Misplaced Pages

#839160

57-510: Geologically the Great Dyke is not a dyke , but is lopolithic and Y-shaped in cross-section. It is a group of layered ultramafic intrusions that extend across Zimbabwe with a strike of about N20°E. The width of the intrusions vary from 3 to 12 km (7.5 mi). The Great Dyke is unusual in that most ultramafic layered intrusions display near horizontal sill or sheet forms. The well-layered lower units of ultramafic rocks comprising

114-438: A central volcano or intrusion. Though they appear to originate in the central intrusion, the dikes often have a different age and composition from the intrusion. These radial swarms may have formed over the intrusion and were later cut by the rising body of magma, or the crust was already experiencing regional tension and the intrusion triggered formation of the fissures. In rock of the oceanic crust, pillow lava erupted onto

171-460: A curtain of fire where lava erupts along the entire length of a fissure several kilometers long. However, the length of erupting fissure diminishes over time, becoming focused on a short segment of less than half a kilometer. The minimum possible width of a dike is determined by the balance between magma movement and cooling. There may be more than one injection of magma along a given fissure. When multiple injections are all of similar composition,

228-497: A sill is a sheet intrusion that forms within and parallel to the bedding. Mafic magma (fluid magma low in silica) usually reaches the surface through fissures, forming dikes. At the shallowest depths, dikes form when magma rises into an existing fissure. In the young, shallow dikes of the Hawaiian Islands, there is no indication of forceful intrusion of magma. For example, there is little penetration of magma into

285-873: A 10% share in the company. In 1968, Implats entered into a prospecting accord with the Bafokeng tribe (now the Royal Bafokeng Nation ) and obtained a lease for 12,000 ha of land, with production kicking off in July 1969. During the 1970s, legislation introducing new vehicle emission standards worldwide, including those introduced and pursued in the U.S. by the Environmental Protection Agency (EPA), increased global demand for platinum. In 1974, Implats began supplying major motor manufacturer General Motors with up to 300,000 t oz  of platinum and 120,000  t oz of palladium

342-425: A consequence of the direction of minimum principal stress changing as the magma ascends from deep to shallow levels in the crust. An en echelon dike set may evolve into single dike with bridges connecting the formerly separate segments and horns showing former segment overlaps. In ancient dikes in deformed rock, the bridges and horns are used by geologists to determine the direction of magma flow. Where there

399-431: A crack then solidifies as a sheet intrusion , either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack. A magmatic dike is a sheet of igneous rock that cuts across older rock beds. It is formed when magma fills a fracture in the older beds and then cools and solidifies. The dike rock is usually more resistant to weathering than

456-418: A few millimeters to hundreds of meters, but is most typically from about a meter to a few tens of meters. The lateral extent can be tens of kilometers, and dikes with a thickness of a few tens of meters or more commonly extend for over 100 km. Most dikes are steeply dipping; in other words, they are oriented nearly vertically. Subsequent tectonic deformation may rotate the sequence of strata through which

513-710: A history of antagonism and violence between the African National Congress -allied National Union of Mineworkers (NUM) and its emerging rival, the Association of Mineworkers and Construction Union (AMCU). At the Marikana platinum mine, operated by Lonmin at Nkaneng near Rustenburg , 3,000 workers walked off the job on 10 August after Lonmin failed to meet with workers. On 11 August, NUM leaders allegedly opened fire on striking NUM members who were marching to their offices. The killing of two miners

570-608: A line extending for 250 km. Individual segments overlap, with the overlapping portions thinner, so that the combined thickness of the two overlapped portions is about the same as the thickness of a single segment. Other examples of en echelon dikes are the Inyo dike of Long Valley, California , US; the Jagged Rocks complex, Arizona , US; and the dikes of oceanic spreading centers . Dikes range in composition from basaltic to rhyolitic , but most are basaltic. The texture

627-543: A ring fracture. Magma rising into the ring fracture produces a ring dike. Good examples of ring dikes and cone sheets are found in the Ardnamurchan peninsula of Scotland. A feeder dike is a dike that acted as a conduit for magma moving from a magma chamber to a localized intrusion . For example, the Muskox intrusion in arctic Canada was fed by a large dike, with a thickness of 150 meters. A sole injection

SECTION 10

#1732797663840

684-411: A shallow magma chamber. Cone sheets form when magma is injected into a shallow magma chamber, which lifts and fractures the rock beds above it. The fractures take the form of a set of concentric cones dipping at a relatively shallow angle into the magma chamber. When the caldera is subsequently emptied by explosive volcanic activity, the roof of the magma chamber collapses as a plug of rock surrounded by

