The Iron Quadrangle ( pt, Quadrilátero Ferrífero) is a mineral-rich region covering about in the central-southern part of the Brazilian state
Minas Gerais
Minas Gerais () is a state in Southeastern Brazil. It ranks as the second most populous, the third by gross domestic product (GDP), and the fourth largest by area in the country. The state's capital and largest city, Belo Horizonte (literally ...
. The area is known for its extensive deposits of
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 met ...
,
diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
s, and
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
, being the source of approximately 40% of all gold produced in Brazil between the years 1500 and 2000. The deposits themselves pertain to the
Minas Supergroup, a sequence of meta-sedimentary rocks initially formed in the
Paleoproterozoic
The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
, about 2.5 Ga. In the 2010s, there have been two collapses of large
tailings dam
A tailings dam is typically an earth-fill embankment dam used to store byproducts of mining operations after separating the ore from the gangue. Tailings can be liquid, solid, or a slurry of fine particles, and are usually highly toxic and potent ...
s, which caused extensive damage and loss of life.
Demographics
The Iron Quadrangle is located in the south-central portion of the state of Minas Gerais and is home to more than 4 million people. The largest city in the region,
Belo Horizonte
Belo Horizonte (, ; ) is the sixth-largest city in Brazil, with a population around 2.7 million and with a metropolitan area of 6 million people. It is the 13th-largest city in South America and the 18th-largest in the Americas. The metropol ...
, acts as the regional economic hub and processes most of the gem and mineral wealth produced by the region. Other large settlements include
Santa Luzia,
Ibirité
Ibirité is a Brazilian municipality located in the state of Minas Gerais. The city belongs to the mesoregion Metropolitana de Belo Horizonte and to the microregion of Belo Horizonte. Its population in 2020 was 182,153.
History
Ibirité was bor ...
, and
Itabira
Itabira is a Brazilian municipality and a major city in the state of Minas Gerais. The city belongs to the Belo Horizonte metropolitan area mesoregion and to the Itabira microregion.
It is currently the twenty-fourth largest city in the state in ...
. The local economy is dependent on mining and associated activities (steel production and metallurgy), tourism and agriculture.
History
The mineral wealth of the Iron Quadrangle has been known since the year 1690. European discovery of gold was followed shortly by discovery of
diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
s. The
Portuguese Empire
The Portuguese Empire ( pt, Império Português), also known as the Portuguese Overseas (''Ultramar Português'') or the Portuguese Colonial Empire (''Império Colonial Português''), was composed of the overseas colonies, factories, and the l ...
exerted strong control over the mining operations, collecting 20% of all production in taxes. During the larger
Brazilian Gold Rush
The Brazilian Gold Rush was a gold rush that started in the 1690s, in the then Portuguese colony of Brazil in the Portuguese Empire. The gold rush opened up the major gold-producing area of Ouro Preto (Portuguese for ''black gold''), then known as ...
of the late 17th to late 19th century, hundreds of thousands of Europeans migrated to the region from both Europe and Northern Brazil, and at least half a million slaves were imported from Africa to work in the mines and
lavra
A lavra or laura ( el, Λαύρα; Cyrillic: Ла́вра) is a type of monastery consisting of a cluster of cells or caves for hermits, with a church and sometimes a refectory at the center. It is erected within the Orthodox and other Eastern Chr ...
s. This concentration of wealth and population shifted the economic and political focus of Portuguese Brazil away from the sugar plantations of the north of the country. After gold production slowed in the late 18th century, the economic focus redirected to production of iron ore and agriculture, especially
coffee
Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulant, stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world.
S ...
and dairy.
Geology
![BIF (jaspilite meta-BIF, Paleoproterozoic Minas Gerais](https://upload.wikimedia.org/wikipedia/commons/1/11/BIF_%28jaspilite_meta-BIF%2C_Paleoproterozoic_Minas_Gerais.jpg)
The iron ore deposits of the Iron Quadrangle are hosted within the
Cauê Formation Caue may refer to:
People
* Cauê Benicio (born 1978), Brazilian midfielder
* Cauê Macris (born 1983), Brazilian politician
* Cauê (footballer, born 1986), Cauê Santos da Mata, Brazilian football midfielder
* Cauê (footballer, born 1987), R ...
