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Here is the second of three posts:
Part 1: Geology, petrology and geotectonic controls
Introduction
A.N. Lecheminant and B.A. Kjarsgaard
Diamonds, though extremely rare, are widely distributed and have been discovered in unconsolidated and consolidated sediments, diverse igneous rocks of upper mantle origin, mantle xenoliths, ophiolites, ultra high pressure metamorphic rocks, meteorites and impact structures. Of these, only diamond-bearing kimberlite and lamproite, and their derived placer and paleoplacer deposits, have proven to be economically viable. Before 1960, >80% of all diamonds were recovered from secondary deposits; by 1990, increased diamond production from kimberlite and lamproite pipes reduced this to <25% (Levinson et a1., 1992). Nearly two thirds of present world production by weight comes from only five pipes located in Australia, Botswana, Russia, and South Africa (Boucher, 1995).
Kimberlites occur in continental shield areas, and economic kimberlite are associated with the stable parts of Archean (>2.5 Ga) cratons. Lamproites, on the other hand, typically intrude demobilized Archean cratons or Proterozoic orogenic belts. For example, the Argyle deposit in Australia, the world's most productive diamond mine, occurs in a lamproite pipe within the Proterozoic Halls Creek mobile belt.
Locations of diamond-bearing rocks in Canada are shown in Figure 1, along with the locations of many commercially important kimberlites and lamproites. This figure also shows the worldwide distribution of Archean and Proterozoic cratons based on a generalized geological map of the world, now available digitally (Kirkham et al., 1994, 1995).
Figure 1. Locations of diamond-bearing kimberlites in Canada, and worldwide locations of kimberlite clusters and primary diamond producers and past producers. The distribution of Archean and Proterozoic rocks is derived from a digital geological map of the world compiled by Kirkham et al. (1994, 1995).
In Canada, Archean and Proterozoic structural provinces are well known and geological maps provide a rigorous framework for examining the distribution of kimberlites and for testing models of diamond formation. Precambrian rocks of the North American craton have been subdivided into numerous crustal domains with distinctive ages, structural trends, and geophysical characteristics (Stockwell, 1961; Hoffman, 1989). Archean cratons, which underlie a large part of the exposed Canadian Shield, were welded together by Proterozoic orogeny, many of which continue in the sub-surface beneath adjacent Phanerozoic sedimentary cover (e.g. Trans-Hudson orogen - Fig. 1: Percival, 1996). The contrast between exposed Archean fields and adjacent thinly covered Proterozoic orogeny is one characteristic that hints at significant differences between Archean and Proterozoic processes of lithosphere formation and preservation (Durrheim and Mooney, 1994). The continental lithosphere, made up of the crust and part of the upper mantle, is a strong persistent layer relative to the underlying convecting asthenosphere. Seismological studies indicate that the lithosphere is thicker beneath Archean cratons, and contains regions of cool lithospheric mantle within the diamond stability field (i.e. depths >150 km; Durrheim and Mooney, 1994; Grand, 1994; Polet and Anderson, 1995). The first three papers in this volume provide a brief summary of the complex history of Archean cratons and flanking Proterozoic orogeny in Canada, and contain key references to maps and reports on the Precambrian geology of Canada.
In this volume, the geology of kimberlites and lamproites is covered in summary papers, accompanied by short papers on analytical methods and specific kimberlite fields in Canada. For comprehensive summaries of the petrology of kimberlites, lamproites, and lamprophyres the reader is referred to books by Mitchell (1986; 1995), Mitchell and Bergman (1991) and Rock (1991). Extensive additional information can be obtained from the Proceedings Volumes for the five International Kimberlite Conferences published up to 1994, and in the Abstracts Volume for the Sixth International Kimberlite Conference (*Proceeding Volumes, 1979-1994; Abstracts Volume, 1995).
Kimberlites resemble and can be spatially associated with other alkaline rocks, some of which originate at depths >150 km, and are therefore potentially diamond-bearing. Diamonds have been reportedly recovered from lamprophyre diademe breccias in the Canadian Cordillera, from lamprophyre dykes and diatremes south of Baker Lake, N.W.T., and from the Ile Bizard diatreme breccia in Quebec. These diamond-bearing rocks and several other alkaline intrusions in Canada are described in a series of short Papers.
Kimberlite, lamproite and other alkaline magmas originate as small volume melts of deep-seated origin. The triggering mechanism for generation of kimberlites is unknown, and correlation of kimberlite magnetism with mantle plumes, with flexure of the lithosphere, or with specific plate tectonic processes has not been adequately demonstrated worldwide (Mitchell, 1986; Haggerty, 1994). Furthermore, no viable theory has accurately predicted the location of kimberlite fields within a craton, although many fields have preferred orientations that suggest pre-existing structural controls are important. Lamproites and lamptophyres originate from shallower sources than kimberlites, and magmas are derived from partial melting of subcontinental lithospheric mantle that has a long and complex metasomatic history (Mitchell and Bergman, 1991).
