Kimberlite Fields of Nunavut

ABSTRACT

    Exploration for and sampling of kimberlite bodies within what is now regarded as Nunavut has been ongoing for over a quarter of a century. Blackadar and Christie (1963) identified a basic intrusive of possible lower Paleozoic age approximately 40 kilometres north of Creswell Bay on Somerset Island.
Field exploration (*** see below) by Cominco and consulting geologists ensued and Mitchell and Fritz (1973) subsequently classified this diatreme as kimberlite, which was staked and sampled by Diapros in 1973 and 1974. Cominco was also very active in this region at that time, however results of Cominco's sampling has not been made public. These events marked the true onset of kimberlite exploration and sampling within the high Arctic, in a region that is still very much the focus of exploration and discovery (e.g. Opus kimberlite of north Baffin). At present over 60 kimberlite intrusions are known in four broad, although disparate, regions of Nunavut. For the purpose of this discussion the four geographic regions are defined as: Keewatin, North Slave, Victoria Island, and Somerset/Brodeur. Other occurrences of diamond-bearing rock types will also be briefly discussed.

            Keewatin/western Churchill
    Kimberlite dykes in the Rankin Inlet area have been described by Miller et al. (1998), although the exact location and diamond content of these occurrences are not currently in the public domain. The kimberlite dykes intrude greenschist grade Archean metasedimentary supracrustal rocks and appear to orient themselves along pre-existing regional east-southeast and northwest fracture patterns. To date only xenoliths of host supracrustals have been identified within the dykes. An age determination of 2143+/-1 (Rb-Sr, phlogopite) for the kimberlite represents the only expression of Late Triassic kimberlite magmatism of known Phanerozoic kimberlites in North America (Miller et al. 1998).

            North Slave
    The North Slave region encompasses portions of the Slave Craton and Proterozoic Epworth Group platform rocks within southwest Nunavut. Within this geographic region there are at least 10 known kimberlite occurrences. The most important of these is the cluster referred to as the Jericho Project. A cluster of three Jurassic kimberlites (172 Ma; Heaman et al., 1997) were discovered in 1995 near the northern part of Contwoyto Lake. This cluster and other known occurrences in this geographic region are dominated by hypabyssal and diatreme facies kimberlite. The Jericho Project, owned and operated by Tahera corporation is now in the environmental review process, and the land based JD/01 kimberlite has a calculated open-pit mineable resource of 2.345 million tonnes grading 1.13 ct/t with an average modeled diamond valuation of US$74/ct (Tahera press release, 01/31/00). Kimberlite occurrences in the region of Kikerk Lake are characterized by indicator mineral (glacial dispersal) trains dominated by ilmenite, whereas kimberlites west of Contwoyto Lake are characterized by indicator trains dominated by garnets, ilmenite, and locally chrome diopside. Mantle xenolith studies suggest a transition from lithosphere to asthenosphere at depths of 190 km below Jericho (Koypolova et al., 1998). Isotopic Re-Os dating of xenoliths suggests some spinel facies peridotites are Archean, garnet facies peridotites may have been Archean or Proterozoic (re-worked Archean?) ages, and model ages for wherlite and some garnet peridotites are circa 450 Ma (Irvine et al., 1999).

            Victoria Island
     The central portion of Victoria Island has been the focus of concerted kimberlite exploration over the past three years. Much of the attention has been focused on the joint venture property of Major General and Ascot, where Monopros, as operator, discovered 7 kimberlite bodies and an extensive kimberlite dyke network. Other companies active include Aber, Diamet, and Monopros. The cluster of pipes intrude Ordovician limestones and lie fairly proximal to exposed Archean (Slave) basement rocks within the Wellington Inlier (Lecheminant et al, 1996) and 250 kilometres to the east of the diamond unfriendly focal point of the Mackenzie dyke swarm (Helmstaedt and Gurney, 1995). The Snowy Owl pipe has returned the most significant diamond counts and crater facies kimberlite has been intersected in drill core (Major General  press release). No dates have been published for these pipes; however the presence of crater facies material is in contrast to the lack of crater material within the Cretaceous Somerset kimberlites.