, part of the Minas Supergroup. The Minas Supergroup was initially deposited approximately 2.5 Ga ago on the edge of the
São Francisco craton
The São Francisco Craton is an ancient craton in the eastern part of South America. The craton crops out in the Brazilian states of Minas Gerais and Bahia.
It includes a number of blocks of Archean basement, separated by orogenic belts. The be ...
, the geologic core of southern Brazil. The larger Iron Quadrangle contains the following five
lithostratigraphic
Lithostratigraphy is a sub-discipline of stratigraphy, the geology, geological science associated with the study of stratum, strata or rock layers. Major focuses include geochronology, comparative geology, and petrology.
In general, strata are ...
units:
[Alkmim & Marshak, 1998]
* Archean Basement rock (3.2–2.61 Ga):
Gneiss
Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
and
migmatite
Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock tha ...
s of the São Francisco craton. At the time, this craton would have been connected to the
Congo craton
The Congo Craton, covered by the Palaeozoic-to-recent Congo Basin, is an ancient Precambrian craton that with four others (the Kaapvaal, Zimbabwe, Tanzania, and West African cratons) makes up the modern continent of Africa. These cratons were fo ...
and would not have been associated with the rest of South America. Also included in this unit are
calc-alkaline
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic mag ...
and
granitic
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quart ...
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s emplaced during the Late
Archean
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth
Earth ...
.
*
Rio das Velhas Supergroup (2.86–2.6 Ga):
Greenstone and intercalated sedimentary rocks, including
Algoman-type
banded iron formation
Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness a ...
s (BIF)
* Minas Supergroup (2.6–2.12):
Metamorphosed
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
sedimentary units deposited on the shelf of the São Francisco craton. The lowermost units (
Tamanduã and
Caraça Groups) show a typical
marine regression
A marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land.
Evidence of marine ...
sequence, with
conglomerates and
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
s fining upward into marine
pelite
A pelite (Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, ...
s. These are overlain by the Cauê Formation, Lake Superior-type banded iron formations which are in turn overlain by
carbonates
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
of the
Gandarela Formation and shallow water to
deltaic
A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rare ...
sediments of the
Piracicaba Group. Lastly, the whole sequence is
unconformably
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
overlain by the
Sabará Group, a mix of
turbidite
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites were ...
s,
volcaniclastic
Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...
s, conglomerates and
diamictite
Diamictite (; from Ancient Greek ''δια'' (dia-): ''through'' and ''µεικτός'' (meiktós): ''mixed'') is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that ra ...
s.
* Post-Minas intrusives (various ages):
Pegmatitic
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
dikes
Dyke (UK) or dike (US) may refer to:
General uses
* Dyke (slang), a slang word meaning "lesbian"
* Dike (geology), a subvertical sheet-like intrusion of magma or sediment
* Dike (mythology), ''Dikē'', the Greek goddess of moral justice
* Dikes, ...
and
granitoid
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz ...
plutons which intruded into the Minas Supergroup. It is these pegmatites which host the gemstones being mined in the region.
*
Itacolomi Group (2.1 Ga): coarse sandstone and conglomerates which overlie the Minas Supergroup. This unit contains banded iron formation clasts which might be derived from the Cauê Formation.
The area shows evidence for a collisional event (the
Transamazonian Orogeny The Transamazonian orogeny was a mountain building event in the Paleoproterozoic, affecting what is now the São Francisco Craton and Guyana Shield. During the orogeny from 2.14 to 1.94 billion years ago two small Archean proto-continents—includin ...
) approximately from 2.2 to 1.8 Ga, based on mapped
folds and
thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Thrust geometry and nomenclature
Reverse faults
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.
If ...
s. It has been suggested that this event relates to other contemporaneous
orogenies
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
around the world, leading to the assemblage of the
Supercontinent Columbia.
