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4th FEBRUARY, 2010
AMOO K. OBAFEMI 10234452
DIAMOND OCCURRENCE ON THE GEOLOGIC CONTINENT OF AFRICA
Diamonds, even from ancient times, have been sought for their extraordinary hardness (they are
the hardest substance known), transparency, luster, dispersion of light, color and their brilliance,
especially in the colorless transparent gemstone variety. Before discussing on their occurrence on
the African continent; I will attempt to illustrate the characteristics of this wonderful gemstone, a
well as their generally believed mode of formation and occurrence.
Diamond crystal structures are created when carbon is subjected to great pressures (between 45
and 55kilobars) and high temperatures (1,050°C to 1,200°C) corresponding to depths of 150 –
200 km – known as the diamond stability field. Rarely, micro-diamonds can be found at
meteorite impact sites, where shock derived pressures and temperatures are sufficiently intense
to transform carbon into diamonds. They are found in rare intrusive ultramafic igneous rocks
(peridotites or eclogites) that have originated at depth or, as the result of erosion and
transportation, in sand and gravel deposits of river and marine environments. The ultramafic
igneous rocks that contain diamonds at the Earth’s surface are known as kimberlites or
lamproites. Kimberlites are volatile-rich (containing H20 and CO2) potassic rock, ultra-basic
rocks which have variable grain size, with magmatic crystals set in a fine-grained matrix.
Lamproites are ultrapotassic, magnesium-rich rocks that, unlike kimberlites, do not contain CO2.
The volatiles contained in kimberlites allow them to intrude explosively from the lower crust or
upper mantle to the Earth’s surface at great speeds, possibly at the speed of sound. Magmas pick
up diamonds from their host rock in the diamond stability field and act as a conduit, rapidly
bringing the diamonds to the Earth’s surface. Essentially, diamonds go through the pressure-
temperature transition from the depth to surface so quickly that they can’t revert to graphite. The
great hardness of diamonds allows them to survive the explosive intrusion.
It is hence known that diamond deposits could be primary or secondary. Primary deposits occur
as diamondiferous pipes, which are ‘pipes’ of mineral-rich volcanic rock containing uplifted
diamonds. These pipes are the solidified cores of kimberlite, lamproites and komatiite volcanoes.
Secondary deposits are those that contain diamonds which have travelled some distance from
Diamond occurrence on the geologic continent of Africa
their original source usually surviving many cycles of erosion because of their resistance to
chemical and physical erosion. Their high density (3.5) allows them to be concentrated in the
base of river or beach deposit. Because the weathering of the hard-rock host is gentle and
prolonged, alluvial deposits can host some of the larger and least flawed stones.
Fig: formation of kimberlites in various schemes
Fig: Alluvial (placer) diamonds
Diamond has been found in all five continents, but not to the same extent in each. It has been
longest known in Asia, where the famous old Indian deposits have probably been known and
worked from the earliest times; now, however, they are almost completely exhausted. The focus
of this text is on the African continent which is at the present time, by far the most important
source of diamonds, which have been collected here since the late sixties in ever increasing
numbers, far surpassing the yield from any other region. To date, Africa has produced over 75%,
in value, of the world's diamonds with more than 1.9 billion carats worth an estimated $US 158
billion mined.
Diamond occurrence on the geologic continent of Africa
I will like to sub-discuss the regions diamonds as regards the kind of deposits hosted rather than
geographically, even though there is a likelihood of similar deposits or more than one type may
occurring in same regions.
Primary deposits are mostly centered on the Archaean crusts of South Central Africa, with
diamonds being produced from kimberlite mines in countries as South Africa, Angola, DRC,
Tanzania, Lesotho and Botswana.
Diamond bearing kimberlite in some parts of South Africa is black in color. Most kimberlite is
called "blue-ground" kimberlite or "yellow-ground" kimberlite and can be found worldwide. The
name "Kimberlite" was derived from the South African town of Kimberly where the first
diamonds were found in this type of rock conglomeration. A 478-carat white diamond, reputed to
be the 20th-largest ever unearthed, was been discovered in Africa's Lesotho in 2008, other large
stones were the Lesotho Promise at 603 carats, the Lesotho Brown at 601 carats and the 493-
carat Letseng Legacy.
Fig: mining kimberlite pipes Fig: diamond in kimberlite matrix
Young Mesozoic Kimberlites occur in some parts of West Africa notably Sierra Leone and
Liberia, which are important diamond producers as well as northern areas of Ivory Coast,
southwestern Mali and in Guinea. There has also been a recent discovery of potential
diamondiferous kimberlites in Mauritania.
