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Lac de Gras. Ekati - Panda, Koala, Fox and Misery. Diavik - A154 South, A154 North, A418 and A21 |
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Northwest Territories, Canada |
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Diamonds
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Super Porphyry Cu and Au
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The Lac de Gras kimberlite field, comprises >235 kimberlitic bodies, including those supporting the producing diamond mines at Ekati and Diavik. The kimberlite field is located within the central part of the Archean Slave Province, with the main mines being 300 km NE of Yellowknife in the Northwest Territorries of Canada (#Location: 64° 43' 21"N, 110° 35' 59"W).
The kimberlite pipes of the field appear to have been emplaced over a time period from 86 Ma (Cretaceous) to 47 Ma (Eocene), although all of the viable economic kimberlites delineated to date have been dated within a relatively restricted time window of between 55 and 52 Ma.
While the Lac de Gras kimberlites are all in the age range detailed above, older, Jurassic kimberlites are known further to the north (Contwoyto Lake field; e.g. Jericho), and further to the south, Cambrian age pipes occur in the Kennady Lake and Snap Lake areas. Further to the southwest again, and west of the Lac de Gras kimberlite field, there are Ordovician aged kimberlites (e.g. Dry Bones Bay, Cross).
The kimberlites at Lac de Gras are composed mainly of volcaniclastic facies (ie., redeposited kimberlitic volcanics and pyroclastics) and hypabyssal facies kimberlite. Diatreme (or vent) facies, are known within the field, but are rare. The kimberlites appear to have been emplaced in a variety of settings, including terrestrial, marginal marine and marine.
The intruded country rocks are typical of the central Slave Province. Three main Archean units are observed in the vicinity of the Diavik and Ekati pipes, namely: i). NW-SE trending, steeply dipping greywacke-mudstone metaturbidites, ii). 2610-2600 Ma tonalite-quartz diorite intrusives, and iii). 2590 - 2580 Ma two-mica granites. Three phases of Proterozoic dolerite (diabase) dykes cut the sequence, ranging in age from 2.23 to 1.27 Ga. All of these rocks are overlain by glacial deposits and are partially covered by current lakes.
Ekati
The Ekati mines include the Panda, Koala, Fox and Misery mines and fall within the swarm of at least 150 kimberlites on the project area. Misery is some 20 km SE of the other 3, and 7 km NE of Diavik. Panda, Koala and Fox lie along a generally NE alignment over an interval of 10 km.
The Ekati kimberlites are relatively small, generally covering an area of 5 ha, with the largest of 16 ha. They are steep sided pipes and in some cases have round to elliptical shapes in plan, although more commonly they are complex and irregular shaped, often clearly fault controlled. The pipe walls taper inwards with depth, generally at angles of 75 to 85°, although some have overhanging walls. The pipes almost exclusively comprise apparent extrusive volcaniclastic kimberlite, which in some cases have been delineated to depths of more than 500 m. Root zones have not been intersected to date and, the pipes are generally devoid of tuffisitic kimberlite breccia, while a few are either partially or wholly filled with magmatic kimberlite. The volcaniclastic fill comprises both primary and redeposited kimberlitic material, ranging from massive and homogeneous to well bedded, with minor (<1 to 5%) crustal xenoliths of granitoids, mudstone, lesser biotite schist and quartz arenite, and local zones of breccia. Carbonised wood is found to depths of 400 m. Magmatic kimberlite is commonly found as narrow peripheral dykes that do not cut the kimberlite pipes. The internal geology of the pipes is complex and variable, with a range of volcaniclastic types, inter-realtionships and geometries.
The exploited Ekati Panda pipe is roughly circular with a diameter of 200 m and covers an area of around 3 ha. It has a steep sided inverted cone shape, but in detail is irregualr, asymmetric, has a strong fault control and in places has faulted margins. It has been delineated to a depth of around 550 m at which point it has tapered to a width of around 20 m. It is predominantly filled with a complex mix of extrusive volcaniclastic kimberlite which has been reworked to varying degrees and incorporates surficial mudstone, as both lithified clasts and as dark unconsolidated fine-grained matrix material and contains carbonised wood fragments. The volcaniclastics occurs as highly irregular wedges or lenses that dip inwards towards the pipe centre at steep angles, generally >70°. More homogenous primary volcanicalstics are found at depths of >350 m. Minor hypabyssal intrusions occur as narrow (usually <1 m wide) peripheral dykes.
