The Great Dyke is a linear geological feature that trends nearly north-south through the centre of

Zimbabwe Zimbabwe (), officially the Republic of Zimbabwe, is a landlocked country located in Southeast Africa, between the Zambezi and Limpopo Rivers, bordered by South Africa to the south, Botswana to the south-west, Zambia to the north, and ...

passing just to the west of the capital,

Harare Harare (; formerly Salisbury ) is the capital and most populous city of Zimbabwe. The city proper has an area of 940 km2 (371 mi2) and a population of 2.12 million in the 2012 census and an estimated 3.12 million in its metropolitan ...

. It consists of a band of short, narrow ridges and hills spanning for approximately . The hills become taller as the range goes north, and reach up to above the

Mvurwi Range The Mvurwi range of mountains is located in northern Zimbabwe. It stretches about 160 km from Lake Manyame, just west of Harare, north to the Zambezi Escarpment. Its high point is at about 1728 metres. It is an important chromium Chromium ...

. The range is host to vast ore deposits, including

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...

and

asbestos Asbestos () is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, each fibre being composed of many microscopic "fibrils" that can be released into the atmosphere b ...

Geology and Soils

Geologically the Great Dyke is not a dyke, but is

lopolith A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body. The term was first defin ...

ic 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 . The Great Dyke is unusual in that most ultramafic layered intrusions display near horizontal

sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, ...

or sheet forms. The well-layered lower units of ultramafic rocks comprising 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.Wilson, A.H. and Prendergast, M.D., 1989. The Great Dyke of Zimbabwe-I: tectonic setting, stratigraphy, petrology, structure, emplacement and crystallisation. In: Prendergast, M.D., Jones, M.J. (Eds.), ''Magmatic sulphides-the Zimbabwe volume''. Institution Mining Metallurgy, London, 1–20. Stratigraphically each sub-chamber is divided into a lower Ultramafic Sequence of

dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, w ...

harzburgite Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene (enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a f ...

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...

bronzitite Bronzite is a member of the pyroxene group of minerals, belonging with enstatite and hypersthene to the orthorhombic series of the group. Rather than a distinct species, it is really a ferriferous variety of enstatite, which owing to partial alte ...

s and

pyroxenite Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the we ...

s together with narrow layers of

chromitite Chromitite is an igneous cumulate rock composed mostly of the mineral chromite. It is found in layered intrusions such as the Bushveld Igneous Complex in South Africa, the Stillwater igneous complex in Montana and the Ring of Fire discovery in O ...

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 Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

-rich rocks, such as

norite Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name ''norite'' is derived from ''Norge'', the Norwegian name for Norway. Norite also known as orthopyrox ...

s, gabbronorites and olivine gabbros.Prendergast, M.D. and Wilson, A.H. 1989. The Great Dyke of Zimbabwe-II: Mineralisation and mineral deposits. In: Prendergast, M.D., Jones, M.J. (Eds.), ''Magmatic sulphides-the Zimbabwe volume''. Institution Mining Metallurgy, London, 21-42. The dyke lies within the

Zimbabwe Craton The Zimbabwe Craton is an area in Southern Africa of ancient continental crust, being a part of the ancient continent of Western Gondwana, with rocks dating back to the early Archean Eon, possibly as early as 3.46 billion years ago ( Ga.). The cra ...

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 A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

had stabilised by the time the Dyke intruded. Two

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

dykes, the East and Umvimeela, flank the Great Dyke to the east and west respectively.

Volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

surface manifestation of the Great Dyke event has not been recorded and have probably been

eroded Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...

. Soils on the Great Dyke are strongly influenced by the underlying bedrock. Shallow, infertile

Lithosol Orthents are soils defined in USDA soil taxonomy as entisols that lack due to either steep slopes or parent materials that contain no permanent weatherable minerals (such as ironstone). Typically, Orthents are exceedingly shallow soils. They ar ...

s 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 is a strategic economic resource with significant quantities of chrome and platinum.

Chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...

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

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...

, and

platinum group metals The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...

(PGM). The base metals occur as disseminated inter-cumulus Fe-Ni-Cu sulfides within an interval referred to as 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).

Development

The Great Dyke was first reported in 1867 by the explorer

Karl Mauch Karl Gottlieb Mauch (7 May 1837 – 4 April 1875) was a German explorer and geographer of Africa. He reported on the archaeological ruins of Great Zimbabwe in 1871 during his search for the biblical land of Ophir. Exploration and Great Zimbabwe ...

. However the existence of

was not realized until 1918, and mining of oxidized ores did not start until in 1924.

is mined throughout the Dyke, especially in the

Darwendale Darwendale is a small town in southern Zvimba District of Mashonaland West, Zimbabwe, on the Gwebi River. It is located about west of Harare. According to the 1982 census, the village had a population of 3,264. Darwendale is regarded as one o ...

Lalapanzi Lalapanzi is a village in Midlands (Zimbabwe), Midlands province in Zimbabwe. It straddles the Great Dyke, a mineral-rich geological formation that runs north-south down the center of the country. The name is also applied to the areas surrounding ...

and

Mutorashanga 300px, Entrance of an adit of a chromite mine, Mutorashanga">chromite.html" ;"title="adit of a chromite">adit of a chromite mine, Mutorashanga Mutorashanga is a small ferrochrome mining town in Mashonaland West province in Zimbabwe. Situated on Z ...

areas. The three largest chrome mining companies are

Maranatha Ferrochrome __NOTOC__ ''Maranatha'' (Aramaic: ') is an Aramaic phrase which occurs once in the New Testament (). It also appears in Didache 10:14. It is transliterated into Greek letters rather than translated and, given the nature of early manuscripts, t ...

, Zimalloys and Zimasco, although most mines are worked by the tribute system.

Platinum group metals The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...

are currently mined at Ngezi Mine south of Selous by Zimplats of the

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 is the ...

Group, at

Unki Mine The Unki mine is an underground mine located in the central part of Zimbabwe in Shurugwi, Midlands Province. Unki represents one of the largest platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense ...

near

Shurugwi Shurugwi, formerly Selukwe, is a small town and administrative centre in Midlands Province, southern Zimbabwe, located about 350 km (220 miles) south of Harare, with a population of 22,900 according to the 2022 census. The town was establ ...

by Anglo American and at

Mimosa Mine ''Mimosa'' is a genus of about 590 species of herbs and shrubs, in the mimosoid clade of the legume family Fabaceae. The generic name is derived from the Greek word (''mimos''), an "actor" or "mime", and the feminine suffix -''osa'', "resem ...

near

Zvishavane Zvishavane (known until 1982 as Shabani) is a mining town in Midlands Province, Zimbabwe. Surrounded by low hills, it lies west of Masvingo, on the main Bulawayo-Masvingo road. Other roads lead from Zvishavane to Gweru, north, and Mberengwa, ...

by Zimasco for Impala Platinum. The Hartley Platinum Mine near

Makwiro Makwiro is a settlement in Mashonaland West province in Zimbabwe Zimbabwe (), officially the Republic of Zimbabwe, is a landlocked country located in Southeast Africa, between the Zambezi and Limpopo Rivers, bordered by South Africa to ...

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.Oberthür, T., Melcher, F., Buchholz, P., Locmelis, M. 2012. The oxidized ores of the Main Sulfide Zone, Great Dyke, Zimbabwe: Turning resources into minable reserves – mineralogy is the key.''In: Platinum 2012, The Southern African Institute of Mining and Metallurgy (SAIMM), Symposium Series'', S72, 647-672

/ref>

References

{{Authority control Economic geology Geology of Zimbabwe Layered intrusions Neoarchean magmatism

Geology and Soils

Development

See also

References