Diamond occurrence on the geologic continent of Africa
Secondary alluvial diamonds occurs in Angola, CAR, Namibia and South Africa and some parts
of West Africa. Most of West Africa's diamond production originates from fluviatile placers and
only on a minor scale from eluvial deposits or from altered kimberlite pipes. The diamonds that
are now found in Namibia have travelled over 1,000 miles from their original source in southern
Africa, transported by the Orange River.
Fig: paleoplacer diamonds
The world's largest known gem quality alluvial diamond deposits are located along the Namib
Desert coastline of southwestern Africa, known as the Sperrgebiet or "forbidden territory," and
along the Orange River near Alexander Bay. Namibia's placer diamond deposits are between 40
and 80 million years old carried from their primary origination point on the Kaapvaal Craton, in
central South Africa and Botswana. Alluvial diamond mining in Angola takes place along a
meandering stretch of the Cuango River flood-plain which is also along the south-western
coastline of Africa. Some of the largest and highest gem-quality diamonds produced from
alluvial placer diamond mining have come from this region, including Angola's two largest
diamonds at 105.9k and 101.8k.
The principal source of these diamonds are the many kimberlites that intrude the old Archaean
crust and which have then been eroded by the Vaal and Orange Rivers, taken to the coast and
then separated from the accompanying sediment into rich placer deposits. The Congo or Lena
River also cuts through this Archaean crusts and taps the kimberlitic diamonds.
The Birim field in Ghana is also an important diamond area with annual production of 1-
1.5million carats emanated from Proterozoic sediments. The diamonds occur within Birimian
and Tarkwaian rocks that are some 2000Ma old, no kimberlites have been found and may have
Diamond occurrence on the geologic continent of Africa
been destroyed during the Eburnean orogeny or may lie concealed beneath the Voltaian
sediments to the north. In this region, except for the Sierra Leonean primary kimberlite, most
other sources have not been found. Virtually all mines are relatively small-scale operations
mainly run by artisanal miners, except for the Akwatia mine in Ghana and the Aredor project in
Guinea.
Offshore marine diamonds are found in South Africa and Namibia. Diamond grades in
kimberlite pipes are known to decrease towards the root of the intrusion. It is estimated that the
uppermost 1,400 meters of the pipes in the Orange/Vaal catchment area in South Africa were
removed and transported into the westward drainage systems. An estimated 3 billion carats have
been delivered to the sea, the majority lying north of the present day Orange River mouth
offshore Namibia. Onshore deposits may represent less than 10% of this total resource.
Marine deposits could only occur if shoreline and near-shore sedimentation had occurred in the
area. Diamonds in marine areas are typically trapped in bedrock depressions eroded from various
structures such as bedding planes, faults or joints in hard bedrock. These form gullies, potholes,
depressions, channels and other trap-sites for diamonds.
On the African continent; the principal mining methods of these diamonds include:
Artisanal Mining, which dominates alluvial deposits especially in West Africa sub-
region.
Hard Rock Mining, which involves tunneling underground and creating underground
"rooms" or "stopes" supported by timber pillars of standing rock.
Marine Mining, employing both "vertical" and "horizontal" techniques to extract
diamonds from offshore placer deposits
Open Pit Mining, method of extracting rock or minerals from the earth by removal from
an open pit or burrow.
Placer Mining, used for extracting diamonds and minerals from alluvial secondary
deposits.
The method used depends principally on the scale of miners, environment and nature of deposit
found as well as proximity to the surface.
Diamond occurrence on the geologic continent of Africa
Case Study: Conflict Diamonds of Sierra Leone and Angola
Conflict diamonds are diamonds that originate from areas controlled by forces or factions
opposed to legitimate and internationally recognized governments, and are used to fund military
action in opposition to those governments, or in contravention of the decisions of the United
Nations Security Council.
On 1st December 2000, the United Nations General Assembly adopted, unanimously, a resolution
on the role of diamonds in fuelling conflict, breaking the link between the illicit transaction of
rough diamonds and armed conflict, as a contribution to prevention and settlement of conflicts
(A/RES/55/56). In taking up this agenda item, the General Assembly recognized that conflict
diamonds are a crucial factor in prolonging brutal wars in parts of Africa, and underscored that
legitimate diamonds contribute to prosperity and development elsewhere on the continent. In
Angola and Sierra Leone, conflict diamonds continue to fund the rebel groups, the National
Union for the Total Independence of Angola (UNITA) and the Revolutionary United Front
(RUF), both of which are acting in contravention of the international community's objectives of
restoring peace in the two countries.
REFERENCES
De Beers,(2008) Origin and Formation of Diamonds
Kennedy, G.C., Nordlie, B.E., 1968. The genesis of diamond deposits. Economic
Geology. vol. 63
Mitchell, R.H., 1991. Kimberlites and lamproites: primary sources of diamond.
Geoscience Canada
Wright, J.B, Geology and Mineral Resources of West Africa,