Kimberlites have chemical and isotopic signatures suggesting they originated from sources in the asthenosphere, although the magmas can be modified by interaction with metasomatized regions in the overlying continental lithosphere (Haggerty, 1994; Ringwood et al., 1992; Tainton and McKenzie, 1994).
Much of our knowledge of the deep crust and upper mantle is inferred indirectly from geophysical methods. Mantle and crustal xenoliths and xenocrysts transported to surface by deep-seated magmas are actual samples of the mantle and deep crust, and can be used to test the geophysical models and study the lithosphere beneath Canada. Pilot studies provide a glimpse of the rich potential provided by xenolith suites in kimberlites and related rocks, discovered as a result of the continued success of diamond exploration across Canada. Age determinations on diamond inclusions and on primary minerals in diamond-bearing kimberlites indicate that diamonds are xenocrysts in the kimberlite magma. Kimberlites act only as transportation agents, bringing diamonds and mantle xenoliths from within the diamond stability field to the surface. In general, diamonds are disserllinatead throughout the kimberlite host, although xenoliths of diamond-bearing source rocks, such as eclogite and rare peridotite, are known.
Of unique interest in some kimberlites are down-dropped blocks of country rocks. Emplacement processes involved in near-surface diadems and crater formation produced kimberlites containing numerous country rock fragments, some of which are the only preserved evidence for stratigraphic units that have now been removed by erosion. Papers on fossil-bearing xenoliths recovered from Lac de Gras kimberlite summarize new and surprising information about the age and geological setting of pipe formation.
Mafic magmatic events generated by mantle plumes and rifting are potentially destructive to the cool diamond-bearing roots of continental lithosphere (Helmstaedt and Gurney, 1994). Large-scale mafic dyke swarms are a surface record of these thermal events, and their age, source, and distribution provide evidence about possible selective destruction or preservation of diamond-bearing mantle roots.
Conceptually, diamond formation and preservation is linked to areas of thick and cool continental lithosphere which extend into the diamond stability field in the mantle (depths >150 km). However, diamonds apparently unrelated to such old cratoric nuclei have been discovered associated with Phanerozoic collisions orogeny, such as in New South Wales, Eastern Australia (Barron et a1., 1994), and diamonds occur in other off-craton localities. Diamonds have been recovered from alluvial sources proximal to technically emplaced ultramafic massifs in collisions orogeny, and microscopic diamonds have been discovered in situ in fault-bounded ultra high pressure metamorphic massifs. In addition, microscopic diamonds occur in carbon-bearing rocks at impact sites worldwide and, to date, no systematic search has been made of the twenty-six known impact structures in Canada. Three papers in this volume summarize these associations and outline the Canadian context. Although none of these unusual diamond occurrences worldwide has proven to be of major economic importance, interest is piqued by rocks from ophiolites, such as the Beni Bousera ult.ramafic massif, that has been interpreted to have initially contained up to 15% diamond (Nixon et a1., 1991).
References
Barrow, L.M., LishmunG S.R., Oakes, G.M., and Barrow B.J.
1994: Subduction diamonds in New South Wales: implications for exploration in eastern Australia;
Geological Survey of New South Wales, Quarterly Notes, v. 94, p. 1-23.
Boucher, M.A.
1995: Diamonds;
1994 Canadian Minerals Yearbook, Chapter 20, p. 20.1-20.12.
Durrheim, R.J. and Mooney, W.D.
1994: Evolution of the Precambrian lithosphere; seismological and geochemical constraints;
Journal of Geophysical Research, v. 99, p. 15359-15374.
Grand, S.P.
1994: Mantle shear structure beneath the Americas and surrounding oceans;
Journal of Geophysical Research. v. 99, p. 11591-11621.
Haggerty, S.E.
1994: Superkimberlites: a geodynamic diamond window to the Earth's core;
Earth and Planetary Science Letters, v. 122, p. 57-69.
Helmstaedt, H.H. and Gurney, J.J.
1994: Geotetonic controls on the formation of diamonds and their kimberlitic and lamproitic host rocks: Applications to diamond exploration;
Proceedings of the Fifth Intemational Kimberlite Conference,
Diamonds: Characterization, Genesis and Exploration,
H.O.A. Meyer and O.H. Leonardos (ed.), v. 2, p. 236-250.
Hoffman, P.F.
1989: Precambrian geology and tectonic history of North America;
The Geology of North America - an overview,
A. W. Bally and A. R. Palmer (ed.);
Geological Society of America The Geology of North America v. A, p. 447 - 511.
Kirkham, R.V., Chorlton, L.B., and Carriere, J.J.
1994: Generalized geology of the World (1:35 000 000);
Geological Survey of Canadaa Open File 2915a (paper map).
