Diamond Geology

2. Scientists believe that the earth originated from a massive cosmic event, the Big Bang, some 12 billion years ago. The earth is the
result of consolidation of this cosmic debris and was complete 4.5 billion years ago. The earth has a crudely concentric structure,
mostly the result of chemical fractionation and cooling to the present. The outer crust cooled and split into a series of plates.
Towards the centre is a solid inner core surrounded by a liquid outer core. The core is made up of the elements nickel and iron
and has a radius of 3470 kilometres. A solid lower and upper mantle, composed of rock of different types, surrounds the core.
Relatively cold mantle is called lithosphere and hot mantle is called aesthenosphere. The dominant rocks of the upper mantle are
peridotites and eclogites. The mantle has a radius of 2900 kilometres. The very outermost skin of the earth is called the crust and
this can reach a maximum thickness of 70 kilometres. Continents and smaller islands ride piggyback on cold lithosphere. The
shapes of landmasses and seas are constantly changing and these processes are largely driven by forces, mainly heat, originating
from the core. Plate tectonics is the current popular theory used to explain many large-scale geological features.
Earth Structure

4. Plate tectonics is the theory used to explain observations such as sea floor spreading, rift valleys, subduction zones and related mountain belts, major faults, most
volcanoes and earthquakes. The outermost mantle (lithosphere) is relatively cold, rigid and brittle and does not move easily without breaking. Faults form when
brittle rock breaks. Below the lithosphere is hotter mantle (aesthenosphere) that moves like putty as it is driven by heat-generated convection currents from the lower
mantle and core. Cutting through these currents are isolated jets of heat called hot spots and plumes. These convection currents break the earth’s outer lithosphere
into a series of plates that then move about relative to each other. Major transform faults form where plates slide past each other as is happening today at the San
Andreas fault near San Francisco in the USA. Spreading centres, rift valleys and volcanoes form where plates split and separate and this is normally the start of new
seas and oceans. An example is Iceland, an island formed by volcanoes of the North Atlantic spreading rift. All major oceans today have active spreading rifts.
Subduction zones, mountain belts and volcanoes form where plates converge and collide. The Himalayas, Alps and Andes are examples of collisional mountain belts.
Subduction zones recycle lithosphere back into deeper hotter mantle like a conveyor belt. Earthquakes occur at all sites of plate movement activity but especially so at
subduction zones.
Plate Tectonics

6. Diamonds are much older than their host volcanoes (kimberlites) and are carried to surface as accidental fragments, called xenocrysts and xenoliths, from depth.
Diamond is the high pressure mineral form of the element carbon and generally forms in nature at depths greater than 150 kilometres and temperatures greater
than 10000C within the upper mantle of the earth. Diamonds are old with ages ranging from 1000 to 3300 million years. Most carbon for diamond formation
was derived from very deep earth fluids rich in CO2 (carbon dioxide) and CH4 (methane) and other carbon-bearing species. This carbon may be directly derived
from the deep earth or be recycled from shallower depths in subduction zones. Kimberlite is the rock formed by a kimberlite volcanic eruption or intrusion and
is the result of processes that caused the mantle to melt at great depth and this melt to be intruded to the earth’s surface. Kimberlites are younger than the
diamonds they contain. The pipes of Kimberley are only 90 million years old and contain diamonds ~3000 million years old. Diamonds occur in patches at depth
in mantle rocks called peridotites (blue and green) and eclogites (orange) and therefore not all kimberlites contain diamonds. Kimberlites found within very old
(>2500my) cold continental crust that is underlain by relatively cold mantle, have the highest chance of containing diamonds. These areas on earth are prime
targets for diamond exploration programs.
Diamond and Kimberlite Volcano Sources

8. This is a reconstruction of what the landscape around Kimberley may have looked like 90 million years ago, looking from east to west. The uppermost blackish
rocks are like those seen at the Drakensberg mountains today. Bultfontein pipe is exploding; next to it is the Du Toitspan volcano that exploded some thousands
of years before; in the background are the quiet but still smouldering volcanoes of the De Beers (right) and Kimberley-Big Hole (left) pipes. Kimberlites blow a
big hole in the land surface (the crater) and cut through solid country rock to form a steep pipe (the diatreme) that narrows and may become complex with
depth (the root). Kimberlites are extremely violent eruptions and incorporate fragments of deeper mantle and shallower crustal rocks. Diamonds are liberated
from broken diamond-bearing mantle fragments (xenoliths) during the intrusion and eruption of the liquid kimberlite magma. Diamonds can be found at all
levels within a kimberlite. Erosion of the land surface of Kimberley from 90 million years to the present has removed about 1000m of rock off the top of this
picture. The present day level of the city of Kimberley is at the lowermost blackish rock level in the picture.
Eruption of a Kimberlite Volcano

10. This is a reconstruction of the animals and plants that lived and grew in the violent volcanic land around Kimberley some 90 million years ago. Dinosaurs
were dominant and co-existed with smaller mammals, amphibians, fish, insects and the first flowering plants. The large hole in this scene is the result of a
kimberlite eruption that dug out a crater surrounded by low hills of kimberlite debris (pyroclastics) that settled out of eruption clouds. Water accumulated in
the crater and kimberlite pyroclastics and other rocks were re-deposited as layers into the crater lake by collapsing unstable crater walls. In the foreground lie a
variety of rocks and minerals of interest: rocks with purple to red garnets are called peridotites and those with orange garnets are eclogites. These are the rocks
from the deep earth in which diamonds originally grew some 3000 million years ago. Note the diamond in the eclogite fragment (xenolith) and the large
diamond enclosed in a piece of layered kimberlite. Grains of red garnet and black ilmenite lie scattered on the surface and these together with other mantle
minerals like green diopside and black chromite, as well as diamonds, are used by geologists to track down and find a kimberlite pipe during exploration.
Other methods to find a kimberlite pipe include the use of geophysics, for example magnetic surveys.
Detail of a Kimberlite Crater

12. Kimberlites, like all rocks are continually being worn down, broken and dispersed (erosion) by climatic agents like rainwater, rivers and wind. Track the
scenes in this poster clockwise from top left to bottom left in the following descriptions:
a kimberlite pipe being cut and eroded by a large river as is happening in Angola today
as diamonds and other mantle minerals are dispersed into rivers they tend to accumulate, because they are heavy, in between pebbles and boulders
forming the gravel bed of a river. These deposits are called alluvial gravels
alluvial gravels fringe major rivers and become stranded at levels higher than the river as the active river cuts into the landscape. The area around Barkly
West is famous for diamondiferous gravels associated with the Vaal river
diamonds are more frequently trapped in coarser gravels either in specific parts of alluvial bars or particularly if the river has a rough bottom with lots of
potholes and other trap sites
after hundreds or even thousands of kilometres rivers end up in seas and the transported diamonds are deposited on gravely beaches. Sea or land level
changes can result in diamondiferous marine deposits both onshore and offshore as is the situation along the west coast of South Africa and Namibia
only the best diamonds survive this long journey and that is why alluvial and marine diamonds are better in quality than average kimberlite pipe
diamonds
Erosion of the Pipes from 90million years ago to Today