GEOLOGY 101 — Diamond deposits, Pt. 1

.BDerek Wilton

Diamond, which is pure carbon, is the hardest substance known, yet diamonds can be broken relatively easily. Paradoxically, graphite, one of the softest minerals, is a polymorph of diamonds. The essential difference between the relative strengths of these two minerals lies in their crystal structures: in diamonds, carbon atoms are linked in an isometric form, whereas atoms in graphite form linked hexagonal sheets.

Diamond crystal structures are created when carbon is subjected to great pressures (between 45 and 55 kilobars) and high temperatures (1,050 C to 1,200 C). The zone of formation of diamonds, therefore, is below the Earth’s crust, in the upper mantle. Rarely, microdiamonds can be found at meteorite impact sites, where shock-derived pressures and temperatures are sufficiently intense to transform carbon into diamonds. Diamonds can also be produced synthetically.

Diamond crystals can revert to graphite if subjected to changes in pressure and temperature over time.

On the Earth’s surface, diamonds are found in unusual intrusive ultramafic igneous rocks or in placer and paleoplacer concentrations. In these deposits, diamonds are not hosted by the upper mantle rocks, namely peridotite or eclogite, in which they primarily formed.

The ultramafic igneous rocks that contain diamonds at the Earth’s surface are kimberlites or lamproites. Kimberlites are volatile-rich (containing H2O and CO2) potassic, ultrabasic rocks which have an unequigranular grain size, with macrocrysts (magmatic crystals and rock-crystal fragments measuring 0.5 to 15 mm across) and megacrysts (greater than 2 cm and up to 20 cm across) set in a fine-grained matrix. Lamproites are ultrapotassic, magnesium-rich rocks which, unlike kimberlites, contain no CO2.

The feature of kimberlites essential to their containing diamonds is their volatile content, as volatiles cause magmas to intrude explosively from the lower crust or upper mantle to the earth’s surface.

Diamonds are carried in kimberlites and lamproites as xenolithic crystals, or xenocrysts. While in transit from their melt sources, magmas pick up diamonds from their host rock and carry them upward. The magma essentially acts as a high-speed elevator, 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. It is the great hardness of the diamonds that allows them to survive the explosive intrusion.

Kimberlite and lamproite systems have distinctive intrusive architectures.

The uppermost part of an intrusive body in a kimberlite is carrot-shaped, and has its roots in dykes and sills (hypabyssal or medium-depth intrusive rocks).

Pipes, which are generally up to 2 to 3 km wide, originate at a root zone, rise through diatreme facies to the crater facies, where the kimberlite actually breaches the Earth’s surface. The walls of the diatreme dip at angles of 75 to 85 from the horizontal; the crater walls exhibit shallower dip. The crater is the widest part of the pipe, but it seldom exceeds 2 km in diameter or 250 ha in area.

In contrast, lamproites are not pipe shaped and consist of crater facies fewer than 500 metres in depth. Diamondiferous lamproite craters are up to 1.25 km in diameter with an area of about 125 ha.

Kimberlites and lamproites also contain xenocrysts of garnet and spinel.

Diamonds are actually a very minor component of kimberlite and lamproite magmas, whereas other xenocrysts appear in greater abundance.

Placer and paleoplacer (Geology 101, T.N.M., Jan 18-25/98) deposits, also known as alluvial deposits, form through the weathering and erosion of diamond-bearing kimberlites or lamproites. Diamonds form detrital placer grains due to their great hardness (they can withstand the erosional processes) and higher density compared with other detrital material.

Diamondiferous kimberlites and lamproites are essentially secondary concentrations, whereas placer and paleoplacers deposits are tertiary.

— The author is a professor of geology at Memorial University in St.

John’s, Nfld.