Diamond Fever

What is a Diamond?

Natural diamond crystals
A diamond is a crystal which is composed of the element, carbon. In nature, diamond crystals display a crystal habit, but often the crystal shape has been rounded or abraided during emplacement. Diamonds typically occur in a rock called Kimberlite and in related cousins (Lamproite).

What is a Kimberlite?

Origin of Diamond

Although diamond crystals are found in Kimberlite and related rocks, the origin of diamond is more closely related to the fragments of peridotite and eclogite which are derived from the upper mantle, below cratonic (shield) areas. In order for diamonds to form, they require extremely high pressures and temperatures which are only found in these deep levels of the earth. It is here that the rock, eclogite, forms consisting of red pyrope garnet and green clinopyroxene; diamond crystals develop alongside the garnet and pyroxene crystals.

Peridotite fragments (xenoliths) composed of garnet, olivine, and orthopyroxene also contain diamonds and are similarly derived from the upper mantle. However, these fragments commonly disaggregate during the emplacement process resulting in a matrix containing the disaggregated minerals of olivine, pyroxene, and diamond (xenocrysts).

Eclogite

Sample of eclogite consisting of red (pyrope) garnet and green pyroxene. When found unaltered, eclogite may be considered a gem material due to the combination of the two attractive gem minerals.

Click to view 3D animation of diamondiferous eclogite xenolith

Types of Diamond Crystals

Because diamonds have a close affinity to the type of fragment (eclogite, peridotite) and their respective source areas, they can be subdivided into those which have a peridotitic origin (P-type) and those which have an eclogitic origin (E-type). Even though a diamond may be found as a xenocryst (single crystal) within the Kimberlite matrix, inclusions or flaws within the diamond crystal can identify its origins. These flaws or inclusions within diamonds can consist of associated minerals such as garnet, pyroxene, olivine, chromite, and sulphides (pyrrhotite). The chemistry of the mineral inclusion can be compared with that of those present in the eclogite and peridotite fragments to determine their source. These inclusions also provide valuable information which allows for the determination of pressure and temperature of formation as well as age.

Chemistry of Diamond inclusions

After Kirkley, M. B. et. al. (1991)

Age of Diamonds

By analysing the diamond inclusions such as pyroxene and garnet, which contain measureable quantities of radioactive elements (U/Pb), it is possible to establish a radiometric age for the inclusion and by extension for the the diamond containing it. Similarly radiometric age dates can be established for the minerals which crystallized during the emplacement of the Kimberlite host rock. Comparing the age of the diamond with that of the host rock, we see that the diamond crystals are older than the time at which they were emplaced. In many cases diamond crystals have been resident in the upper mantle for billions of years prior to their more recent emplacement (~100 my age). This geochronological evidence supports the view that diamonds are formed in the upper mantle of the earth and do not crystallize as part of a Kimberlite 'magma' as once had been believed.

Age of diamonds and age of emplacement
Location (Mine)	Age of Diamond	Age of Kimberlite	Type of Inclusion
Kimberley, South Africa	~3,300	~100	Peridotitic
Finsch, South Africa	~3,300	~100	Peridotitic
Finsch, South Africa	1,580	~100	Eclogitic
Premier, South Africa	1,150	1,100-1,200	Eclogitic
Argyle, Australia	1,580	1,100-1,200	Eclogitic
Orapa, Botswana	990	~100	Eclogitic
After Kirkley, M. B. et. al. (1991)

Carbon Sources

The classification of diamond crystals based on peridotitic (P-type) and eclogitic (E-type) host source rocks is also reflected in the carbon isotopic composition of the diamond. Carbon has isotopic characteristics that can guide us in determining the source of the carbon and as such provide some insight into how the diamond and its source rock formed.

Carbon Isotopic Composition of Diamonds
Carbon isotopic studies, generally speaking, measure the Carbon 13 to Carbon 12 ratio (expressed as delta 13 C). These Carbon isotopic ratios indicate that diamonds of a peridotitic origin have a smaller range (-10 to 0) than diamonds of an eclogitic origin   (+3 to -34). Modified from Kirkley, M. B. et. al. (1991)

The explanation for this difference is based on the differing origins for these rocks and their contained carbon. The eclogite is thought to form as the residue from partial melting of a subducting oceanic slab which contains rocks with a wider range of carbon isotopic values. This carbon may be derived from carbonates and hydrocarbons near surface. On the other hand, the peridotitic diamonds have carbon isotopes that have not been significantly fractionated (smaller range) and are probably derived from a more homogeneous source representing original constituents of the primitive earth's mantle.

