Diamond Geology
Kimberlite Pipes
Diamonds form at a depth greater than 93 miles (150 kilometers) beneath the earth's surface. After their formation, diamonds are carried to the surface of the earth by volcanic activity. A mixture of magma (molten rock), minerals, rock fragments, and occasionally diamonds form pipes shaped like champagne flute glasses as they approach the earth's surface. These pipes are called kimberlites (see diagram below). Kimberlite pipes can lie directly underneath shallow lakes formed in the inactive volcanic calderas or craters.
Kimberlite is an igneous-rock matrix composed of carbonate, garnet, olivine, phlogopite, pyroxene, serpentine, and upper mantle rock, with a variety of trace minerals. Kimberlite occurs in the zone of the Earth's crust in vertical structures known as kimberlite pipes (above, right). Kimberlites are found as "dikes" and "volcanic pipes" which underlie and are the source for rare and relatively small volcanoes or "maars" (above, left). Kimberlite pipes are the most significant source of diamonds, yet only about 1 in every 200 kimberlite pipes contain gem-quality diamonds. Many kimberlite pipes also produce alluvial diamond placer deposits.
Photo (left) Attribution - Unknown/Public Domain
Diamond bearing kimberlite in some parts of South Africa is black in color (above, right). Most kimberlite is called "blue-ground" kimberlite (above, left) or "yellow-ground" kimberlite and can be found worldwide. The name "Kimberlite" was derived from the South African town of Kimberly where the first diamonds were found in this type of rock conglomeration (see section on "Kimberley - North Cape" below).
Photo (left) Attribution - Unknown/Public Domain
Lamproite Pipes
Lamproite pipes produce diamonds to a lesser extent than kimberlite pipes. Lamproite pipes are created in a similar manner to kimberlite pipes, except that boiling water and volatile compounds contained in the magma act corrosively on the overlying rock, resulting in a broader cone of eviscerated rock at the surface. This results in a martini glass shaped deposit as opposed to kimberlite's champagne flute shape.
Alluvial (Placer) Diamonds
The location of alluvial (or placer) diamond deposits is controlled by the surrounding topography. Alluvial diamond deposits are usually located within river terrace gravels that have been transported from their location of origin, usually from kimberlite deposits.
The alluvial terrace gravels (below, left) and marine gravels of the south-western coastline of Africa represent the some of the world's largest placer diamond deposits. The world's largest known gem quality alluvial diamond deposit is located along the Namib Desert coastline of southwestern Africa, known as the Sperrgebiet or "forbidden territory". Namibia's placer diamond deposits are up to 40 million years old.
Alluvial diamond mining in Angola takes place along a meandering stretch of the Cuango River flood-plain which is also along the south-western coastline of Africa. Some of the largest and highest gem-quality diamonds produced from alluvial placer diamond mining have come from this region, including Angola's two largest diamonds at 105.9k and 101.8k.
Many of these alluvial diamond deposits occur in Pleistocene and Holocene successions (1.8 million to 10,000 years ago). The diamonds within these deposits were transported from deeply eroded kimberlites or, to a lesser extent, from olivine lamproites formed during the Cretaceous or Permo-Triassic period. Westward draining river systems transported these diamonds to Africa's continental coastline for final deposition within on-shore marine terrace gravels. Diamonds that were transported downstream, but were not deposited on land, made their way to the sea bed just offshore. Diamonds in marine areas are typically trapped in bedrock depressions such as gullies, potholes, depressions, channels or other trapsites for diamondiferous deposits.
Morphology and volcanology<
Kimberlites occur as carrot shaped, vertical intrusions termed 'pipe'. This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma that inherit a large proportion of both CO2 and H2O in the system which produces a deep explosive boiling stage that causes a significant amount of vertical flaring (Bergman, 1987). Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement and Skinner 1985, and Clement, 1982).
The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within 1.5-2 km of the surface the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved but is usually similar to a maar volcano. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer.
Petrology
Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source.
Historically, kimberlites have been subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who re-named these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that Group II kimberlites actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion.
Group I kimberlites
Group I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromian pyrope, almandine-pyrope, chromian diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture cause by macrocrystic (0.5-10 mm) to megacrystic (10-200 mm) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite in a fine to medium grained groundmass.
The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.
Group II kimberlites
Group-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of orangeites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.
Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopside rimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite); Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite); potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite.
Kimberlitic indicator minerals
Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (a pyroxene), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.
These indicator minerals are generally sought in stream sediments in modern alluvial material. Their presence, when found, may be indicative of the presence of a kimberlite within the erosional watershed which has produced the alluvium.
