1. Diamond Genesis and Petrology Fact Sheet 5/11/14
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Introduction
• Diamond is defined as ‘a naturally occurring, high pressure form of carbon,
valued as an industrial mineral because of it hardness and as a gem.’[1]
• Diamonds ultimately derive from kimberlite and lamproite, which are shown
in figure 1, and form placer deposits. [1]
• Diamond is the hardest known mineral, as well as having perfect octahedral
cleavage (figure 4) and adamantine lustre. [2]
Petrogenesis
Diamond is principally obtained from terrestrial igneous rocks (kimberlite and
lamproite), however it is also found in meteorites and placers.
Placers
• Diamonds are found in placer deposits of different ages and genetic types.
These deposits originate either from ancient fossil placers, which
commonly consist of clastic rocks at different metamorphic stages or
directly due to igneous parent rocks. [5]
Meteorites
• A good example of a diamondiferous impact structure is found at Sudbury,
Ontario and struck the earth 1.85 billion years ago. [2]
• Microscopic diamonds were found in the ejecta from the explosion. They
were formed during the passage of the shock wave, where pressures
reached above 103GPa, comfortably into the diamond stability field. [2]
Terrestrial Igneous Rocks
• The majority of economically mined diamonds are derived from kimberlitic
and lamproitic deposits. [9]
• There are over 5000 occurrences of kimberlites worldwide, compared to 24
known occurrences of compositionally similar lamproite. [10]
• The diamondiferous magmas that bring diamonds to the surface intrude
into stable continental crusts of ages between >2500-1500Myr, usually the
older end of the scale. [3]
• The diamonds found in kimberlitic host rocks are typically older than the
kimberlite that brought them there, with ages of 1500-3000Ma, indicating
that they reside in the mantle for a while before their eventual eruption. So
diamonds don’t crystallise from kimberlite and are instead xenocrysts
within the magma, as shown by figure 4. [11]
Chemical Composition
• Diamond is chemically composed of pure carbon.
• Its crystal structure, shown in figure 3, displays each carbon atom
surrounded by four neighbouring carbons in a tetrahedral manner with
covalent bonding. [2]
• It is also worth noting the 𝐶12/𝐶13 ratio for diamonds, as this plays a part in
determining the genesis of the mineral. [5]
• The minimum absolute value of 𝐶12/𝐶13 was 89.24, taken from a South
African specimen. [6]
• The maximum value found was 89.78, for a diamond from the Mir pipe. The
average ratio was established at 89.44. [7]
Figure 1: Global
occurrence of
diamond deposits
in kimberlite
(squares) and
lamproite (circles).
Archean cratons
are shown as
shaded areas. [3]
Figure 2: An enlarged map of the African
Kaapvaal Craton, displaying clusters of
kimberlite and orangeite diatremes as well
as their ages in Ma.[4]
Figure 3: The location of
carbon atoms is shown for
a diamond. This is an
example of a Bravais
lattice, which is a distinct
lattice type which when
repeated can fill a whole
space. [2, 8]
Figure 4: A clear
octahedron of
diamond (top
centre) in a matrix
of kimberlite. [2]
• These xenocrysts occur in two ways:
 Isolated single crystals in kimberlitic magma (as above).
 Minerals in discrete xenoliths of peridotite(P-type diamonds) or eclogite(E-
type diamonds).
• High pressure phase relations (figure 5) provide expected mineral
assemblages that indicate diamonds are derived from sub-cratonic
lithosphere, >200km thick, that extend below Archean shield areas. [9]
Figure 5: Phase relations
in mantle peridotite.
Diamond is relatively low
temperature, but great
depth and high pressure.