Emplacement

Diamond-bearing Kimberlites and related rocks represent constituents of the upper mantle which have been emplaced into shallow levels of the earth's crust. A basic requirement for their emplacement is the presence of fractures which extend below the base of the craton. Once a fracture taps these deep levels, the combination of high pressure, temperature and high content of volatile constituents (H2O and CO2) result in the release of a Kimberlite 'magmatic fluid' from depths of at least 100 kilometres. The initial ascent is rapid, with speeds estimated in the order to 10-30 kph. Within 2 - 3 km of surface, the velocity of the magmatic fluid increases to several hundred kph ( Kirkley, M. B. et. al. , 1991). This rapid increase is due to a combination of low pressure and interaction with high-level ground water, resulting in explosive brecciation of wallrocks and the formation of a kimberlite 'pipe'.

Kimberlite Pipes

The explosive emplacement of the volatile-rich kimberlite magmatic fluid results in the development of near surface kimberlite 'pipe'. These pipes have a 'carrot' shape ranging in diameter from several hundred metres near surface, narrowing to 1 -10 metres at depth. The carrot-shape pipes generally extend over a vertical distance of 2 - 3 kilometres.

Idealized Model of a Kimberlite Pipe
Modified from Kirkley, M. B. et. al. (1991)	An idealized model of a kimberlite pipe can be subdivided into three zones: the root, diatreme, and crater. The root zone is characterized by crystallized kimberlite magma with typical intrusive textures and containing xenoliths (fragments) and xenocrysts (crystals). The root zone extends into a feeder dike or fractures along which the magmatic fluid passed through. The diatreme zone is the main source of diamonds due to its larger size and volume of kimberlite rock. It contains the bulk of the xenoliths derived from the mantle as well as framents of wall rock. This is the zone where rapid expansion of the kimberlite magmatic fluid occurs with its attendant explosive behaviour due to the contained volatiles. With expansion, rapid cooling also takes place such that there are few thermal contact metamorphic effects and diamond crystals are able resist conversion to graphite.
When a kimberlite pipe is emplaced, the surface expression is that of a small explosive volcanic eruption consisting of fragments and hot gases (pyroclastic). This volcanic explosion results in the formation of a small volcanic edifice consisting of a crater (Maar) and a pyroclastic (tuff) ring. Kimberlite volcanoes have not been documented mostly because they tend to be small in size and are easily eroded.	Shape of the A154South Diavik Kimberlite Pipe, Reference: Aber Diamond Corporation.

Putting it all together

Although diamond crystals form in the upper mantle below cratonic areas, they can only remain stable at these high pressures and temperatures. The mantle xenoliths and diamond crystals that are brought quickly to surface in a Kimberlite magmatic fluid are able to survive near surface in a 'quenched' or 'meta-stable' state. If the intrusion of kimberlite is delayed during its rise to surface or is trapped in the lower crust, diamond crystals will not be stable in the P-T environment and will revert to graphite.

It is under shield areas or cratons that the diamond crystals can remain stable at shallower depths due to the low geothermal gradient related to the sub-cratonic keel beneath continental crust. This P-T environment has been referred to as the diamond 'storage area' (Kirkley, M. B. et. al., 1991). The keel area is an optimal source for diamonds since fractures below the craton are more likely to tap this area and remain accessible to the surface.

Diamond-Graphite Stability & Geothermal Gradients Modified from Kirkley, M. B. et. al. (1991)

Peridotitic diamonds are most likely to be tapped from this storage area, but if the fractures extend further, they may also include eclogitic diamonds (K1). Eclogitic diamonds form at greater depth than peridotitic diamonds and represent the residue from the partial melting of subducted oceanic slabs that underplate the cratonic/upper mantle 'keel' (K2). These areas also give rise to Lamproite (Kimberlite cousin) intrusions closer to the edges of the craton (L1)

Idealized model of sources of diamonds beneath the craton

Modified from Kirkley, M. B. et. al. (1991)

Under oceanic crust, temperatures increase more rapidly than under shield areas (high geothermal gradient). The equilibrium line between diamond and graphite intersects the oceanic geothermal gradient at depths of greater than 200 kilometres. This depth is considered to be unsuitable for the production of diamonds, likely due to the inability of fractures to propagate to that depth in an oceanic envirnment (too hot). However, recent discoveries of diamond-bearing 'Lamproite-like' rocks have been identified in Archean volcanic terrane (Michipecoten Greenstone Belt). It remains to be seen whether these micro and macro-diamonds in Archean-age rocks will carry diamonds of a sufficient size and quantity to be economic.