1995: Generalized geology of the World and linked databases;
Geological Survey of Canndn. Open File 2915d (CD-ROM).
Levinsow A.A., Gurney, J.J., and Kirkley, M.B.
1992: Diamond sources and production: past, present, and future;
Gems and Gemology, v. 28, p. 234-254.
Mitchell, R.H.
1986: Kimberlites - Mineralogy, Geochemistry, and Petrology;
Plenum Press, New York, 442 p.
1995: Kimberlites, Orangeites and Relate Rocks;
Plenum Press, New York, 410p.
Mitchell, R.H. and Bergman, S.C.
1991: Petrology of Iamproites;
Plenum Press, New York, 447 p.
Nixon, P.H., Pearson, D.G., and Davies, G.R.
1991: Diamonds: the oceanic lithosphere connection with special reference to Beni Bousera North Morocco,
Ophiolite Genesis and Evolution of the Oceanic Lithosphere,
TJ. Peters, A. Nicolas and R.G. Coleman (ed.);
Kluwer Academic Publisbers, p. 275-289.
Percival, J.A.
1996: Archean Cratons;
Searching for Diamonds in Canada;
A.N. LeCheminant, D.G. Richardson, R.N.W. DiLabio and K.A. Richardson (ed.);
Geological Survey of Canada, Open File 3228, p. 11-15.
Polet, J. and Anderson, D.L.
1995: Depth extent of cratons as infered from tomographic studies;
Geology, v. 23, p. 205-208.
Ringwood, A.E., Kesson, S.E., Hibberson, W., and Ware, N.
1992: Origin of kimberlites and related magmas;
Earth and Planetary Science lctters, v. 1 13. p. 521-538.
Rock, N.M.S.
1991: Lamprophyres;
Van Nostrand Reinhold, New York, 285 p.
Stockwell, C.H.
1961: structural provinces, orogenies, and time classificadon of rocks of the Canadian Precambrian Shield;
Age Determinations by the Geological Survey of Canada, J.A Lowden (ed.);
Geological Survey of Canada; Paper 61-17, p. 108-118.
Tainton, K.M. and McKenzie D.
1994: The generation of kimberlites, lampmites, and their source rocks;
Journal of Petrology, v. 35, p. 787-817.
* Proceedings Volumes for the First five international Kimberlite Conferences and Abstracts Volume for the Sixth International Kimberlite Conference:
· Proceedings of the First International Kimberlite Conference.
Physics and Chemistry of the Earth, v. 9.
· Proceedings of the Second International Kimberlite Conference,
v. 1 . Kimberlites, Diatremes and Diamonds: their Geology, Petrology and Geochemistry, H.O.A. Meyer and F.R. Boyd (ed.);
American Geophysical Union, Washington D.C., 1979.
· Proceedings of the Second International Kimberlite Conference,
v. 2. The Mantle Sample: Inclusions in Kimberlites and Other Volcanics, H.O.A. Meyer and F.R. Boyd (ed.);
American Geophpical Union, Washington D.C., 1979.
· Proceedings of the Third International Kimberlite Conference,
v. 1. Kimberlites I: Kimberlites and Related Rocks, J. Kornprobst (ed.);
Developments in Petrology 11A, Elsevier, Amsterdam, 1984.
· Proceedings of the Third International Kimberlite Conference,
v. 2. Kimberltes II: The mantle and Crust-Mantle Relationships, J. Kornprobst (ed.);
Developments in Petrology 11A, Elsevier, Amsterdam, 1984.
· Proceedings of the Fourth International Kimberlite Conference,
v. 1. Kimberlites and Related Rocks: Their Composition, Occurence, Origin and Emplacement, J. Ross (ed.);
Geological Society of Autralia Special Publication14,
Blackwell Scientific Publications, Oxford, 1989.
· Proceedings of the Fourth International Kimberlite Conference,
v. 2. Kimberlites and Related Rocks: Their Mantle/Crust Setting, Diamonds and Diamond Exploration, J. Ross (ed.)
Geological Society of Autralia Special Publication14,
Blackwell Scientific Publications, Oxford, 1989.
· Proceedings of the Fifth International Kimberlite Conference,
v. 1. Kimberlites, Related Rocks and Mantle Xenoliths, H.O.A. Meyer and O.H. Leonardos (ed.);
Companhia de Pesquisa de Recuros Minerals - Special Publication 1/B Jan/94, Brazilia 1994.
· Proceedings of the Fifth International Kimberlite Conference,
v. 2. Diamonds: Characterization, Genesis and Exploration, H.O.A. Meyer and O.H. Leonardos (ed.);
Companhia de Pesquisa de Recuros Minerals - Special Publication 1/B Jan/94, Brazilia 1994.
· Extended Abstracts Volume - Sixth International Kimberlite Conference, Novosibirsk, Russia August 1995, 707p.
***
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