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Kimberlite Cabochon Diamond bearing rock Kimberley Mine South Africa Kimberlit
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Country/Region of Manufacture: | Germany | Kimberlite Kimberley Mine SA: | Diamond bearing kimberlite ultramafic igneous rock |
Product Description
Size : 16 x 12.3 x 6.9 mm Weight : 15.03 carat
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Kimberlite, also called blue ground, a dark-coloured, heavy, often altered and brecciated (fragmented), intrusive igneous rock that contains diamonds in its rock matrix. It has a porphyritic texture, with large, often rounded crystals (phenocrysts) surrounded by a fine-grained matrix (groundmass). It is a mica peridotite, and its most abundant mineral constituent is olivine. Other abundant minerals include phlogopite mica; chromium- and pyrope-rich garnet phenocrysts; chrome-bearing diopside, particularly in the African kimberlite; ilmenite; serpentine; calcite; and magnetite. Kimberlite, along with a similar rock called lamproite, is important for delivering diamonds to the crust through magmatic intrusions that solidify into pipelike structures.
Kimberlite occurs in the uplifted centres of continental platforms. In the Kimberley district, South Africa, it forms pipes (funnels, more or less oval in cross section, that become narrower with increasing depth) and, occasionally, dikes. Other occurrences include the dikes at Ithaca, N.Y., the Kimberley and Lake Argyle regions of Australia, and the lavas in the Iswisi Hills, Tanzania.
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Big Hole
The Big HoleLocation Kimberley HoleLocation KimberleyProvince Northern CapeCountry South AfricaCoordinates 28°44′19″S 24°45′31″E / 28.73861°S 24.75861°E / -28.73861; 24.75861Coordinates: 28°44′19″S 24°45′31″E / 28.73861°S 24.75861°E / -28.73861; 24.75861Production Products DiamondsType Open-pit, undergroundHistory Opened 1871 (1871)Closed 1914Owner Company DeBeers
The Big Hole, Open Mine, Kimberley Mine or Tim Kuilmine[1] (Afrikaans: Groot Gat) is an open-pit and underground mine in Kimberley, South Africa, and claimed to be the largest hole excavated by hand, although this claim is disputed.[2]
ContentsThe first diamonds here were found on Colesberg Kopje by members of the "Red Cap Party" from Colesberg on the farm Vooruitzigt belonging to the De Beers brothers, in 1871. The ensuing scramble for claims led to the place being called New Rush, later renamed Kimberley.[3][when?] From mid-July 1871[3] to 1914 up to 50,000 miners dug the hole with picks and shovels,[4] yielding 2,720 kilograms (6,000 lb) of diamonds. The Big Hole has a surface of 17 hectares (42 acres) and is 463 metres (1,519 ft) wide. It was excavated to a depth of 240 metres (790 ft), but then partially infilled with debris reducing its depth to about 215 metres (705 ft). Since then it has accumulated about 40 metres (130 ft) of water, leaving 175 metres (574 ft) of the hole visible. Once above-ground operations became too dangerous and unproductive, the kimberlite pipe of the Kimberley Mine was also mined underground by Cecil Rhodes De Beers company to a depth of 1,097 metres (3,599 ft).[5]
There is currently an effort in progress to register the Big Hole as a World Heritage Site.[6]
ExcavationIn 1872, one year after digging started, the population of the camp of diggers grew to around 50,000. As digging progressed, many men met their deaths in mining accidents. The unsanitary conditions, scarcity of water and fresh vegetables as well as the intense heat in the summer, also took their toll. On 13 March 1888 the leaders of the various mines decided to amalgamate the separate diggings into one big mine and one big company known as De Beers Consolidated Mines Limited, with life governors such as Cecil John Rhodes, Alfred Beit and Barney Barnato. This massive company further worked on the Big Hole until it came to the depth of 215 metres, with a surface area of about 17 hectares and perimeter of 1.6 kilometres. By 14 August 1914, when over 22 million tons of earth had been excavated, yielding 3,000 kilograms (14,504,566 carats) of diamonds, work on the mine ceased after it was considered the largest hand-dug excavation on earth. By 2005, however, it was reported that a researcher had re-examined mine records and found that the hand-dug portions of the Jagersfontein and Bultfontein diamond mines, also in South Africa, may have been deeper and/or larger in excavated volume.[2] There are other, larger, mine excavations, but these were created using earth-moving equipment rather than manual labour.
Mine Museum A sign next to the hole, reading "If all the diamonds recovered from the Kimberley Mine could be gathered together they would fill three cocopans such as these".Mining operations having been closed down in 1914, the open pit became an attraction for visitors to the city and by the 1960s a gathering together of relics of Kimberleys early days, including old buildings and sundry memorabilia, began to be organised into a formal museum and tourist attraction. In 1965 De Beers appointed Basil Humphreys as museum consultant, with the museum being substantially up-graded as an open-air representation of early Kimberley, with streetscapes and dioramas, and exhibits of mining technology and transport. There was an official opening during the Kimberleys centenary celebrations in 1971. One of the attractions was the Diamond Hall. The Mine Museum went through subsequent further upgrades. Between 2002 and 2005 De Beers invested R50 million in developing the Big Hole into a world-class tourism facility, based on the idea of creating "a lasting legacy for the people of Kimberley." The new facility, the Big Hole Kimberley, elaborating a theme of Diamonds and Destiny, was expected to double visitor numbers to the Big Hole.[7][8]
See alsoKimberlite pipesMir diamond pipeUdachnaya pipeReferences- Jump up ^ "Griekwastad Tourism Brochure" (PDF). Mary Moffat Museum. Retrieved 3 September 2015.
