Bootstrap/Capstone/Tara Mine, Bootstrap Mining District, Elko County, Nevada, USAi

Regional Level Types
Bootstrap/Capstone/Tara Mine	Mine
Bootstrap Mining District	Mining District
Elko County	County
Nevada	State
USA	Country

Structure: Roberts Mountains Thrust Fault A complex fault zone is exposed along the crest of the Bootstrap window. Steep north trending faults have been intruded by dikes.

Alteration: Host rocks have been affected by pervasive hydrothermal alteration: sericitization, silicification, and argillization. Dikes are sericitized. Within carbonate sediments, cherty replacement silicification along faults and joints diminishes laterally in intensity and grades into argillic alteration. Argillized zones grade into fresh country rock.

Commodity: Ore Materials: gold Gangue Materials: clay, quartz, carbonate

Deposit: Disseminated gold mineralization occurs along a NNE-trending , west-dipping fault zone over two miles long. Ore is localized on a north-striking, high angle fault developed along a competency contrast between massive Devonian Bootstrap carbonates and platform-slope talus debris within the Bootstrap Limestone. West-dipping, late Cretaceous to Tertiary dikes intruded along the fault. Later tectonism accentuated initial structural breaks, enhancing permeability and porosity.Gold occurs as microscopic particles along fractures in quartz latite and dacite dikes, chert, siliceous mudstone, calcareous siltstone, and shale.Host rocks have been affected by sericitization, silicification, and argillization. Dikes are sericitized. Within carbonate sediments, cherty replacement silicification along faults and joints diminishes laterally in intensity and grades into argillic alteration. Argillized zones grade into fresh country rock. Where several dikes are closely spaced, the ore is mineable by open pit methods.

Deposit type: Sediment-hosted Au

Development: The first reported gold production from the Bootstrap deposit was in 1914 from a small underground operation near the NW corner of the present Bootstrap pit. In 1918, approximately 500 tons of stibnite ore were shipped to the railhead at Dunphy, NV. Gold production resumed in 1958 when Harry Treweek and Marion Fischer began mining a small dike immediately west of the present Bootstrap pit. It was the largest gold producer in Elko County in 1958. In 1967, the property was acquired by Newmont, who completed a development drilling program of 76 holes totalling 4,394 meters in 1969. Newmont began mining in 1974, producing 20,000 oz. gold over the next five years by milling and heap leaching. Intermittent mining of high grade zones continued from 1976-1984, produciing an additional 100,000 oz. gold. It was listed as an active open pit mine employing 2 persons in 1980. Capstone was discovered in 1985 by Newmont and heap-leaching of low grade ore continued in 1987. Additional exploration resulted in the discovery of Tara in 1990. The property was inactive in 1991. Production resumed in 1992 with the opening of the contiguous Capstone deposit, and mining began at the Tara deposit in 1997.

Geology: The Bootstrap deposits are aligned along the crest of a N-S-trending horst block, comprised primarily of carbonate rocks of the Devonian Popovich Formation and Siluro-Devonian Roberts Mountains Formation, and siliciclastic rocks of the Devonian Rodeo Creek unit. Within the horst block, rock strata are folded into a broad, S20E-trending gently plunging antiform. Resistant carbonate units and silicified rocks forming the east limb of the antiform form a 2-mile-long, 3000-foot wide, 600-foot high ridge known as the Bootstrap Window. Most of the west limb of the antiform has been down-droped by faults and is only exposed on the west side of the Tara deposit. Dikes are up to 15' wide. Gold occurs as microscopic particles along fractures in the dikes, and in the cherts & shales. Limestone in contact with the dike is hornfelsed and partially marbleized

Ore(s): Ore is localized on a north-striking, high angle fault developed along a competency contrast between massive Devonian Bootstrap carbonates and platform-slope talus debris within the Bootstrap Llimestone. West-dipping, late Cretaceous to Tertiary dikes intruded along the fault. Later tectonism accentuated initial structural breaks, enhancing permeability and porosity. Mineralization is strongly controlled by strata (particularly at Tara) however the dominant control is structural.

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Link to USGS MRDS:	10310520

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References