diamond occurence in africa

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.


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.


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


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.


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,

diamond occurence in africa

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