- ^ Jump up to: a b http://www.news24.com/SouthAfrica/News/Big-Hole-loses-claim-to-fame-20050519
- ^ Jump up to: a b Roberts, Brian. 1976. Kimberley, turbulent city. Cape Town: David Philip & Kimberley Historical Society
- Jump up ^ Williams, Gardner F. (1904). The diamond mines of South Africa. New York: B. F. Buck & company. p. 199.
- Jump up ^ Show mines of South Africa: Kimberley Mine - Big Hole
- Jump up ^ Bid to plug Big Hole worldwide, News24
- Jump up ^ The Big Hole Kimberley - Diamonds and Destiny
- Jump up ^ Re-envisioning the Kimberley Mine Museum:De Beers’ Big Hole Project
Kimberlite
Kimberlite from U.S.A. QEMSCAN mineral map of kimberlite from South Africa
Kimberlite is an igneous rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.70 g) diamond called the Star of South Africa in 1869 spawned a diamond rush, eventually creating the Big Hole.
Kimberlite occurs in the Earths crust in vertical structures known as kimberlite pipes as well as igneous dykes and sills. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), potentially from anomalously enriched exotic mantle compositions, and they are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.
Kimberlite has attracted more attention than its relative rarity might suggest that it deserves. This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earths surface. Its probable derivation from depths greater than any other igneous rock type, and the extreme magma composition that it reflects in terms of low silica content and high levels of incompatible trace element enrichment, make an understanding of kimberlite petrogenesis important. In this regard, the study of kimberlite has the potential to provide information about the composition of the deep mantle and about melting processes occurring at or near the interface between the cratonic continental lithosphere and the underlying convecting asthenospheric mantle.
ContentsMany kimberlites are emplaced as carrot-shaped, vertical intrusions termed pipes (kimberlite pipes). This classic carrot shape is formed due to a complex intrusive process of kimberlitic magma which inherits a large proportion of CO2 (lower amounts of 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 (0.93–1.24 mi) 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 (up to 0.6 mile).
Two Jurassic kimberlite dikes exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners.[1]
PetrologyBoth 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 renamed 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 kimberlitesGroup-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by primary forsteritic olivine and carbonate minerals, with a trace mineral assemblage of, magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic (0.5–10 mm or 0.020–0.394 in) to megacrystic (10–200 mm or 0.39–7.87 in) 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, is dominated by carbonate and significant amounts of forsteritic olivine, with lesser amounts of pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.
Olivine LamproitesOlivine Lamproites were previously called Group II kimberlite or orangeite in response to their mistakenly being considered to only occurring in South Africa. Their occurrence and petrology, however, are identical globally and should not be erroneously referred to as kimberlite.[2] Olivine lamproites are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O). The distinctive characteristic of olivine lamproites is phlogopite macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Al-poor phlogopite requiring Fe to enter the tetrahedral site). 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 mineralsKimberlites are peculiar igneous rocks because they contain a variety of mineral species with chemical compositions that indicate they formed under high pressure and temperature within the mantle. These minerals such as chromium diopside (a pyroxene), chromium spinels, magnesian ilmenite, and pyrope garnets rich in 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 may indicate the presence of a kimberlite within the erosional watershed which produced the alluvium.
GeochemistryThis section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2007)The geochemistry of Kimberlites is defined by the following parameters:
Ultramafic; MgO >12% and generally >15%Ultrapotassic; Molar K2O/Al2O3 >3Near-primitive Ni (>400 ppm), Cr (>1000 ppm), Co (>150 ppm)REE-enrichment[3]Moderate to high LILE enrichment; ΣLILE = >1,000 ppm- LILE = large ion lithophile elements[4]
Kimberlites are the most important source of primary diamonds. Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. About 6,400 kimberlite pipes have been discovered in the world, of those about 900 have been classified as diamondiferous, and of those just over 30 have been economic enough to diamond mine. [5]
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.
See also Mir Mine and Udachnaya pipe, both in Sakha Republic, Siberia.
The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 19th century accidentally cut into the blue ground and found gem quality diamonds in quantity. The economic importance of the time was such that, with a flood of diamonds being found, the miners undercut each others prices and eventually decreased the diamonds value down to cost in a short time.[6]
Related rock typesLamproite, Lamprophyre, Nepheline syeniteUltrapotassic igneous rocksKalsititic rocksNotes- Jump up ^ Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.
- Jump up ^ Francis and Patterson, 2009
- Jump up ^ Nixon, P.H., 1995. The morphology and nature of primary diamondiferous occurrences. Journal of Geochemical Exoloration, 53: 41-71.
- Jump up ^ Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland
- Jump up ^ "Diamond Investing FAQ", Mining.com, February 18, 2014.
- Jump up ^ "South Africa: A New History of the Development of the Diamond Fields" (1902): New York Times Archives [PDF file], New York Times.
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