Geochemistry
The geochemistry of Kimberlites is defined by the following parameters;
- Ultramafic; MgO >12% and generally >15%
- Ultrapotassic; Molar K2O/Al2O3 >3
- Near-primitive Ni (>400 ppm), Cr (>1000 ppm), Co (>150 ppm)
- REE-enrichment
- Moderate to high LILE enrichment; SLILE = >1,000 ppm
- High H2O and CO2
Economic importance
Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. However, only about 1 in 200 kimberlite pipes contain gem-quality diamonds.
The deposits occurring at Kimberley, South Africa were the first recognized and the source of the name. The Kimberley diamonds were originally found in weathered kimberlite which was colored yellow by limonite, and so was called yellow ground. Deeper workings encountered less altered rock, serpentinized kimberlite, which miners call blue ground.
The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 1800's accidentally cut into the blue ground and found gem quality diamonds in quantity. The economic importance of the time is that with flood of diamonds being found, the miners were undercutting each other's price of the diamonds and eventually decreased the diamonds value down to cost in a short time.
Jagersfontein Big Hole Open Mine (Jagersfontein)
The Jagersfontein Mine is a huge open cast mine, and is the biggest man made hole on Earth. If you are confused now, that's okay: it is always said that the Big Hole at Kimberly was the biggest man made hole. But they never really checked this, it was a convenient assumpion and good for tourism, and so they used it and still use it although it is reportedly untrue. Jagersfontein mining started three years earlier than Kimberly, the mine is the oldest of its kind. The world's biggest vertical man-made hole and was dug using only pick, shovel and dynamite. It produced two of the ten largest natural diamonds of the world, the 972-carat Excelsior diamond and the 637-carat Reitz diamond, today called the Jubilee. The mining continued for about one century, with slow downs during the World Wars. But today mining has ended and the mines are closed.
Polish up on diamond history at Jagersfontein Mining Museum - The little town of Jagersfontein is an hour's drive outside of Bloemfontein yet stretches back centuries in time as the world's oldest diamond mining town. Contrary to its simple modern day existence, Jonkersfontein was once the most glamourous town in the Free State thanks to the discovery of its natural diamond assets by the 18th century pioneers. As the first town in the Free State to have piped water and electricity it subsequently had a social life for high-flyers of the day boasting Sir Herbert Baker buildings, its own newspaper, bank, town hall and theatre. The farm that this mining town was established on in 1870 was once the property of Grigua Jacobus Jagers, hence the name Jagersfontein (Jager's Fountain); colloquially referred to as 'Jaggers'. None other than Al Capone and Elizabeth Taylor are amongst the world's celebrities who have proudly adorned themselves with so-called 'Jagger' diamonds world renown for their size, clarity, quality and colour. The term 'Jagers' has subsequently been coined to denote the pale blue tint distinctive of diamonds mined in Jaggersfontein.
In South Africa the diamond mining industry was born before the gold mining era and thus Jagersfontein is thought of as the 'birthplace of the South Africa mining industry'. The Jagersfontein Open Mine (Big Hole) boasts the biggest man-made hole on earth which was dug by the hands of enthusiastic diamond diggers aided only by simple pick and shovel. Olden day transport and original mining instruments archived in displays at this mining village's museum details the history of this local mine that produced eight of the worlds biggest two dozen diamonds. 1893 Saw the discovery of the Excelsior Diamond measuring 972 carats and second only in size to the Cullinan Diamond. Formed from the purest water this gem now described as a blue white diamond is estimated to be worth R1.2 Billion Rand today. In 1895 an additional history making diamond of 637 carats was excavated on site and was originally named after Francis William Reitz, the state president of the Orange Free State at the time, namely the Reitz Diamond. However the following year marked the 60th anniversary of Queen Victoria's coronation (her diamond jubilee) and this prized gem was renamed for the occasion becoming the Jubilee Diamond in her honour. The Excelsior and Jubilee Diamonds are two of the world's ten biggest diamonds in history. The Jagersfontein Mining Museum pays homage to the first place in the world where diamonds were discovered in Kimberlite; their mother stone or blue ground. The Jagersfontein mining area is known as a Kimberlite Pipe. Geologists confirm that this abandoned open-pit mine is prime locality for finding mantle xenoliths originating 300 - 500km below the earth's surface.
Enjoy the great facts of the little place that individually produced the greatest volume of the world's biggest diamond's. Discover the interesting trivia of South Africa mining that led the world in the appreciation of flawless twinkling beauty. Take a stroll and see the mining site of history at the neighbouring Big Hole, the site is no longer active as a mine but the outlook is more impressive than ever; afterall this is the place that had the foresight to notice a diamond in the rough.