741-631: A year for devices to reduce exhaust pollution. On 26 January 1973, Bishopsgate Platinum Limited, of which Implats was a wholly owned subsidiary, was listed on the Johannesburg Stock Exchange (JSE). On 19 October 1978, Bishopsgate changed its name to Impala Platinum Holdings Limited (Implats). In 1990, Implats acquired an effective interest in Western Platinum and Eastern Platinum (collectively Lonhro Platinum Division, or Lonplats). A full merger agreement with Lonplats

798-544: Is a dike injected along a thrust fault plane, where rock beds were fractured and thrust up over younger beds. Clastic dikes (also known as sedimentary dikes) are vertical bodies of sedimentary rock that cut off other rock layers. They can form in two ways: Impala Platinum Impala Platinum Holdings Limited or Implats is a South African holding company that owns several companies which operate mines that produce platinum and platinum group metals, as well as nickel , copper and cobalt . Its most significant mine

855-634: Is a strategic economic resource with significant quantities of chrome and platinum. Chromite occurs to the base of the Ultramafic Sequence and is mined throughout the dyke. Below the Ultramafic-Mafic sequences' contact, and in the uppermost pyroxenite (bronzitite and websterite) units are economic concentrations of nickel , copper , cobalt , gold , and platinum group metals (PGM). The base metals occur as disseminated inter-cumulus Fe-Ni-Cu sulfides within an interval referred to as

912-457: Is concentrated on the tip of the propagating fracture. In effect, the magma wedges apart the brittle rock in a process called hydraulic fracture . At greater depths, where the rock is hotter and less brittle, the magma forces the rock aside along brittle shear planes oriented 35 degrees to the sides of the dock. This bulldozer-like action produces a blunter dike tip. At the greatest depths, the shear planes become ductile faults, angled 45 degree from

969-420: Is divided into a lower Ultramafic Sequence of dunites , harzburgites , olivine bronzitites and pyroxenites together with narrow layers of chromitite that constitute the bases of cyclic units and that are extensively mined along the Great Dyke, and an upper Mafic Sequence mainly consisting of a variety of plagioclase -rich rocks, such as norites , gabbronorites and olivine gabbros. The dyke lies within

1026-508: Is not currently operating. Oxidized, near-surface ores are a promising future source for platinum-group metals because of their easy accessibility. They constitute a resource of ca. 160-400 Mt, but mining has so far been hampered due to insufficient recovery rates. Dike (geology) In geology , a dike or dyke is a sheet of rock that is formed in a fracture of a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into

1083-408: Is rapid flow of molten magma through a fissure, the magma tends to erode the walls, either by melting the wall rock or by tearing off fragments of wall rock. This widens the fissure and increases flow. Where flow is less rapid, the magma may solidify next to the wall, narrowing the fissure and decreasing flow. This causes flow to become concentrated at a few points. At Hawaii, eruptions often begin with

1140-918: Is the Impala mine in the North West province of South Africa. The company also owns or has interest in the Two Rivers mine and the Marula mine in the South Africa Bushveld Igneous Complex and the Mimosa mine and Zimplats in Zimbabwe, as well as the Impala Refining Services which smelts and refines metals for other companies. In December 2019, Impala Canada was formed, owned by the holding company, out of

1197-569: Is the Mackenzie dike swarm in the Northwest Territories , Canada. Dike swarms (also called dike complexes ) are exposed in the eroded rift zones of Hawaiian volcanoes. As with most other magmatic dikes, these were fissures through which lava reached the surface. The swarms are typically 2.5 to 5 km in width, with individual dikes about a meter in width. The dike swarms extend radially out from volcano summits and parallel to

SECTION 20

#1732797663840

1254-695: Is the P-wave velocity of the host rock (essentially, the speed of sound in the rock). This formula predicts that dikes will be longer and narrower at greater depths below the surface. The ratio of thickness to length is around 0.01 to 0.001 near the surface, but at depth it ranges from 0.001 to 0.0001. A surface dike 10 meters in thickness will extend about 3 km, while a dike of similar thickness at depth will extend about 30 km. This tendency of intruding magma to form shorter fissures at shallower depths has been put forward as an explanation of en echelon dikes. However, en echelon dikes have also been explained as

1311-415: Is the thickness of the dike; b {\displaystyle b} is its lateral extent; P e x {\displaystyle P_{ex}} is the excess pressure in the magma relative to the host rock; ρ h o s t {\displaystyle \rho _{host}} is the density of the host rock; and V P {\displaystyle V_{P}}