The Ekati Misery kimberlite complex, comprising numerous intrusives and a variety of rock types, was discovered in 1993 by drilling of a conductivity anomaly within an extensive indicator mineral dispersion zone. Delineation drilling and sampling by large diameter reverse circulation cored drill holes was subsequently completed and open-pit mining operations commenced in early 2001. The first kimberlite and diamond production following in early 2002.
The Main Misery kimberlite covers 1.5 ha, and is an elongate, steep-sided pipe of approximately 90x175 m at surface. It cuts across a contact zone between two-mica granite and schistose meta-greywackes in the Archean basement and corresponds to the intersection of this contact and a narrow, approximately north-south trending linear zone of intense fracturing. The Main Misery pipe is filled with volcaniclastic kimberlite, from dark, ash- or mud-rich varieties to coarse-grained, very olivine-rich material. Variable carbonized wood, mudstone clasts, granite and schist fragments, and altered peridotite xenoliths are present. In places well-defined fine-scale bedding is evident, generally characterised by variations in the grain size and abundance of olivine. Bedding angles are highly variable, including both shallow and steeply dipping orientations. Numerous other small kimberlite intrusives are found in the immediate vicinity of, and in some cases abut, the Misery Main kimberlite, including narrow hypabyssal dykes, apparently radiating outwards from the pipe, pipe-like hypabyssal intrusions, and small pipes infilled with volcaniclastic kimberlite similar to that occupying the main pipe.
Production from the Ekati mines for the year of 2003 totalled 4.34 million carats.
Mineral Resources at the end of 2003 were: Ekati Core zone - 88.5 Mt @ 100 cpht for 88.5 million carats of diamonds, and
Ekati Buffer zone - 39.4 Mt @ 210 cpht, for 82.7 million carats.
Ore Reserves at the end of 2003 were: Ekati Core zone - 47.7 Mt @ 80 cpht, for 38.16 million carats of diamonds.
Note: cpht = carats per hundred tonnes. Reserves/resources are for diamonds of >2 mm.
Source of reserve/resource figures: BHP Billiton Annual Report, 2004. At that stage the company had an 80% interest in the Ekati Project, which was divested in 2012.
Diavik
The Diavik Project is based on four main kimberlite pipes, the A154 South, A154 North, A418, and A21 kimberlites, which are aligned along a general NE trend, over a length of around 5 km. There are some 63 kimberlite occurrences on the project area, of which approximately 50% are diamondiferous. Diavik is ~30 km SE of Ekati (#Location: 64° 30' 0"N, 110° 14' 16"W).
The four project kimberlites, are located beneath shallow waters immediately east of the 20 sq. km East Island on which all of the mine infrastructure is located. Mining is planned to span a 20 year period. Both the A154 South and A154 North kimberlites will initially be mined from a combined open pit (protected behind a dyke), followed by additional open pits commencing in 2009 and 2013 respectively to exploit the A418 and A21 kimberlites. Underground mining is also planned for the A154 South and A418 kimberlites. The first diamonds were produced in January 2003.
The Eocene aged Diavik kimberlites occur as small, steep sided, elliptical bodies in plan view. They all have plan surface areas of <2 ha, and are roughly aligned along an approximate 30° trend. A narrow, kimberlite filled fault, which is generally parallel to this trend, extends to the NNE from the A154 South kimberlite. A similar kimberlite filled fault continues along the same trend from the northern margin of the A154 North kimberlite.
The kimberlites at Diavik contain interpreted crater and hypabyssal facies rocks, with pyroclastic and redeposited volcaniclastics being the dominant lithologies. The upper section of the A154 South kimberlite is, appears to be characterised by a bedded sequences of tuffs, tuff breccias, and redeposited volcaniclastics with individual beds varying from 3 to 4 m in thickness. The bedding, although chaotic, generally dips at a shallow angle toward the centre of the kimberlite. Mud clasts are common. Hypabyssal kimberlite is typically found along the margin of the pipes, as observed along the eastern side of the A154 South pipe where it is from 1 to 3 m thick and also dips steeply towards the centre of the kimberlite. Dilution by country rock xenoliths, other than Cretaceous mudstone seems to be minimal, particularly in the A21 and A418 kimberlites. The contact between the kimberlite and host rocks is often sharp, with visible alteration restricted to a 1.0 to 1.25 m thick zoned contact.