Classification of Diamonds
Background
When selecting a diamond, most jewellers refer to the 4 C’s:
· Colour
· Carat or weight
· Clarity
· Cut
These four factors largely determine the value of a diamond.
Colour
Diamonds come in a range of colours from faint yellow or brown, through to very rare pinks, blues, greens and other colours known as fancies. The best colour for a diamond is no colour, i.e. a diamond that allows white light to pass through it effortlessly and be dispersed in a rainbow of colours.
The best colourless stones carry a grading allocated ‘D’. From here, grades are designated letters ranging all the way through the alphabet down to Z, for diamonds which may be light yellow, brown or grey. Colour gradings are difficult compare/determine without the correct conditions and equipment.
The colour of diamonds is determined by the presence of trace elements present in the atomic structure of the diamond. The more intense the colour, the further the grading descends down the scale.
Some colours occur extremely rarely and are calssified by a different scale. Such colours may include pinks, blues, greens, amber and red. As mentioned above, these colours are called ‘fancy colours’, with classifications dertermined by hue and saturation.
Carat or Weight
The weight of diamonds is measured in ‘carats’ (often abbreviated to ‘ct’), with diamonds being available in various weights and fractions of a carat. One carat is divided up into 100 points such that 0.75 carat is the same as ¾ carat.
The word carat is derived from the carob tree or Ceratonia Siliqua, which produces seeds of high uniformity and consistent weight. Diamonds and other gemstones were originally weighed against these seeds. More recently, one carat (or 100 points) has been standardised at 0.2grams
While size is a major determining factor in the price of a diamond, quality can also play a significant role. It should also be noted that the larger a diamond, the rarer it is, which will also have a bearing on its value.
Clarity
Almost all diamonds contain inclusions (often called birthmarks) which affect its clarity. There are an internal flaws or inclusions. External flaws also exist and may consist of surface irregularities or blemishes which also affect clarity.
Imperfections may not be seen by the naked eye but can be seen using microscopes operated at 10x magnification. Clarity of diamonds is graded using a complex method which evaluates the size, location and visibility of the inclusions.
The fewer the number of inclusions, the rarer the diamond and hence the higher its desirability and value. The type, size and number of imperfections influences a diamonds’ grade. These grades are summarised in table 1.
Table 1. Classifications of diamonds.
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Classification
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Description
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Flawless and Internally Flawless
F1 & FL
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Flawless is the highest grade. These diamonds contain no detectable flaws when viewed with a microscope at 10x magnification.
Internally flawless diamonds contain no visible internal flaws when viewed with a microscope at 10x magnification, but have minor surface blemishes.
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Very Very Slightly Included
VVS1 & VVS2
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The classification is divided into 2 categories. They contain minute inclusions often only seen by experienced graders under a microscope at 10x magnification.
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Very Slightly Included
VS1 & VS2
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Again, this category is divided into 2 categories. They contain slight imperfections which are still difficult to detect under a microscope.
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Slightly Included
SI1 & SI2
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Also divided into 2 categories. These diamonds contain slight imperfections that can be seen under a microscope at 10x magnification.
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Included
I1, I2 & I3
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This category is divided into 3 classes, with increasing number indicating flaws that are more obvious, with some being visible to the naked eye.
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Cut
Often confused with the shape of a diamond. Diamonds are cut into many shapes, with the shape normally dictated by the shape of the rough diamond. This is only factor that is not dictated by nature.
When cut by a skilled diamond cutter, not only is the resultant diamond a sight to behold, but it is also better able to handle light, producing more scintillation and sparkle or brilliance.
A well cut diamond reflects light from one facet to another and then out through the top. A diamond cut too deep (called a nailhead) or shallow (called fish eye) will allow light to escape through the bottom or pavillion portion.
Diamonds are usually cut with 58 flat surfaces of facets. The size and location (in relation to other facets) are determined by a precise mathematical formula, designed to maximise a diamonds’ brilliance.
Some common cuts of diamond include:
· Round
· Princess
· Oval
· Marquise
· Emerald
· Heart
Address: From Bloemfontein take R706 then take the R704 and travel 108km to Jagersfontein.
Telephone: +27 (0)51 411-4300
Opening hours: Call to confirm viewing hours.
Scources:
http://www.khulsey.com/jewelry/kh_jewelry_diamond_mining.html
http://www.answers.com/topic/kimberlite
To qualify:
1. Upload a photo of any feature of the mine from the viewpoint. Uploading a photo of yourself is not a requirement anymore and is optional.
2. Name the digging methods used to create this huge hole?
3. Name the 4 C's in the classification process of diamonds.
3. Estimate the width, length and depth of the Big Hole.
4. What is the diamond bearing substance called and where did the name came from.
5. How much carats of diamonds were mined at the Jagersfontein open mine?
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