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Bootstrap, Capstone

Nevada, USA

Main commodities: Sb Au
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The contiguous Bootstrap and Capstone gold deposits are located immediately north of the boundary between Eureka County to the south, and Elko County to the north, in north-eastern Nevada, USA.

They are located on the margin of the Bootstrap Window, is towards the northern end of the Carlin Trend and some 45 km to the NNW of the town of Carlin. The Bootstrap/Capstone deposits are less than 2.5 km to the south-east of the Dee orebody and 6 km to the north-west of the Post/Goldstrike mine.

Published reserve and production figures for Capstone and Bootstrap include:

     ~8.8 Mt @ 3 g/t Au = 26.5 t Au (Bootstrap, Pre-mining resource, Jory, 2002)
     ~3.1 Mt @ 3 g/t Au = 9.3 t Au (Capstone, Pre-mining resource, Jory, 2002)
     ~10 Mt @ 1.8 g/t Au = 18.5 t Au (Tara, pre-mining resource, Jory, 2002)
      25.5 Mt @ 1.19g/t Au (Total geological reserve, 27/12/89, McFarlane, 1991)
      21 Mt @ 1.27 g/t Au = 26 t Au (Proven+Probable Reserve, 31 Dec. 1992, Christensen, 1993)
      6.8 t Au (Prod. 1918 to 1984, Baker, 1990)
      20.6 Mt @ 1.77 g/t Au = 36.45 t Au (Production+reserves+resources, 2002, includes Tara also,
          Theodore, et al., 2003)

Mining was first recorded in the Bootstrap district in 1918 when 500 t of stibnite were shipped out. Gold production commenced in 1958 with mining of a small dyke immediately to the west of the present Bootstrap pit. Total production to the end of 1959 amounted to 3 t of Au. In 1967 Newmont acquired the mineral leases at Bootstrap and in 1969 commenced a development drilling program. This work amounted to 76 holes with a cumulative length of 4394 m. Initial mining by Newmont commenced at Bootstrap in 1974 with 0.6 t of Au being produced over the succeeding five years from both milling and heap leaching. Intermittent mining of the higher grade zones continued between 1976 and 1984 with an additional 3 t of Au being produced. The Capstone deposit was discovered by exploration drilling conducted by Newmont in 1986 as part of a program to expand the reserves at Bootstrap. Production was expected to commence at Capstone in 1992 (Baker, 1990).

Geology

Within the Bootstrap Window, silty limestones of the Siluro-Devonian Roberts Mountains Formation underlie relatively high energy carbonate facies of the Devonian Bootstrap Formation, an equivalent of the Popovich Formation which is found in the Lynn Window to the south. Thinly interbedded siliceous and calcareous sediments of the Upper Devonian Rodeo Creek Unit overlie, or are in high angle fault contact with the carbonates. Gold mineralisation is localised along north-trending high angle faults, and primarily occurs within Rodeo Creek lithologies peripheral to the faults. Altered Cretaceous (?) dykes of intermediate composition fill some faults and are commonly mineralised. Silicification is the dominant alteration type. Gold is associated with moderate to very dense silicification in the form of quartz stockworks and silica flooding. In places argillised siltstones and dykes, as well as hematitic fault gouge, host gold. The Roberts Mountains Formation and Bootstrap limestones are un-altered, except for the presence of jasperoid breccias along structures (Flaherty & King, 1991).

For more detail of the setting see the Carlin Trend Geology and Carlin Trend Mineralisation records.

The Bootstrap and Capstone deposits are located on the northern half of Round Mountain, a prominent hill on the margin of the Bootstrap Window. Round Mountain represents a fault bounded uplift of calcareous and siliceous sediments ranging in age from Silurian to late Devonian. These are bounded to the east and north by Ordovician and Devonian silici-clastics, while to the north and west Tertiary volcanics cap the Palaeozoic sediments (Baker, 1990).