            Somerset/Brodeur
    A minimum of 36 kimberlites are known to occur on Somerset Island and have been the topic of numerous published papers, additional kimberlites occur on the Brodeur Peninsula and more recently have been discovered on north Baffin bringing the total number of occurrences so greater tan 40. The majority of these pipes were discovered by Diapros and Cominco in the period 1973-1975. Diapros established a 1-ton/hr processing facility in the vicinity of the Batty kimberlites (n = 21) in the summer of l974,a total of 262.3 tons of kimberlite were sampled and and 215.1 tonns were processed from the Diapron, Batty. Nord, Oucat, Ham. and Elwin kimberlites. Diamonds were recovered from the Nord (0.142 et): Diapros (total etws of 0.0015); and Batty-KI (0.024 and 0.129 et). Kimberlites are typically hypabyssal or diatreme facies and range in age from 105 to gg Ma Studies conducted on mantle xenoliths define a geotherm of approximately 44 mW/m^2 (Kiaragased and Peterson, 1992:
    Sebmidheger and Francis, 1999). The transition from lithosphere to asthenosphere at depths of 140 kilometres beneath Somerset Island is suggested by an inflection in the temperature and pressure array defined by mantle peridotites (Selansidherger and Francis, 1999). The presence of diamonds also indicates that kimberlites tapped lithosphere within the diamond stability field, although the lithospheric root is thinner under Somerset Island than in the central Slave craton. Preliminary Re-Os dating of mantle xenoliths have returned some model Arebean dates for material sampled (Irvine et at., 1999).

    Several other notable diamondiferous occurrences are located within Nunavut. The Parker Lake dyke is a minette lamprophyre (1832±/-28 Ma: MacRae et al., 1996) that appears to be synchronous with Christopher Island Formation potassic magmatic activity. The Parker Lake dvke produced a prodigious number of microdiamonds from a small sample (>1500 diamonds in 22kg sample: Macrae et al., 1995). Activity within the Tomgas region bodes well for portions of southeastern Nunavut and the north coast of Baffin Island. The Torngat dykes at 346 +/-l2 Ma (Digonnet et al., 1996) are analogous to the kimberlite dykes of west Greenland. The voluminous Christopher Island Formations phreatomagmatic volcanic rocks and associated breccias have also seen exploration for diamonds. The extent of Proterozoic metasomatism within the lithopshere beneath the western Churchill is still poorly documented and its effects on diamond stability are as yet not clearily known.

        References

Armstrong,J.F. 1999 Kimberlite Indicators and Diamond Database (K~D)DlAND NWT Gcology Open File 1999-03.

Assessment filings housed by DIAND Archives: 80216, 80218, 80219, 80220, 80222, 80223, 80224, 80226 80229 80601 80637, 81987, 81988, 83263-83274 incl. 83325,83636

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Brummer, H.H. 1978. Diamonds in Canada. CIM Bulletin. 64-79.

Digonnet, S., Goulet, N., Bonnie, J.H. and Stevenson, R. 1996. Genesis and comparison of kimberlitic dykes from the Ungava Bay area. northern Quebec, and from west Greenland. Lithoprobe Report No. 57, 38-43.
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LeCheminant , A.N., Rainbird, R.H., and Villeneuve, ME. 1996. Precambrian geology of northern Wellington Inlier, Victoria Island. Northwest Territories. IN Current Research 1996-C; Geological Survey of Canada, 1-10.

MacRae. ND., Armitage, AR., Miller, A.R., Roddick, J C.. Jones, A.L., and Murdy, M.P. 1996. The diamondiferous Akluilak lamprophyre dyke, Gibson Lake area, N W.T. IN Open File 3228; Geological Survey of Canada, 191-107

Macrae. N. D., Armitage, A. R., and Miller, AR. 1995. A diamondiferous lamprophyre, Gibson Lake area, Northwest Tenitortes. Int. Geol. Rev. 37, 212-229.

Miller. AR , Seller. M. H., Armitage. A. E., Davis, W. J., and Barnett, R.L., 1998. Late Triassic kimberlite magmatism, western Churchill Structural Province, Canada. IN Extended Abstracts, 7th Intenational Kimberlite Conference, Cape Town. S.A.

Mitchell, R. H., and Fritz, P., 1973 Kimberlite from Somerset Island, District of Franklin, N.W.T. Can. Jour. Earth Sci., 10, 384-3 93.

Schmidberger. S S. and Francis, D., 1999 Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites. Lithos, 48, 195-216.