1368-483: Is typically slightly coarser than basalt erupted at the surface, forming a rock type called diabase . The grain size varies systematically across the dike, with the coarsest grains normally at the center of the dike. Dikes formed at shallow depth commonly have a glassy or fine-grained chilled margin 1 to 5 cm thick, formed where the magma was rapidly cooled by contact with the cold surrounding rock. Shallow dikes also typically show columnar jointing perpendicular to

1425-416: Is usually dense, with almost no vesicles (frozen bubbles), but vesicles may be seen in the shallowest part of a dike. When vesicles are present, they tend to form bands parallel to walls and are elongated in direction of flow. Likewise, phenocrysts (larger crystals) on the margins of the dike show an alignment in the direction of flow. In contrast to dikes, which cut across the bedding of layered rock,

1482-729: The Impala Platinum mine shaft accident when a cage carrying miners fell 200m down the shaft. Seventy-five were injured. In 2012, what started as a peaceful protest resulted in a massacre. The Marikana massacre was the killing of thirty-four miners by the South African Police Service (SAPS) on 16 August 2012, during a wildcat strike at the Lonmin platinum mine in Marikana , Rustenburg , North West province , South Africa. The violence started because of

1539-637: The Zimbabwe Craton and has been dated at 2.575 billion years old. The Great Dyke acts as a strain-marker for the craton: The fact that it has been undeformed since intrusion (excluding the Musengezi area) shows that the craton had stabilised by the time the Dyke intruded. Two mafic dykes, the East and Umvimeela, flank the Great Dyke to the east and west respectively. Volcanic surface manifestation of

1596-530: The Clapham Shaft. In 2009, nine Implats employees were killed in a fall-of-ground incident at the Impala 14 Shaft at its Rustenburg mine. In 2012, Implats was charged with culpable homicide over the death of one of its workers five years prior. The charge was based on the report of a mining inspector, who claimed that mine bosses had not taken proper measures to ensure that the worker was safe when he

1653-653: The Dyke, especially in the Darwendale , Lalapanzi and Mutorashanga areas. The three largest chrome mining companies are Maranatha Ferrochrome , Zimalloys and Zimasco , although most mines are worked by the tribute system. Platinum group metals are currently mined at Ngezi Mine south of Selous by Zimplats of the Impala Platinum Group, at Unki Mine near Shurugwi by Anglo American and at Mimosa Mine near Zvishavane by Zimasco for Impala Platinum. The Hartley Platinum Mine near Makwiro

1710-515: The Earth's crust , since large numbers of dikes ( dike swarms ) are formed when the crust is pulled apart by tectonic forces. The dikes show the direction of extension, since they form at right angles to the direction of maximum extension. The thickness of a dike is much smaller than its other two dimensions, and the opposite walls are roughly parallel, so that a dike is more or less constant in thickness. The thickness of different dikes can range from

1767-428: The Great Dyke are locally overlain by erosional remnants of gabbroic rock . These mark the centres of the four sub-chambers within the Great Dyke magma system, namely (from north to south) Musengezi, Darwendale, Sebakwe and Wedza. Each of these sub-chambers has an elongate, doubly plunging synclinal structure, and was fed by a feeder dyke continuous below almost the entire Great Dyke. Stratigraphically each sub-chamber

Great Dyke - Misplaced Pages Continue

1824-540: The Great Dyke event has not been recorded and have probably been eroded . Soils on the Great Dyke are strongly influenced by the underlying bedrock. Shallow, infertile Lithosols over ultramafic rock dominate the northern and southern ends. Best for agriculture are those reddish brown granular clays developed over mafic rock in the central area. Ultramafic rock in the central area supports a complex association of moderately deep to deep dark self-churning clay, and moderately deep reddish brown kaolin-rich clay. The Great Dyke

1881-563: The Lac des Iles mine would be operated by the new company called Impala Canada Limited. Since August 2018, Impala Platinum has been led by company directors Nico Muller ( CEO ) and Meroonisha Kerber ( CFO ). Kerber joined the company in August 2018 after her predecessor Brenda Berlin, who had served as CFO since 2011, resigned in February. In 2007, two Implats employees were killed in a blast at

1938-466: The South African platinum industry. The GDP of South Africa contracted in the first quarter of 2014, pulled down by the steepest drop in mining production (25% of which 19% was directly attributable to the strike) in 50 years. It was the first contraction since 2009. Workers, most of whom already lived in poverty, lost around 11 billion rands ($ 1 billion) in wages. In 2017, an AMCU treasurer