The sequence at Bootstrap and Capstone is as follows, commencing from the structural base:

Siluro-Devonian, Roberts Mountains Formation - composed of thin bedded, laminated, calcareous siltstone with sandy, pelletal limestone interbeds. These beds are vertically transitional with the overlying Devonian Bootstrap Formation limestones (Baker, 1990).
Devonian, Bootstrap Formation - which is an equivalent of the Popovich Formation of the Lynn Window and the Devonian Limestones of the Carlin Window. The Bootstrap Formation can be divided into three distinct units as follows:
- Lower Bootstrap Unit - laminated, platy, calcareous siltstones interbedded with sandy, pelletal limestones, commonly containing rip-up clasts of laminated calcareous siltstones, bioclastic limestones, oolitic limestones and skeletal debris. These coarse clastic beds typically display cross-bedding, pinch and swell bedding, channelled sands, crudely graded bedding, load casts, flame structures and plastic, soft sediment deformation features (Baker, 1990);
- Upper Bootstrap Unit - a limestone unit characterised by thick beds of oolitic and pelletal carbonates with minor silty to sandy detritus. They contain sporadic well preserved coral heads, bryazoa, brachiopods and crinoid fragments. Debris flow assemblages, as exposed in the Bootstrap pit, occur as wedges within the upper limestone unit (Baker, 1990);
A third unit, which appears laterally transitional and coeval with the Upper Bootstrap Unit, has been identified on the southern margin of the Bootstrap Window, but not in the orebody area. This unit comprises a thinly bedded, poorly laminated, clay rich, weakly sandy, calcareous siltstone to silty limestone. Minor sandy to silty, medium-bedded limestone with graded bioclastic debris occurs as thin interbeds near the top. Thick to massive bedded, sandy to silty limestone, with debris flow lenses and diagenetic black chert seams and beds from 3 to 60 cm thick, occur at the top of the unit (Baker, 1990).
Upper Devonian, Siliceous Sequence - This sequence conformably overlies the Bootstrap Formation calcareous sandy siltstones. It comprises typically fine grained siliceous mudstones with siliceous and variable calcareous siltstone interbeds. Adjacent to the Bootstrap and Capstone deposits these sediments are structurally juxtaposed against the Bootstrap Formation carbonates (Baker, 1990). This unit is a probable equivalent of the Rodeo Creek Unit of the Lynn and Carlin Windows (Flaherty & King, 1991)
Intrusives - North striking dykes are present in the mine area, emplaced along steeply dipping fault and shear zones, commonly localised at the contact between carbonate and silici-clastic rocks. These intrusives, which are probably of late Cretaceous to Tertiary age, have been extensively altered, although relict textures and compositions suggest an original dioritic to granodioritic composition (Baker, 1990).
Tertiary, Carlin Formation - composed of water lain tuffs and ash with minor sand and gravel bars. These sediments are found along the northern and eastern margins of the Round Mountain area (Baker, 1990).

Structure

The dominant structures in the district are north-south high angle faults that are cut and off-set by north-west and north-east striking high angle normal faults. East-west striking high angle faults cut and off-set all structural regimes (Baker, 1990).

Mineralisation and Alteration

The two contiguous orebodies, Capstone and Bootstrap, are faulted sections of the same deposit. Capstone was down-dropped to the north with an approximate 150 m dextral offset. Both orebodies occur within the same stratigraphic sequence, although the occurrence of gold and the associated alteration assemblages appear to be different.

Bootstrap - This is a lens shaped, vein-like deposit localised on a north striking, high angle fault structure, apparently developed along a competency contrast between massive Bootstrap carbonates and talus debris within the Bootstrap Formation limestones. West dipping late Cretaceous to Tertiary dykes are emplaced along these structures, which were subsequently reactivated, enhancing permeability and porosity (Baker, 1990).

Alteration consists primarily of cherty replacement silicification along faults and joints diminishing laterally in intensity and grading to argillic alteration. The argillised zones in turn grade into relatively fresh country rock (Baker, 1990).

Mineralisation is continuous along strike with very little post-ore disruption. Gold grades do however, vary markedly across the alteration zones. The high degree of structural control required a tight drill testing pattern on an orthogonal grid of 15 m east-west and 30 m north-south spacings (Baker, 1990).

Capstone - The Capstone mineralisation is localised at the structural intersection of 335° and 35° fault sets and is hosted by variably silicified upper Devonian silici-clastics and mid to upper Devonian carbonate sediments. The ore deposits overlie the massive limestones of the Bootstrap Formation. Ore distribution is controlled by structures, typified by steep, west dipping faults which are intruded by dykes, and to a lesser degree by lithology with gold being found in thin bedded siliceous sediments where they occur immediately above massive limestone (Baker, 1990).

Alteration is dominated by replacement silicification of both carbonate and silici-clastic sediments. Late stage argillisation, accompanied by gold overprinted the silicification near dykes and faults (Baker, 1990).

The most recent source geological information used to prepare this summary was dated: 1996.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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