Rex Diamond Mining Corporation Ltd.
The Genesis of the Diamond (by Dr. Luc Rombouts)
This page was written by Dr. Luc Rombouts
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Diamond is the high pressure variety of carbon. Laboratory studies have shown that diamond crystallises out
from liquid carbon at pressures of a least 50 kilobar (50,000 times the atmospheric pressure) and at
temperatures above 900 °C. At lower pressures or temperatures, graphite is formed instead of diamond. At
higher temperatures, above 1,200 °C, graphite forms again. At pressures of about 50 kbar, diamond formation
is limited to a temperature window of 900 °C to 1,200 °C.
Pressures of 50 kbar are encountered at depths of 150 km or more in the earth. Usually at these depths,
temperatures exceed 1,200 °C, except underneath the old stable Archean cratons, where temperatures are in
the range of 900 °C to 1,200 °C at 150 to 200 km depth. These old cratons were formed more than 2.5 billion
years ago and have since then been unaffected by a major tectonic event. They form the nuclei of the old
continents. Examples are the Slave craton in Canada, the Siberian craton in Russia, the Kalahari craton in
Southern Africa, the Kasai craton in Angola and Congo-Zaire, and the West African craton. Diamonds have
been formed underneath these cratons for billions of years. Sudden, deep-seated volcanic eruptions can bring
the diamond from depths of 150 to 200 km to the surface. Such deep volcanoes in the cratons are rare and
hardly known today. They existed, however, at several episodes in the past. These deep-seated volcanoes are
called kimberlites or lamproites, depending on their mineralogical and chemical composition.
Kimberlites erupted 1,100 million years ago (e.g. Premier, Argyle, Mali, India), 520 million years ago
(Venetia, Russia), 250 to 90 million years ago (most African kimberlites), 50 million years ago (Lac de Gras,
Tanzania) and 20 million years ago (Ellendale in Western Australia). The kimberlite magma comes to the
surface in a few hours time, following deep fractures in the earth, known as fissures. When the magma cools,
the kimberlite forms "dykes", i.e. walls, in the deep fractures. A few kilometres from the surface, the
confining pressure of the overlying rocks is no longer sufficient to contain the gas-rich kimberlite magma and
a violent eruption follows. This eruption creates a carrot-shaped explosion pipe (diatreme) topped by a crater
of pulverized rock pieces that after being ejected into the air fell back into the explosion pipe, forming layered
tuffs.
The diatreme and crater are a mixture of kimberlite and pulverized country rocks, containing minute amounts
of diamonds that were brought to the surface by the kimberlite magma (at best the order of 1 carat or 0.2 g
per tonne of rock). Once the kimberlite volcano is emplaced in the landscape, erosion by rain, wind and rivers
starts, and over millions of years the volcano is broken down to its root level. If erosion continues, the entire
volcano disappears and only the kimberlite dykes, solidified at depths greater than a few kilometres at the
time of emplacement, remain. The relatively young Lac de Gras and Tanzanian kimberlites (only 50 million
years old) usually still have a large part of the crater with tuff layers preserved. More erosion, such as in the
Kimberley pipes in South Africa, can remove the crater, and the kimberlites are exposed at the diatreme or
explosion pipe level.
Deep mining since the last century has removed most of the diatreme in Kimberley and the root of the
kimberlite explosion has now been reached, with dykes becoming more abundant. In the Bellsbank area,
erosion was more intense, and the kimberlite diatreme was entirely removed, with only kimberlite dykes
remaining.
Not all kimberlites are diamond-bearing though. Often the kimberlite magma takes too long to come to the
surface. Diamonds tend to turn into graphite or burn into carbon dioxide if the magma stays too long at
shallow depths where pressures and temperatures are too low to keep diamonds stable. On average, about
1% contain diamonds in economic quantities. Diamond-bearing kimberlites with grades better than 0.1
carat/tonne are extremely rare. Presently about 30 major kimberlites are being mined world-wide.
Diamonds are very rare crystals that are continuously being depleted. Finding new diamond-bearing
kimberlites is a major technological challenge, as most easily discovered pipes have been found already. With
ever increasing demand for diamonds, a serious shortfall of supply is expected to develop after the year 2000.
The erosion of the kimberlites frees the diamonds, along with other minerals formed at great depths and
brought to the surface by the kimberlite magma from the kimberlite rock, then dispersing them in the rivers.
Minerals formed together with diamonds at depths of 150 to 200 km are for instance pyrope garnets,
ilmenites, chrome-diopsides and chromites. These minerals are more abundant than diamonds and are
therefore more easily detected in samples taken in the rivers downstream of the pipes. For this reason, they
are called kimberlite indicator minerals and are an important exploration tool. Apart from sampling for
indicator minerals in the rivers, other exploration techniques applied to kimberlites are magnetic surveys. The
earth tends to be more iron-rich at greater depths and the deep magma brought to the surface by kimberlites
is often richer in magnetic iron-bearing minerals than the rocks at the surface. As a result, the kimberlites
often are more magnetic than the surrounding rocks. These magnetic anomalies can be detected by
measuring systematically the magnetic field along survey lines from the air (by aeroplane or helicopter) or on
the ground.
Because of their higher density than sand, diamonds tend to settle in the bottom gravels of river deposits. If
their concentration to those bottom gravels is high enough (say 1 carat per ten cubic metres), they can
constitute economic alluvial diamond deposits. The diamonds are eventually carried by the rivers to the sea,
where they can accumulate in beach deposits (e.g. Namaqualand and Namibia). During transport in the
rivers, only the best diamonds with little or no imperfections or cleavages survive the continuing wear and tear
over millions of years. Therefore, diamonds in river or beach deposits tend to have a better average quality
than the diamonds in the original source rocks (the kimberlites). The alluvial and beach deposits are easily
mined and reserves have been seriously depleted this century. Mainly lower grade or deeper offshore
deposits remain. With diamonds continuously being depleted at a rate of more than 100 million carats per
year, around US$400 million is now spent annually by mining and exploration companies on their world-wide
diamond exploration efforts. |