Diamond crystals may in fact be fairly common below cratonic and oceanic areas at the required P-T conditions for their formation. Those below cratons have produced economic diamond mines, likely due to their long residence time which perhaps gives the diamond the opportunity to grow to gem quality.

Exploration for Diamonds in Ontario

Much of the shield area of Ontario is highly fractured, with large fractures and faults providing access ways for deep-level igneous rocks as exemplified by the presence of carbonatite and accompanying alkalic intrusions. Most kimberlite pipes discovered to date are about 150 my old and follow older structures that have had repeated movement over time (Kapuskasing Structural Zone); a few pipes are mid-Proterozoic in age. Notable, the kimberlite pipe discoveries tend to have been found within Paleozoic cover rocks overlying the craton. This may simply reflect the use of geophysical techniques for exploration and the high magnetic contrast between the sedimentary rocks and the kimberlite pipes. There is also the view that the change from igneous and metamorphic crust into the overlying sedimentary cover rocks may also influence pipe development.

Kimberlite Indicator Minerals (KIM) in the Batchewana Area, Ontario
Analysis of surficial deposits of recent river gravels and sands indicate that kimberlite indicator minerals (KIM) are present over wide areas of Ontario. These indicator minerals are the mineralogical and chemical signature that identify a kimberlite source rock. The major problem for the prospector and explorationist is that these samples represent transported (glacial) overburden, such that the original KIM source is very difficult to trace. Note: pie size proportional to KIM #, Colour = KIM type
Lithology Legend: Green = Greenstone Belt Yellow = Gneisses Purple = Granites Grey = Metasediments Black& Blue = Mafic Intrusions

Prospectors and explorationists require knowledge of the glacial geology, transport distance of various glacial materials, and the drainage basins from which the samples were taken. It is not simply a matter of staking a claim around a KIM anomaly.

One place to start is by prospecting around these KIM anomalies, in the country rocks for any signs of brecciation, breccia zones, and the presence of exotic fragments (eclogite and peridotite). Geological prospecting can be supplemented by detailed magnetic surveys since many kimberlite pipes will have a stronger magnetic signature than the surrounding rocks and will have a small, generally circular shape.

Not all Kimberlites are equal

Finally, not all kimberlite pipes have diamonds of economic value. Discovery of a kimberlite pipe does not in itself make a mine, but it certainly raises the diamond potential of an area. The presence of micro diamonds or even macro diamonds serve as another significant indicator of the diamond potential of a diamond prospect. From results of diamond exploration in Nunavut, the real economic value comes from the presence of larger stones with a higher weight per ton of rock.

Major World Diamond Deposits

Reference: Aber Diamond Corporation web site (www.aber.ca); Green represents Aber deposits

Diavik A154South pipe diamond reserves, Nunavut, Canada
	Tonnes (millions)	Grade (carats/ tonne)	Carats (millions)
Measured	8.2	5.6	45.3
Indicated	3.2	5.1	16.7
Inferred	0.6	3.2	1.9
Total	12.0	5.3	63.9
Reference: Aber Corporation web site (www.aber.ca)

Reference: Aber Diamond Corporation web site (www.aber.ca)

Note the large quantities of micro and macro diamonds in these economic diamond deposits (exponential scale).

References:

Cannon, W.F. and Mudrey, M.G.Jr. 1981: The Potential for Diamond-bearing Kimberlite in Northern Michigan and Wisconsin, Geological Survey Circular 842, U.S. Department of the Interior.

Kirkley, M. B., Gurney, J.J., Levinson, A.A., 1991: Age, Origin, and emplacement of Diamonds: Scientific Advances in the last Decade; Gems and Gemology, Vol. 27, No. 1, pp2-25, Gemological Institute of America.