OVERVIEW ON PRIMARY DIAMOND DEPOSITS - INTRODUCTION
Primary diamond deposits such as kimberlites and lamproites have produced over 50% of the world's diamonds, whereas the remaining 50% are derived from recent to ancient placer deposits that have formed from the erosion of kimberlite and/or lamproite. Although diamondiferous kimberlite/lamproite make up most of the economic diamond deposits, other diamond-bearing rocks have also been discovered and are the subject of numerous academic papers. Such diamond-bearing rocks include ultramafic lamprophyres (aillikites) in Canada and volcaniclastic komatiites in French Guiana (Capdevila et al. 1999). It has been established by the scientific community that diamonds are not genetically related to kimberlite or lamproite but that kimberlite and lamproite serve as a transport mechanism for bringing diamonds to surface (Kirkley et. al., 1991).
Clifford (1966) and Janse (1994) have stated that a majority of economic diamondiferous kimberlites occur in stable Archaean age cratonic material that has not undergone any thermal or deformational event since 2.5 Ga. Such Archaean age cratons include the Kaapvaal, Congo and West African Cratons in Africa, Superior and Slave Provinces in Canada, East European Craton (Russia, Finland, etc.), and the West, North and South Australia Cratons. The only exceptions, to date, are the Argyle and Ellendale Mines of Australia, which occur in Proterozoic age remobilized cratonic material.
To date, over 6,000 known kimberlite and lamproite occurrences
have been discovered in the well-known diamondiferous
kimberlite/lamproite mines currently being mined include Argyle (lamproite) in Australia, Orapa
and Jwaneng (kimberlite) in Venetia (kimberlite) in South
Botswana, Africa, and
Jubilee, Udachnaya and Mir (kimberlites) Ekati (kimberlite) and Diavik (kimberlite) in
in Russia, Canada.
Economic diamond-bearing kimberlite and/or lamproite pipes range from less than 0.4 ha to 146 ha in size, with the maximum size being greater than 200 ha (i.e. Catoca, Angola). Economic kimberlite diamond grades can range from 1.3 cpht to 600 cpht.
KIMBERLITE HOSTED DIAMOND DEPOSITS
The following discussion of kimberlite types and deposits is taken directly from a publication on ore deposit models by Mitchell (1986).
Kimberlites remain the principal source of primary diamond despite the discovery of high grade deposits in lamproites. Mineralogical and Nd-Sr isotopic studies have shown that two varieties of kimberlite exist:
Group 1, or olivine-rich monticellite-serpentine-calcite kimberlites; and
Group 2, or micaceous kimberlites (predominantly occur in southern Africa).
Group 1 Kimberlites
Group 1 kimberlites are complex hybrid rocks consisting of minerals that may be derived from (1) the fragmentation of upper mantle xenoliths (including diamond), (2) the megacryst or discrete nodule suite, and (3) the primary phenocrysts and groundmass minerals. The
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