Diluted mineable reserves at Diavik prior to mining were as follows:
A154 South - 11.7 Mt @ 480 cpht for 55.2 million carats of diamonds,
A154 North - 2.9 Mt @ 310 cpht for 9.1 million carats of diamonds,
A418 - 8.6 Mt @ 340 cpht for 30.7 million carats of diamonds,
A21 3.9 Mt @ 300 cpht for 11.7 million carats of diamonds,
Total 27.1 Mt @ 390 cpht for 106.7 million carats of diamonds.
Note: cpht = carats per hundred tonnes
The resource is to mean sea level, 415 m below lake level.
The average diamond value, based on the 2000 valuation, is USD 62.00 per carat.
Production from the Diavik mines for the year of 2003 totalled 3.83 million carats.
Mineral Resources at the end of 2003 were: All Diavik Pipes - 11 Mt @ 240 cpht for 26.4 million carats of diamonds.
Ore Reserves at the end of 2003 were: All Diavik Pipes - 25.5 Mt @ 380 cpht, for 97 million carats of diamonds.
Rio Tinto has a 60% interest in the Diavik Project. Source of information, Rio Tinto Annual Report, 2003.
Production from the Diavik mines for the year of 2017 totalled 7.486 million carats.
Mineral Resources at the end of 2017 were: All Diavik Pipes - 2.9 Mt @ 290 cpht.
Ore Reserves at the end of 2017 were: All Diavik Pipes - 15 Mt @ 280 cpht, for 25.5 million carats of diamonds.
Rio Tinto has a 60% interest in the Diavik Project. Source of information, Rio Tinto Annual Report, 2017. Reserves are additional to resources.
The most recent source geological information used to prepare this decription was dated: 2004.
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.
Ekati Diavik
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Selected References:
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Armstrong R A and Moore R O, 1998 - Rb-Sr ages on kimberlites from the Lac de Gras area, Northwest Territories, Canada : in S. Afr. J. Geol. v101 pp 155-158
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Donnelly C L, Stachel T, Creighton S, Muehlenbachs K and Whiteford S, 2007 - Diamonds and their mineral inclusions from the A154 South pipe, Diavik Diamond Mine, Northwest territories, Canada: in Lithos v98 pp 160-176
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Fedortchouk Y and Canil D 2004 - Intensive Variables in Kimberlite Magmas, Lac de Gras, Canada and Implications for Diamond Survival : in J. of Petrology v45 pp 1725-1745
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Fedortchouk Y , Canil D and Carlson J A 2005 - Dissolution forms in Lac de Gras diamonds and their relationship to the temperature and redox state of kimberlite magma: in Contrib. to Mineralogy & Petrology v150 pp 54-69
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Gurney J J, Hildebrand P R, Carlson J A, Fedortchouk Y and Dyc D R 2004 - The morphological characteristics of diamonds from the Ekati property, Northwest Territories, Canada: in Lithos v77 pp 21-38
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Klein-BenDavid O, Izraeli E S, Hauri E and Navon O, 2007 - Fluid inclusions in diamonds from the Diavik mine, Canada and the evolution of diamond-forming fluids: in Geochimica et Cosmochimica Acta v71 pp 723-744
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Krebs, M.Y., Pearson, D. G., Stachel, T. Stern, R. A. Nowicki, T. and Cairns S., 2016 - Using Microdiamonds in Kimberlite Diamond Grade Prediction: A Case Study of the Variability in Diamond Population Characteristics Across the Size Range 0.2 to 3.4 mm in Misery Kimberlite, Ekati Mine, NWT, Canada : in Econ. Geol. v.111 pp. 503-525
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Lockhart G, Grutter H and Carlson J 2004 - Temporal, geomagnetic and related attributes of kimberlite magmatism at Ekati, Northwest Territories, Canada: in Lithos v77 pp 665-682
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Menzies A, Westerlund K, Grutter H, Gurney J, Carlson J, Agnes Fung and Nowicki T 2004 - Peridotitic mantle xenoliths from kimberlites on the Ekati Diamond Mine property, N.W.T., Canada: major element compositions and implications for the lithosphere beneath the central Slave craton: in Lithos v77 pp 395-412
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Westerlund K J, Shirey S B, Richardson S H, Carlson R W, Gurney J J and Harris J W, 2006 - A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re-Os isotope systematics : in Contrib. to Mineralogy & Petrology v152 pp 275-294
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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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