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Great Dyke
The Great Dyke is a linear geological feature that trends nearly north-south through the centre of Zimbabwe passing just to the west of the capital, Harare. 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 range is host to vast ore deposits, including gold, silver, chromium, platinum, nickel and asbestos. Geology and Soils 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 . 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 the Great Dyke are locally overlain by erosional remnants of gabbroic rock. These mark the centres of the four s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 Mozambique to the east. The capital and largest city is Harare. The second largest city is Bulawayo. A country of roughly 15 million people, Zimbabwe has 16 official languages, with English, Shona, and Ndebele the most common. Beginning in the 9th century, during its late Iron Age, the Bantu people (who would become the ethnic Shona) built the city-state of Great Zimbabwe which became one of the major African trade centres by the 11th century, controlling the gold, ivory and copper trades with the Swahili coast, which were connected to Arab and Indian states. By the mid 15th century, the city-state had been abandoned. From there, the Kingdom of Zimbabwe was established, followed by the Rozvi and Mutapa empires. The British Sout ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term ''gabbro'' may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite. Etymology The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. It was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany. Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Mafic materials can also be described as ferromagnesian. History The term ''mafic'' is a portmanteau of "magnesium" and "ferric" and was coined by Charles Whitman Cross, Joseph P. Iddings, Louis Valentine Pirsson, and Henry Stephens Washington in 1912. Cross' group had previously divided the major rock-forming minerals found in igneous rocks into ''salic'' minerals, such as quartz, feldspars, or feldspathoids, and ''femic'' minerals, such as olivine and pyroxene. However, micas and aluminium-rich amphiboles were excluded, while some calcium minerals containing little iron or magnesium, such as wollastonite or apatite, were ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle. Terminology The term ''craton'' is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons are composed of two layers: A continental '' shield'', in which the basement rock crops out at the surfa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 craton is named after the country of Zimbabwe where the majority of the craton is. The rocks of the Zimbabwe Craton are separated from the rocks of the Kaapvaal Craton to the southeast by the wide Limpopo Belt of granulite facies tectonites. The Limpopo belt formed contemporaneously with the Zimbabwe and Kaapvaal cratons, but remained geologically active until much later. It was only in the late Archean, ca. 2.8-2.5 Ga., that the two cratons were stabilized together and that high-grade metamorphism ceased in the Limpopo Belt. North of the Zimbabwe Craton is the Zambezi Belt. Origin The Zimbabwe Craton formed from the suture of two smaller blocks, the Tokwe Segment to the south and the much smaller Rhodesdale Segment (aka Rhodesdale gne ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 orthopyroxene gabbro. Norite may be essentially indistinguishable from gabbro without thin section study under the petrographic microscope. The principal difference between norite and gabbro is the type of pyroxene of which it is composed. Norite is predominantly composed of orthopyroxenes, largely high magnesian enstatite or an iron bearing intermediate hypersthene. The principal pyroxenes in gabbro are clinopyroxenes, generally medially iron-rich augites. Norite occurs with gabbro and other mafic to ultramafic rocks in layered intrusions which are often associated with platinum orebodies such as in the Bushveld Igneous Complex in South Africa, the Skaergaard igneous complex of Greenland, and the Stillwater igneous complex in Montana. Norit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect. Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the hig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 Ontario. Chromitite typically forms as orthocumulate layered lenses in peridotite rocks, at times intergrown with other oxides such as magnetite and ilmenite, and silicates such as olivine, pyroxene, plagioclase feldspar (mainly anorthite), and garnet crystals. The chromium rich garnet uvarovite forms in interbedded layers with chromitite. Chromitite is the main economic source for the mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ... of chromite and subsequent source of the metal chromium. References *Guilbert, John M. and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 websterites which contain both types of pyroxenes (see diagram below). Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles. They are essentially of igneous origin, though some pyroxenites are included in the metamorphic Lewisian complex of Scotland. The pyroxene-rich rocks, which result from the type of contact metamorphism known as pyroxene-hornfels facies, have siliceous sediment or basaltic protoliths, and are respectively metapelites and metabasites. Intrusive and mantle pyroxenites Igneous pyroxenites are closely allied to gabbros and norites, from which they differ by the absence of feldspar, and to peridotites, which are distinguished from them by containing more tha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 alteration has acquired a bronze-like sub-metallic luster on the cleavage surfaces. Enstatite is magnesium silicate, MgSiO3, with the magnesium partly replaced by small amounts (up to about 12%) of Fe+2. In the bronzite variety, (Mg,Fe)SiO3, the iron(II) oxide ranges from about 12 to 30%, and with still more iron there is a passage to hypersthene.Klein, Cornelis and Cornelius Hurlbut, Jr. (1985) ''Manual of Mineralogy,'' Wiley, 20th ed., The ferriferous varieties are liable to a particular kind of alteration, known as schillerization, which results in the separation of the iron as very fine films of oxide and hydroxides along the cleavage cracks of the mineral. The cleavage surfaces therefore exhibit a metallic sheen or schiller, which is e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 quickly on the surface. For this reason, olivine has been proposed as a good candidate for accelerated weathering to sequester carbon dioxide from the Earth's oceans and atmosphere, as part of climate change mitigation. Olivine also has many other historical uses, such as the gemstone peridot (or chrysolite), as well as industrial applications like metalworking processes. The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: ) and fayalite (Fe-endmember: ). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (''e.g.'', Fo70Fa30). Forsterite's melting temperature is unusually high at atmospheric pressure, almost , while ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 few percent chromium-rich spinel as an accessory mineral. Garnet-bearing harzburgite is much less common, found most commonly as xenoliths in kimberlite. Harzburgite typically forms by the extraction of partial melts from the more pyroxene-rich peridotite called lherzolite. The molten magma extracted from harzburgite may then erupt on the surface as basalt. If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-poor peridotite called dunite. Harzburgite may also form by the accumulation of olivine and low-Ca pyroxene in large magma chambers of basalt deep in continental crust (layered intrusions). Occurrence Harzburgite is the most commonly found var ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