1995-573: The acquisition of North American Palladium and its mine in Ontario , Canada . Implats was formed in 1966 as a subsidiary of Union Corporation , which established a platinum mine in Rustenburg with an initial capacity of 100,000 oz per year. It received technical advice from Canadian company Inco , while British bank Hambros provided financial advice. The two, along with South African state-owned Industrial Development Corporation, each took

2052-536: The base metal subzone, below which is a sublayer enriched in platinum group metals called the PGE subzone. The base metal and PGE subzones together, make up the Main Sulphide Zone (MSZ). The Great Dyke was first reported in 1867 by the explorer Karl Mauch . However the existence of platinum was not realized until 1918, and mining of oxidized ores did not start until in 1924. Chromite is mined throughout

2109-691: The center of the dike. If the previous dike rock has cooled significantly, the subsequent injection can be characterized by fracturing of the old dike rock and the formation of chilled margins on the new injection. Sometimes dikes appear in swarms, consisting of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event. Dike swarms are almost always composed of diabase and most often are associated with flood basalts of large igneous provinces . They are characteristic of divergent plate boundaries . For example, Jurassic dike swarms in New England and Paleogene swarms in

2166-483: The center of the rift zone before abruptly dropping to very few dikes. It is likely that the number of dikes must increase with depth, reaching a typical value of 300 to 350 per kilometer at the level of the ocean floor. In some respects, these dike swarms resemble those of western Scotland associated with the flood eruptions that preceded the opening of the Atlantic Ocean. Dikes often form as radial swarms from

2223-438: The dike is described as a multiple dike . However, subsequent injections are sometimes quite different in composition, and then the dike is described as a composite dike . The range of compositions in a composite dike can go all the way from diabase to granite , as is observed in some dikes of Scotland and northern Ireland. After the initial formation of a dike, subsequent injections of magma are most likely to take place along

2280-493: The dike propagates so that the dike becomes horizontal. It is common for a set of dikes, each a few kilometers long, to form en echelon . This pattern is seen in the Higganum dike set of New England. This dike set consists of individual dikes that are typically four kilometers in length at the surface and up to 60 meters wide. These short segments form longer groups extending for around 10 km. The entire set of dikes forms

2337-470: The government of Zimbabwe implemented laws which required local ownership of mining companies. Following this news, there were falls in the share prices of companies with mines in Zimbabwe, including Implats. In late January 2014, thousands of employees belonging to Impala Platinum and other platinum mines in South Africa went on strike , demanding a basic salary of R 12,500 (1,180 $ ). The strike,

Great Dyke - Misplaced Pages Continue

2394-415: The long axis of the volcanic shield. Sills and stocks are occasionally present in the complexes. They are abruptly truncated at the margins of summit calderas. Typically, there are about 50 to 100 dikes per kilometer at the center of the rift zone, though the density can be as high as 500 per kilometer and the dikes then make up half the volume of the rock. The density drops to 5 to 50 per kilometer away from

2451-436: The longest in the history of South Africa, ended in late June 2014 when the Association of Mineworkers and Construction Union (AMCU) signed a  three-year settlement deal with Impala Platinum and other platinum mine owners, which saw workers earning less than R 12,500 get a wage increase of R 1,000 ($ 95) per month for two years and R 950 ($ 90) per month in the third year. In October 2019, Implats announced it would acquire

2508-688: The magma fractured and disintegrated the rock at its advancing tip rather than prying the rock apart. Other dikes may have formed by metasomatism , in which fluids moving along a narrow fissure changed the chemical composition of the rock closest to the fissure. There is an approximate relationship between the width of a dike and its maximum extent, expressed by the formula: 2 w 2 b = 2.25 P e x ρ h o s t V P 2 {\displaystyle {\frac {2w}{2b}}={\frac {2.25P_{ex}}{\rho _{host}V_{P}^{2}}}} Here w {\displaystyle w}

2565-401: The margins. Here the dike rock fractures into columns as it cools and contracts. These are usually 5- to 6-sided, but 3- to 4-sided columns are also common. These are fairly uniform in size within a single dike, but range from a few centimeters to over 0.3 meters across in different dikes, tending to be thicker in wider dikes. Larger columns are likely a consequence of slower cooling. Dike rock

2622-521: The outstanding shares in Canadian-based North American Palladium Limited for $ 758 million. North American Palladium Ltd. was purchased by Johannesburg-based Impala Platinum Holdings Limited for CA$ 1 billion in 2019. Under the terms of the deal, Brookfield Business Partners was paid $ 570 million for its 81% stake in the company and minority shareholders were paid $ 19.74 per share. From that point on,

