The Merensky Reef is a layer of

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

in the

Bushveld Igneous Complex The Bushveld Igneous Complex (BIC) is the largest Layered intrusion, layered igneous intrusion within the Earth's Crust (geology), crust. It has been tilted and Erosion, eroded forming the outcrops around what appears to be the edge of a great Ba ...

(BIC) in the North West,

Limpopo Limpopo () is the northernmost Provinces of South Africa, province of South Africa. It is named after the Limpopo River, which forms the province's western and northern borders. The term Limpopo is derived from Rivombo (Livombo/Lebombo), a ...

Gauteng Gauteng ( , ; Sotho-Tswana languages, Sotho-Tswana for 'place of gold'; or ) is one of the nine provinces of South Africa. Situated on the Highveld, Gauteng is the smallest province by land area in South Africa. Although Gauteng accounts f ...

and

Mpumalanga Mpumalanga () is one of the nine provinces of South Africa. The name means "East", or literally "The Place Where the Sun Rises" in the Nguni languages. Mpumalanga lies in eastern South Africa, bordering Eswatini and Mozambique. It shares bor ...

provinces of

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

which together with an underlying layer, the Upper Group 2 Reef (UG2), contains most of the world's known reserves of

platinum group The platinum-group metals (PGMs) are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5 and 6). The six platinum-group ...

metals A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against no ...

(PGMs) or platinum group elements (PGEs)—

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

rhodium Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isot ...

ruthenium Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chem ...

iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...

and

osmium Osmium () is a chemical element; it has Symbol (chemistry), symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a Abundance of elements in Earth's crust, trace element in a ...

. The Reef is 46 cm thick and bounded by thin chromite seams or stringers. The composition consists predominantly of cumulate rocks, including leuconorite,

anorthosite Anorthosite () is a phaneritic, intrusive rock, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic ...

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 ...

, and melanorite.

Composition

The UG2 Reef, the composition of which is relatively consistent throughout the BIC, is rich in

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 Iron, FeChromium, Cr2Oxygen, O4. It is an oxide mineral belonging to the spinel group. The ...

. However, the UG2 Reef lacks the Merensky's

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

copper Copper is a chemical element; it has symbol Cu (from Latin ) 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 pinkish-orang ...

and

nickel Nickel is a chemical element; it has 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 slo ...

by-products, though its reserves may be almost twice those of the Merensky Reef. Overall, the Merensky Reef is observed to be a lower layer composed of

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

with a thin layer of chromitite over top. In addition, there is commonly a layer overlying both composed of feldspathic

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 w ...

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 ...

layers occur commonly in large

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 ...

layered intrusions. A current theory suggests chromitites form as a result of introduction and mixing of chemically primitive

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

with a more evolved

, which results in supersaturation of chromite in the mixture and the formation of a nearly monomineralic layer on the

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

floor. The leading theory regarding the formation of the Merensky reef is that of crystals originating from a main magma source accumulated and cooled as the magma rose resulting in crystallization.Mathez, E. A. (1995). MagmaUc metasomaUsm and formation of the Merensky reef, Bushveld Complex. Contrib Mineral Petrol 119, 277-286. Yet the nature of crystallization is complex.

Layers

The Merensky Reef is composed of five different layers. The first layer is mottled

which is

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

oikocrysts and is described as dark colored bands. The mottled anorthosite is composed of traces of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

titanite Titanite, or sphene (), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals including cerium and yttrium; calcium may be partly rep ...

and

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

minerals. The second layer is Merensky chromitites which are highly irregular grains of basal chromitite. The third layer is similar to the second layer with Merensky chromitites, however the basal chromitite is compact and smaller in size. The fourth layer is Merensky pegmatite and is composed of coarse-grained

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

with an approximate thickness of 2.6 cm thick. In the fourth layer, chromite is sparse and sulfides are present. The fifth layer is Merensky melanorite and is a fine-grained

chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a Mohs scale, hardness of 3.5 to 4 ...

and

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

rich matrix.

Whole-rock chemistry

Sulfidic Chromitite from Merensky Reef South Africa

The Merensky Reef has a high concentration of

s. However, the chromitites differ from one another in their levels of iridium, ruthenium, rhodium and platinum present. There is reported trace element enrichment shown by

arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...

tin Tin is a chemical element; it has symbol Sn () and atomic number 50. A silvery-colored metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, a bar of tin makes a sound, the ...

and

tellurium Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...

. The Merensky Reef is similar to the Platreef because of the presence of primitive mantle, layered intrusions and levels of

and

. The Merensky-type reef has been separated into two categories, the orthomagmatic and the hydromagmatic. The orthomagmatic group is composed of

element mineralization. The hydromagmatic group is composed of platinum group element mineralization to volatile-rich fluids separating from a solid cumulate pile.

Crystallization

There are several theories that suggest how the crystallization on the Merensky Reef occurred. The first accepted hypothesis of the Merensky Reef suggests the chromite crystallization originated from hybrid melts and significant lateral mixing of new and resident magma. In detail, the first hypothesis suggests the high of PGE concentrations were a result from the sulfide and

melt. The

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

melt plays an important role in this hypothesis because the sulfide melt is dense and the settling of the melt through the magma column to the floor of the chamber allowed such mixing to occur. One theory suggests the chromite crystallization originated from hybrid melts and lateral mixing. Another theory that suggests the crystallization originated from

and sulfide droplets. However, there is also another theory that the crystallization originated from emplaced magma merging with roof-rock melts. In the theory of roof-rock melts, there was contamination between the new magma and silica-rich resident melt. The contamination resulted in chromite and PGM crystallization because the

grains attracted the PGM crystals. After crystallization, the crystals were carried in the collapsing margins and formed the layers of

and PGE.

History

The Bushveld Complex

s were first reported by Hall and Humphrey in 1908. The initial recovery of platinum in South Africa took place on several of the large East Rand gold mines and the first separate platinum mine was a short lived venture near Naboomspruit that worked very patchy quartz reefs. The discovery of the

deposits was made in 1924 by a Lydenburg district farmer, A F Lombaard. It was recorded to be approximately 80 kilometers in length. 'Geology for South African Students. CNA Ltd South Africa''/ref>''Platinum Group Metal Mines In South Africa 2007, S A Dept of Minerals and energy''
This was an alluvial deposit but its importance was recognized by

Hans Merensky Hans Merensky (16 March 1871 – 21 October 1952) was a South African geologist, prospector, scientist, conservationist and philanthropist. Early life and education Johannes "Hans" Merensky was born on 16 March 1871 at the Berlin Missionar ...

whose prospecting work discovered the primary source in the Bushveld Igneous Complex and traced it for several hundred kilometres by 1930. Extensive

mining Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...

of the Reef didn't take place until an upsurge in the demand for platinum group metals used in exhaust

pollution Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...

control in the 1950s, made exploitation economically feasible. Extraction of metals from the UG2 chromitite could only take place in the 1970s with major advances in

metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

. The first mine which successfully concentrated on the extraction of chrome-rich UG2 Chromitite Layer was the Western Platinum Mine.

References

Journal of Petrology
retrieved 2018-01-31 at the Wayback Machine)

{{coord missing, South Africa Proterozoic magmatism Geologic formations of Africa Rock formations of South Africa Geology of South Africa Platinum