[Zhao et al., 2004][Zhao et al., 2002]
The iron ore in the region is of the type known as
itabirite
Itabirite, also known as banded-quartz hematite and hematite schist, is a laminated, metamorphosed oxide-facies iron formation in which the original chert or jasper bands have been recrystallized into megascopically distinguishable grains of quar ...
. Itabirite is a metamorphosed variant of
banded iron formation
Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness a ...
in which the original bands of
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
and
jasper
Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases,Kostov, R. I. 2010. Review on the mineralogical systematics of jasper and related rocks. – Archaeometry Workshop, 7, 3, 209-213PDF/ref> ...
have been recrystallized to macroscopically distinguishable quartz grains. The iron minerals (
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
and
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
) are typically present in thin bands. This material typically produces a high-grade iron ore, as impurities such as
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
or
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
were removed during the metamorphic processes.
IUGS geological heritage site
In respect of the importance of this record of banded ironstone formations and the ferruginous caves which it hosts, the 'Paleoproterozoic Banded Iron Formations of the Quadrilátero Ferrífero' was included by the
International Union of Geological Sciences
The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology.
About
The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
(IUGS) in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an 'IUGS Geological Heritage Site' as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'
Mining disasters
![Desastre de Brumadinho](https://upload.wikimedia.org/wikipedia/commons/c/ca/Desastre_de_Brumadinho.gif)
There have been 2 major mining disasters in the Iron Quadrangle in the 2010s. In both cases,
tailings dam
A tailings dam is typically an earth-fill embankment dam used to store byproducts of mining operations after separating the ore from the gangue. Tailings can be liquid, solid, or a slurry of fine particles, and are usually highly toxic and potent ...
s at mines operated in part by
Vale S.A.
Vale S.A., formerly ''Companhia Vale do Rio Doce'' (the Sweet River Valley Company, referring to the Doce River) () is a Brazilian multinational corporation engaged in metals and mining and one of the largest logistics operators in Brazil. Vale ...
collapsed, leading to flooding and destruction of the downstream areas, as well as severe environmental contamination. On November 5, 2015, the
Fundão dam (part of the Germano mining complex) in the Mariana region collapsed, causing 19 fatalities and the destruction of around 200 homes. On January 25, 2019,
Dam I at the Córrego do Feijão iron mine collapsed, causing the deaths of 165 people, and extensive damage to nearby agricultural lands, bridges and the city of
Brumadinho
Brumadinho () is a Brazilian municipality in the state of Minas Gerais. The city belongs to the Belo Horizonte metropolitan mesoregion and to the microregion of Belo Horizonte. Brumadinho is at an altitude of 880 m. In 2020 the population wa ...
. Between the two events, approximately of mining waste was released to the environment, with much of that waste being washed into nearby major rivers. After the collapse of the Fundão dam, contamination of the
Rio Doce
The Doce River (literally the "sweet river"; pt, Rio Doce ) is a river in southeast Brazil with a length of .
The river basin is economically important. In 2015 the collapse of a dam released highly contaminated water from mining into the river ...
caused water shortages for people living downstream.
In response to the 2019 disaster, Vale S.A. halted operations at other nearby mines and has reconfirmed their commitment to decommissioning all upstream tailings dams within the next three years. Vale SA has also been fined
R$ 250 million by the Brazilian Institute of the Environment and Renewable Natural Resources. Brazilian Authorities have also frozen Vale assets worth R$11.8 billion to ensure funding for recovery efforts.
See also
*
Ferrous Resources
*
Port of Tubarão
The Port of Tubarão is a port in Brazil, near the city of Vitória in Espírito Santo. The port was created in 1966 by the Brazilian mining company Vale to export iron ore extracted from the Iron Quadrangle in Minas Gerais. In 2007, the Port o ...
Notes and references
Notes
References
Bibliography
*
*
*
*
{{Major South American geological formations
Geology of Brazil
Precambrian South America
Iron ore deposits
Metallogenetic provinces
Mining in Brazil
Geography of Minas Gerais
First 100 IUGS Geological Heritage Sites