2679-431: The rock, always opening a path along a plane normal to the minimum principal stress . This is the direction in which the crust is under the weakest compression and so requires the least work to fracture. At shallow depths, where the rock is brittle, the pressurized magma progressively fractures the rock as it advances upwards. Even if the magma is only slightly pressurized compared with the surrounding rock, tremendous stress

2736-429: The sea floor is underlain by sheeted dike complexes that preserve the conduits through which magma reached the ocean floor at mid-ocean ridges . These sheeted dikes characteristically show a chilled margin on only one side, indicating that each dike was split in half by a subsequent eruption of magma. Ring dikes and cone sheets are special types of dikes associated with caldera volcanism. These are distributed around

2793-569: The sides of the dike. At depths where the rock is completely plastic, a diapir (a rising plug of magma) forms instead of a dike. The walls of dikes often fit closely back together, providing strong evidence that the dike formed by dilatation of a fissure. However, a few large dikes, such as the 120-meter-thick Medford dike in Maine, US, or the 500-meter-thick Gardar dike in Greenland, show no dilatation. These may have formed by stoping , in which

2850-411: The surrounding rock, so that erosion exposes the dike as a natural wall or ridge. It is from these natural walls that dikes get their name. Dikes preserve a record of the fissures through which most mafic magma (fluid magma low in silica) reaches the surface. They are studied by geologists for the clues they provide on volcanic plumbing systems . They also record ancient episodes of extension of

2907-400: The walls of dikes even when the walls consist of highly porous volcanic clinker, and little wall material breaks off into the molten magma. These fissures likely open as a result of bulging of the rock beds above a magma chamber that is being filled with magma from deeper in the crust. However, open fractures can exist only near the surface. Magma deeper in the crust must force its way through

SECTION 50

#1732797663840

2964-493: The west of Scotland and running into northern England record the early opening of the Atlantic Ocean. Dike swarms are forming in the present day along the divergent plate boundary running through Iceland. Dike swarms often have a great cumulative thickness: Dikes in Iceland average 3 to 5 meters in width, but one 53-kilometer stretch of coast has about 1000 dikes with total thickness of 3 kilometers. The world's largest dike swarm

3021-458: The workers were exploited and this was a motivation for the violence. It also criticised the high profits when compared with the low wages of the workers. Impala Platinum declared the strike "illegal" and fired 13.000 workers that had participated, almost half of 30.000 employed in the town. In 2014, a five-month-long platinum strike had resulted in the deaths of four people, six stabbings, and 24 billion rands ($ 2.25 billion) in lost revenue for

3078-926: Was attained in 1995, but was subsequently blocked by the European Union in 1996. Between 2000 and 2004, Implats gained mineral rights to establish Marula Platinum and also acquired strategic stakes in Zimbabwean operations Zimbabwe Platinum (Zimplats) and Mimosa Mining Company. It entered into a joint venture with Anglovaal Mining (Avmin) to develop the Two Rivers Platinum project, and sold its stakes in Barplats Mines and Lonplats. In 2003, Implats' parent company Gencor completed its "unbundling" after having amalgamated with Gold Fields in 1998. Shareholders were given 8.8 Implats shares for every 100 Gencor shares they held. In March 2011,

3135-543: Was later released on bail, and the charges against him were eventually dropped. For his decision to reopen the mines, Minister of Mineral Resources and Energy, Gwede Mantashe became the target of the National Union of Mineworkers . In 2021, three Implats employees were killed in a mud-rush at the Impala Rustenburg 6 Shaft, and two were hospitalized. In November 2023, 11 employees lost their lives in

3192-555: Was reported in the South African media as a central reason for the breakdown in trust within the union amongst workers. Despite earlier contradictory reports, the clashes on the 11th are now acknowledged to be the first incidents of violence during the strike. According to the Bench Marks Foundation , the violence erupted against a backdrop of a lack of employment opportunities for local youth, squalid living conditions, unemployment and growing inequalities. It claimed

3249-537: Was sent to install support bars under the overhang that collapsed on him. In 2016, four Implats employees were killed in a fire at the Rustenburg mine. Rescue teams were present, but the four had been overcome by smoke and never managed to escape. In 2020, the CEO of Impala Rustenburg, Mark Munroe was arrested for recalling employees too early, in violation of South Africa's Covid-19 lockdown restrictions . Munroe

#839160