[12]
• Diamonds are now thought to be generated deep in the mantle, in the
transition zone between lower and upper mantle, ~400-650km deep. [9]
• Returning to carbon content, the fertile lower mantle is more likely to be
the origin of carbon in diamonds. This is reinforced by the presence of very
high pressure minerals that occur as inclusions in some diamonds. [9]
• Once formed diamonds are more likely to be preserved in the upper
mantle due to the minerals stability depending on the existence of a
reducing environment. [13]
Case Study – Orapa Diamond Mine
• The mine is situated in North Botswana and is part of a kimberlite, aged as
mid-Cretaceous, which contains some of the richest diamond deposits in
the world. [9]
• The mine is owned by Debswana company, a joint venture between De
Beers company and the Botswanan government. [14]
• In 2003, the volume of diamond production amounted to 16.3 million
carats (3260kg), with the average recoverable ore grade at about 0.87
carats (174mg) per tonne. [14]
• The mine reserves total to 87.7 million carats of rough diamonds, at an
average diamond grade of 0.587 carat per tonne. [14]
• Orapa displays a well preserved crater facies, shown in figure 6 below. [9]
• Two pulses of kimberlite are present in the mine, and they merge into a
single maar 200m below the surface. [15]
• The northern diatreme occurred first, and was succeeded by explosive
volcanic activity, followed by the southern diatreme in a similar style. [15]
• The diatremes comprise of tuffisitic kimberlite breccia, that grade
progressively into crater facies of both epiclastic and pyroclastic kimberlite
debris. All of which are diamondiferous. [15]
Figure 6: Idealized
geometry of a diatreme-
maar type volcano,
displaying the nature of
magma movement
through weak layers or
areas of the crust.
Applicable to the nature
and geometry of
kimberlites. [16]
Conclusion
Diamond formation is the key issue. This is summarily, where plumes transfer
melt and volatiles up from the lower mantle, allowing diamond to precipitate
in the reducing conditions that typify the transition zone.
References[1] Kearey, P (2001) Dictionary of Geology. 2nd ed. London: Penguin Books
[2] Klein, C and Philpotts, A (2013) Earth Materials: Introduction to Mineralogyand Petrology. New York: Cambridge University Press
[3] Haggerty, S E (1999) A diamond trilogy: Superplumes, supercontinents, and supernovae. Science.285. pp 851-860
[4] Mitchell, R H (1995) Kimberlites,orangeites, and related rocks. New York: Plenum
[5] Orlov, Y L (1977) The Mineralogyof the Diamond. US: John Wiley and Sons
[6] Craig,H (1953) The geochemistry of the stable carbon isotopes. Geochim. Cosmochim. Acta. 3, 2-3
[7] Vinogradov, A P, Kropotova, O I and Ustianov, V I (1965) Possible sources of carbon for natural diamonds according to C12/C13 data. Geokhimiya.6 (Russian)
[8] Van Zeghbroeck, B J (1997) Bravais Lattices. [Online] Available from: http://ecee.colorado.edu/~bart/book/bravais.htm Accessed on: 3/11/14
[9] Robb, L (2005) Introduction to Ore-Forming Processes. Oxford: Blackwell Publishing
[10] Nixon, P H (1995) The morphology and nature of primary diamondiferous occurrences. Journal GeochemicalExploration. 53. pp41-71
[11] Richardson, S H et al. (1984) Originof diamonds in old enriched mantle. Nature. 310. pp198-202
[12] Best, M G (2003) Igneous and Metamorphic Petrology. 2nd ed. Oxford: Blackwell Publishing
[13] Wood, S A and Vlassopoulos, D (1996) The dispersion of Pt, Pd and Au in surficial media about two PGE-Cu-Ni prospects in Quebec. In Barnes, S J and Duke J M eds. Advances in the Study of
Platinum-group Elements. Mineralogical Association of Canada. Canadian Mineralogist. 28. pp649-663
[14] rough-polished (2014) Orapa Diamond Mine (Botswana, Debswana). [Online] Available from: http://www.rough-polished.com/en/database/90077.html Accessed on: 4/11/14
[15] Field, M et al. (1997) The geology of the Orapa A/K1 kimberlite, Botswana: further insight into the emplacement of kimberlite pipes. Russian Geologyand Geophysics.38 (1). pp261-276
[16] Smith, C B et al. (1979) Petrochemistry and structure of kimberlites in the Front Range and Laramie Range, Colorado-Wyoming. In Boyd, F R and Meyer, H O A eds. Kimberlites, Diatremes, and
the Diamonds: Their Geology,Petrology, and Geochemistry. Proceedings International Kimberlite Conference. 2. pp178-189
[Background Image] Wallpaper Converter (2014) Diamonds wallpaper.[Online] Available from: http://www.wallconvert.com/converted/19046-desktop-wallpapers-diamonds-140529.html
Accessed on 5/11/14