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Giant Yellowknife |
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Northwest Territories, Canada |
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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 Yellowknife Giant gold deposit is located 5 km north of Yellowknife, on the western shore of Yellowknife Bay on Great Slave Lake, in the Northwest Territorries of Canada. It lies within the 50 km long Yellowknife Greenstone Belt, near the southwestern margin of the Archaean Slave craton in the Northwest Territorries of Canada.
The Slave craton is a preserved fragment of a larger land mass, composed of a basement of Mesoarchaean gneisses, overlain by a Neoarchaean supracrustal assemblage, the 2.8 to 2.6 Ga Yellowknife Supergroup. The latter sequence was deposited over a protracted period and can be subdivided into a number of chronologically and tectonically distinct phases. These include: (i) ~2.73 to 2.70 Ga rifting and mafic volcanism (the greenstone rocks - massive, pillowed and variolitic mafic to intermediate tholeiitic volcanic rocks, with minor volcniclastic and sedimentary rocks); (ii) ~2.69 and 2.67 bimodal arc volcanism; (iii) ~2.66 Ga arc-rifting and turbidite deposition; and (iv) ~2.63 Ga arc-plutonism- volcanism and turbidite deposition (dominated by older volcanic rock detritus with a lesser from Mesoarchaean basement rocks) in a back-arc basin. These supracrustal units were deformed, exhumed and unconformably overlain by late orogenic, <2.6 Ga, molasses-type conglomerates, that were deposited on an incised palaeosurface, and subsequently deformed along first order, crustal-scale fault zones. Widespread Neoarchaean plutons, forming a batholithic complex (the Western Plutonc Complex) extending over a 500 km interval along the western part of the craton, were emplaced in pulses at ~2.70, 2.67, 2.635 to 2.620, and 2.610 to 2.602 Ga, with a final bloom of granitoids at 2.60 to 2.58 Ga which range from two-mica S-type, to hornblendebiotite I-type granites.
Multiple episodes of metamorphism and deformation are evident in the supracrustal and older granitic rocks. In the immediate Yellowknife area, metamorphic grade is generally greenschist facies, with local amphibolite hornfels related to intrusion of 2.63 to 2.62 Ga Defeat Suite plutons and ~2.592 Ga S-type plutons.
In the Yellowknife area, gold mineralisation is principally hosted by second order quartz-carbonate-bearing, north-south striking, west-dipping shear zones that crosscut the ~2.70 Ga mafic volcanic-dominated rocks of the Yellowknife Greenstone Belt, although numerous prospects and past-producing mines are found as quartz lodes within 2.66 Ga greywacke-mudstone turbidites to the east of the greenstone belt. There are also numerous structurally hosted gold prospects associated with banded iron formation that is interbedded with the <2.63 Ga greywacke-mudstone turbidites. The turbidite-hosted mineralisation is spatially related to ~2.592 Ga S-type Prosperous plutons and associated pegmatites and contact metamorphic aureoles. In contrast, the strictly greenstone-hosted deposits lack a spatial relationship with the S-type plutons, although the metal budgets (e.g., As and Sb) and radiogenic isotopic signatures of the ore does indicate some sedimentary or plutonic contribution. Gold mineralisation in these deposits is localised near the ~2.63 Ga amphibolite-greenschist isograd, although the mineralisation postdates this plutonic-metamorphic event (Ootes et al., 2011).
The Yellowknife Giant deposit is a shear hosted lode deposit is located within the amphibolite-facies aureole of the Western Granodiorite Complex that overprints the Yellowknife Greenstone Belt. The principal component of the greenstone belt commences is the Kam Group, comprising a steeply dipping homoclinal sequence of tholeiitic massive and pillowed flows, calcalkaline tuffs and flows, intercalated volcanogenic sediments, and multiple and sheeted gabbroic dykes and sills, all of which are cut by Proterozoic dolerite dykes.
The bulk of the Giant orebodies are hosted within a wedge of Kam Group volcanoc rocks, bounded to the south and west by the West Bay Fault, to the north by the Akaitcho Fault, both of which are sinistral and of Proterozoic age, and to the east by an angular unconformity by conglomerates and sandstones of the Jackson Lake Formation. Splays of these major faults offset the orebodies.
The main ore zones are hosted within major brittle-ductile shear zones, which cross-cut the Kam Group, and appears to be the result of lateral shortening which broadly overlapped the emplcement of the Western Granodiorite Complex. These shear zones strike at ~30° and slightly crosscut the mainly volcanic stratigraphy, with a complex variation in dip. These shears have been traced for >6.5 km at surface and extend to a depth of at least 600 m below the surface.
Gold occurs in irregular lens- and pod-like veins of quartz-carbonate-sulphide enveloped within sericite-chlorite-carbonate schist. The ore zones are generally linear within the plane of the shear zones, concentrated at shear zone bends or where branches of the shear intersect. They have either steep or gentle plunges and are characterised by complex folds and irregularities in the schistosity and in the quartz veins. The veins contain pyrite, arsenopyrite, sphalerite, chalcopyrite, stibnite, sulphosalts and pyrrhotite, with the gold being associated with arsenopyrite and pyrite.
Historic production was from four interconnected shafts spread over an interval of 5 km. Open pit production commenced in 1976 and contributed to the underground production which continued. Between 1948 and 1987, 13.16 Mt of ore grading 18.5 g/t Au were processed. Ore reserves in 1987 totalled 1.05 Mt @ 8.6 g/t Au within a total resource inventory of 2.63 Mt @ 8.5 g/t Au. Production in 1994 was 0.43 Mt @ 8 g/t Au which yielded 3.15 t Au. Reserves at the end of 1994 were 2.395 Mt @ 9.8 g/t Au. Production ceased in 2004, after a total output of 220 t of recovered gold.
The most recent source geological information used to prepare this decription was dated: 2001.
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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Elevatorski E A 1988 - World Gold, Mines-Deposits-Discoveries (Extract): in Minobras Mining Services pp 99-102
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Lewis D W T 1987 - Giant Yellowknife mine overview: in Padgham W A (Ed), Yellowknife Guide Book, Geological Association of Canada (GAC) 1987 pp 191-193
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Ootes L, Morelli R M, Creaser R A, Lentz D R, Falck H and Davis W J, 2011 - The Timing of Yellowknife Gold Mineralization: A Temporal Relationship with Crustal Anatexis? : in Econ. Geol. v.106 pp. 713-720
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Shelton KL, McMenamy TA, van Hees EHP, Falck H 2004 - Deciphering the Complex Fluid History of a Greenstone-Hosted Gold Deposit: Fluid Inclusion and Stable Isotope Studies of the Giant Mine, Yellowknife, Northwest Territories, Canada: in Econ. Geol. v99 pp 1643-1663
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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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