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Gold Bar District - Gold Bar, Gold Pick, Gold Ridge, Goldstone, Gold Canyon |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Gold Bar deposit is located in the south-west of the Roberts Mountains in Eureka County, north-central Nevada, USA. It is approximately 48 km to the north-west of the town of Eureka and some 50 km to the SSW of the Cortez Mine. The nearest significant gold deposit is Tonkin Springs, some 18 km to the north. The deposit is one of a string of occurrences in what is now known as the Gold Bar district, including Gold Bar, Gold Pick, Gold Ridge, Goldstone and Gold Canyon. All are bulk mineable low grade, sediment hosted, disseminated ores.
Gold Bar was discovered in 1983 and has been mined since 1986. It is expected that more than 9.25 t of Au will be mined from the deposit (Masinter, 1990). Production in 1993 was 1.7 t Au (AME, 1995). Most of the production has been from weathered oxidised ores, treated by cyanide leach methods, with approximately 15 tonnes of gold retrieved to 1998.
The published resource figures are:
4.36 Mt @ 2.6 g/t Au (Pre-mining Resource, Gold Bar, 1986 Masinter, 1990).
8.8 Mt @ 2.0 g/t Au = 17.5 t Au (Proven+Probable Reserve, Gold Bar, 1994, AME, 1995).
Total estimated Gold Bar district resource, 1998 - 50 t Au.
Geology
The Gold Bar ore deposit is hosted by a variably argillaceous, silty limestone which has been correlated with the Upper Denay Formation of the Devonian Nevada Limestone (Masinter, 1990). The Nevada Limestone is equivalent to either the upper sections of the Siluro-Devonian Roberts Mountains Formation or the lower parts of the Devonian Wenban Limestone.
Within the Upper Denay Formation, argillaceous micritic limestones overlie coarser grained fossiliferous limestones and are mineralised within north-west striking, steeply dipping fault zones. Limestone beds are gently folded along ENE trending fold axes, forming anticlines and synclines that plunge at 10 to 40° to the east. Hydrothermal alteration zones, related veinlets and gold ore zones were disrupted by displacement on the north-west striking faults which were reactivated subsequent to mineralisation and by east-west and ENE striking faults.
Mineralisation and Alteration
Variations in the calcite, silica and clay content of the wall rock define two different hypogene alteration types, namely:
* Early stratabound silicification affected permeable, intercalated bioclastic rocks and micrites, and coarse grained limestones that compose the footwall of the orebody. Silicified rocks are typically devoid of gold. Pre-ore stratabound silicification produced a zoned alteration pattern that resulted from progressive calcite removal and silica introduction. This alteration type is restricted to permeable limestone sections and exhibits a laterally extensive distribution. The degree of silicification increases over a centimetre to metre scale alteration halo, normal to bedding, from unaltered to jasperoid. Four degrees of silicification may been defined within this transition, as follows: i). partially recrystallised carbonate matrix with minor silica introduction; ii). leached and recrystallised carbonate matrix with moderate silica and minor pyrite introduction; iii). complete replacement of carbonate by aggregate-textured quartz, with sedimentary textures preserved and local occurrence of silica-cemented jasperoid breccia down section from bedded jasperoid. Locally coarse grained bioclastics are intensely silicified relative to intercalated micrites. This latter altered intercalated style is the fourth zone, comprising a mixed ii/iii). zone. The preservation of sedimentary textures implies no volume change during the alteration. The mineral association calcite±barite±Fe oxides occurs as veinlets and cements in brecciated rocks, and cuts all alteration zones that are related to the stratabound silicification. Quartz veinlets are only recognised in the third zone. This silicification is interpreted to have been formed by intergranular migration of a siliceous media along the host bed (Masinter, 1990).
* Later decalcification of carbonates and recrystallisation of clay was structurally controlled, centred on north-west striking bedding plane shears and discordant high angle faults. This latter alteration type is spatially coincident with gold ore and occurs primarily within argillaceous, micritic limestones that overlie the coarser grained footwall limestones. Decalcification and clay recrystallisation increases in intensity towards the north-west striking and east dipping bedding plane shears and discordant faults. This type of alteration resulted from progressive calcite removal, and recrystallisation of authigenic matrix clays in the wall rocks. It produced centimetre to metre scale zoned halos that may be divided into three principal zones, namely: i). partially recrystallised carbonate matrix with locally silicified fossils, minor pyrite and trace gold added; ii). leached and recrystallised carbonate matrix, with partially recrystallised matrix clays, locally silicified fossils, and pyrite and gold added; iii). carbonate matrix pervasively leached with recrystallised matrix clays, collapsed and contorted bedding which resulted from volume reduction, and added pyrite and gold (Masinter, 1990).
Compilation of assay data indicates that moderate to intense degrees of this decalcification/clay alteration has a positive correlation with high gold. Most of the gold is contained within several north-west striking, steeply dipping, anastomosing fault zones up to 20 m wide in which wall rocks are altered to zones ii). and iii). as described above. Gold grades decrease outwards from individual faults that form the core of this zoned alteration halo. Calcite±barite±Fe oxides veinlets commonly cut zone i). and ii). and unaltered rocks in the hangingwall. Veins of calcite±barite±As-sulphides±As-metal locally fill open space in concordant and discordant mineralised faults in decalcification/clay alteration zone iii). but are barren and post date gold mineralisation. Evidence for concurrent hydrothermal alteration and extensional faulting is present in multiple generations of folding in recrystallised clays, and in brecciation and re-deposition of open spaced fillings (Masinter, 1990).
Stratabound silicified rocks are mineralised and display textural and mineralogical evidence of alteration overprint where cut and locally offset by north-west striking faults that control the auriferous decalcified/clay recrystallisation halos. Proximal to such faults, those beds with selectively silicified coarse bioclastics and micrites, exhibit recrystallisation of clays and a decrease in carbonate, and an increase in pyrite and gold in the micritic beds. The occurrence of stratabound gold is restricted to this alteration zone. Pyrite occurs on fracture coatings in silicified beds in this alteration type and in auriferous jasperoids and jasperoid breccias in and adjacent to intersections with north-west striking mineralised faults (Masinter, 1990).
The most recent source geological information used to prepare this decription was dated: 2003.
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.
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Selected References:
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Broili C, French G M, Shaddrick D R and Weaver R R, 1988 - Geology and gold mineralisation of the Gold Bar deposit, Eureka County, Nevada: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno, pp 57-72
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Yigit O, Hofstra A H 2003 - Lithogeochemistry of Carlin-type gold mineralization in the Gold Bar district, Battle Mountain-Eureka trend, Nevada: in Ore Geology Reviews v22 pp 201-224
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Yigit O, Hofstra A H, Hitzman M W and Nelson E P, 2006 - Geology and geochemistry of jasperoids from the Gold Bar district, Nevada : in Mineralium Deposita v41 pp 527-547
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Yigit O, Nelson E P, Hitzman M W 2003 - Structural controls on Carlin-type gold mineralization in the Gold Bar District, Eureka County, Nevada: in Econ. Geol. v98 pp 1173-1188
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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