grading 0.106 opt gold. Cortez Gold Mines mined Little Gold Acres from 1973-1976. Heap leaching and milling of the Gold Acres dumps and stockpiles followed until 1983. Development drilling continued from 1984-1986. In 1987, mining activity was reactivated at the North Pit where a refractory mineable reserve of 2.2 million tons/0.118 opt gold was exploited. In 2002, Cortez Gold Mines announced plans to reopen the old Gold Acres Mine to recover carbonaceous ore that was left unmined in the bottom of the old open pit. The ore was to be mined by a contractor and transported to Barrick?s Goldstrike The property is owned by Cortez Joint Venture: Placer Dome, Inc. (60%); Kennecott Explorations (40%) (2004).

Ore(s): Carbonaceous silty limestone of Roberts Mountains Formation provided a favorable host for gold deposition. Northeast-striking high angle normal faults served as conduits for mineralizing fluids. The majority of gold was deposited in an imbricated thrust zone below the Roberts Mountains Thrust within the Roberts Mountains Formation. The upper skarn may have acted as a caprock.

Other Databases

Link to USGS MRDS:	10310461
Link to USGS MRDS:	10069103

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References