Info on diamondsfrom the Northern Miner, that is borrowed from another board. Because there is Kimberlites does not mean diamonds as only 3% of Kimberlites have diamonds, and only 1% explorable, then there are the different types etc..
Diamond deposits June 1-8, 1998
by Derek 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 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.
Diamonds fall into one of four categories. They are, in order of decreasing value: gem, near-gem, industrial and boart. Individual diamonds are measured in carats (1 carat equals 0.2 gram), whereas the grade of diamondiferous rock is expressed in carats per tonne (or carats per 100 tonnes).
World diamond production is in the order of 100 million carats per year. In 1997, the Argyle deposits of Western Australia (the world's largest producer) produced 40.2 million carats from ore grading 3.7 carats per tonne. Highly variable grades can make the value of ore in US dollars per carat quite unpredictable.
The major diamond producing nations are South Africa, Botswana, Australia, Russia and Zaire. In Canada, production from the BHP Diamonds-Dia Met Minerals operation at Lac de Gras, N.W.T., will begin later this year. The vast majority of diamond production is from kimberlites, with only the Argyle deposits providing substantial production from lamproite sources. About 3% of kimberlite pipes, which can occur in clusters of up to 50, contain diamonds, and only 1% of those occurrences are economically exploitable.
Exploration for diamonds, which occur as xenolithic crystals or fragments within kimberlites or lamproites (both of which are intrusive ultramafic igneous rock types), in heavily glaciated areas is difficult because kimberlites and lamproites are soft compared with other rock types, and are likely to be preferentially eroded as a result. The most useful exploration technique, therefore, is geochemical surveying of till and other alluvium. Positive identification of intrusive rocks from a series of samples collected during an exploration program requires detailed petrographic examination and evaluation of the constituent minerals.
Indicator minerals within kimberlite or lamproite need to be geochemically analyzed and classified to determine the intrusion's potential to contain diamonds. The precious stones are a relatively minor mineralogical constituent in those intrusives, though the indicator minerals are sufficiently abundant to be readily evaluated.
Similarly, the composition of indicator minerals in soils, tills and stream sediments can be analyzed to determine if such detrital material was eroded from an area that contained diamondiferous rocks. Indicator minerals include chromite, garnet and ilmenite, each of which has a distinct geochemical signature in diamondiferous rocks.
Critical to the evaluation of diamond potential is the precise analysis of rocks or detrital material, and their indicator minerals, to define their petrological and geochemical compositions. Based on the composition of the sample analyzed, different preparation techniques are required. In order to evaluate a particular kimberlite or lamproite intrusive, bulk samples of more than 30 kg are usually collected. Indicator minerals and diamonds are separated from the sample, producing a heavy mineral concentrate (HMC) for analysis. Heavy minerals will be similarly separated from large bulk samples of detrital material for analysis. The evaluation of diamond prospects is time-consuming owing to the exacting concentration of the minor constituents from such large samples and the precision required to analyze the HMC. Kimberlite and lamproite intrusives often exhibit circular magnetic (mag) or electromagnetic (EM) geophysical anomalies that reflect the elevated mag or EM properties of the intrusives compared with the country rock, usually returning a bull's-eye pattern. The problem with these surveys is that the craters or pipes cover such a small area that it may be difficult to distinguish the anomalies from regional gradients.
Kimberlite and lamproite craters and kimberlite diatremes are initially mined as open-pit operations because the host rocks are usually friable. Underground production is frequently initiated with increases at depth, ely in diatremes. Alluvial sources are mined as open-pit operations. The ore is crushed and diamonds, because of their hardness, are readily separable. -- The author is a professor of geology at Memorial University in St. John's, Nfld.