El Laco is a

volcanic complex A complex volcano, also called a compound volcano or a volcanic complex, is a mixed landform consisting of related volcanic centers and their associated lava flows and pyroclastic rock. They may form due to changes in eruptive habit or ...

in the

Antofagasta Region The Antofagasta Region (, ) is one of Chile's Administrative divisions of Chile, sixteen first-order administrative divisions. Being the second-largest region of Chile in area, it comprises three provinces, Antofagasta Province, Antofagasta, El ...

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

. It is directly south of the

Cordón de Puntas Negras Cordón is a central ''barrio'' (neighbourhood or district) of Montevideo, Uruguay. Part of the city's central business district, alongside Centro, Montevideo, Centro and Ciudad Vieja, Montevideo, Ciudad Vieja, the 18 de Julio Avenue that runs thr ...

volcanic chain. Part of the

Central Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca plate and Antarctic plate underneath the South Americ ...

of the

Andes The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...

, it is a group of seven

stratovolcanoes A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a ...

and a

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

. It is about two million years old. The main summit of the volcano is a

lava dome In volcanology, a lava dome is a circular, mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions ...

called Pico Laco, which is variously reported to be or high. The edifice has been affected by glaciation, and some reports indicate that it is still fumarolically active. The volcano is known for its

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

-containing lava flows of enigmatic origin. In total, there are four lava flows and two dykes, as well as a formation of uncertain nature. In addition to lava flow structures, pyroclastics containing iron oxide are also found within the complex. The magmas formed within a

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 ...

with a volume of about ; whether the iron-rich lavas are native magnetite lavas or were formed by hydrothermal processes acting on regular rock is under debate. After their discovery in 1958, these iron deposits have been mined. Similar deposits of volcanic iron oxide exist in Australia, Chile, and Iran.

Geography

El Laco is part of the Cordón de Puntas Negras sector of the Central Volcanic Zone, directly south of that volcanic chain. It sits atop a

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

and

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

basement that was lifted from the seaground during the

Acadian orogeny The Acadian orogeny is a long-lasting mountain building event which began in the Middle Devonian, reaching a climax in the Late Devonian. It was active for approximately 50 million years, beginning roughly around 375 million years ago (Ma), wi ...

and is of

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

age. Later,

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

and

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

sedimentation of the Salta Group occurred, which was then buried by

Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...

rhyolites. Two major volcanic lineaments cross in the El Laco area, including the so-called Calama-Olacapato-El Toro lineament. El Hueso volcano to the north is - high and has a basement diameter of . It has a crater with a diameter of . The city of

Antofagasta Antofagasta () is a port city in northern Chile, about north of Santiago. It is the capital of Antofagasta Province and Antofagasta Region. According to the 2015 census, the city has a population of 402,669. Once claimed by Bolivia follo ...

is located west of El Laco. Other close towns are Calama and

San Pedro de Atacama San Pedro de Atacama is a Chilean town and commune in El Loa Province, Antofagasta Region. It is located east of Antofagasta, some 106 km (60 mi) southeast of Calama and the Chuquicamata copper mine, overlooking the Licancabur volcan ...

. The international road through Sico Pass connecting

Salta Salta () is the capital and largest city in the Provinces of Argentina, Argentine province of Salta Province, the same name. With a population of 618,375 according to the 2010 census, it is also the List of cities in Argentina, 7th most-populous ...

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

with Calama in Chile runs close to El Laco. A number of tourist sites are found in the

Atacama Desert The Atacama Desert () is a desert plateau located on the Pacific Ocean, Pacific coast of South America, in the north of Chile. Stretching over a strip of land west of the Andes Mountains, it covers an area of , which increases to if the barre ...

adjacent to El Laco, and the dry climate also makes the area suitable for astronomy facilities.

Geology

The El Laco volcanic complex is formed by about seven minor

stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...

es and lava domes. The complex started its activity in the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58porphyric

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

s formed a stratovolcano. During the Pliocene, ash and

pyroclastic Pyroclast, Pyroclastic or Pyroclastics may refer to: Geology * Pyroclast, or airborne volcanic tephra fragments * Pyroclastic rock, rock fragments produced and ejected by explosive volcanic eruptions * Pyroclastic cone, landform of ejecta fro ...

eruptions formed a caldera with a diameter of , which also contains a central lava dome that formed 6.5 million years ago. Finally, probably during the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

, five iron-rich magmas were extruded, named Laco Sur, Laco Norte and Rodados Negros. Laquito and Cristales Grandes, two abyssal iron magma structures, date back to that era. The volcanic complex is located an altitude of and covers a surface area of with a minimum volume of of volcanic rock. The main summit, Pico Laco or Pitón El Laco, has an altitude of , although a maximum height of has also been reported. Pico Laco is an andesitic lava dome with a height of above the surrounding terrain. The dome, with dimensions of , has two summits: the higher eastern one and a western summit, and has also been described as a

volcanic plug A volcanic plug, also called a volcanic neck or lava neck, is a volcano, volcanic object created when magma hardens within a Volcanic vent, vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if risi ...

. Pico Laco has been described as a resurgent volcano within a

, that is surrounded by secondary vents. Other summits include the northwestern Hueso Chico, a cone with a height of above its surroundings and a crater wide. This cone is of dacitic composition. "Volcano 5009" is heavily eroded, and its core of lava and

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin marg ...

has been exposed. It has a diameter of . Eruptive activity here probably coincided with glacier activity during the Pliocene, as evidenced by

moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a gla ...

s in the area. Ages of 10.5 to 1.6 mya have been estimated via potassium-argon dating of the andesite lavas and subvolcanic rocks. An age of 5.3±1.9 mya on lavas in the northern part of the volcanic complex is the oldest obtained date. Other dating has resulted in ages of 3.9±1.3 mya for Pico Laco's dome, 3.8±0.9 mya for lavas beneath San Vicente Bayo, 3.7±0.9 mya for a lava front next to Laco Norte, 2.6±0.6 for Crystales Grandes, 2.1±0.4 for Hueso Chico, and 1.6±0.5 for "Volcano 5009". Cordon de Puntas Negras has younger dates. Another date from Pico Laco is 2.0±0.3 mya; one study suggested four separate episodes of volcano development. Volcanic activity continued after the emplacement of the magnetite bodies. Later alteration included hydrothermal alteration and glacial erosion; the former has left bleached rocks and exhalation deposits. Some minor metasomatic alteration occurred at the contact sites between andesites and iron-containing rocks. Hydrothermal alteration has also been described for lower portions of the volcanic pile and probably occurred because of gases escaping from intruded magma. Moraines found west of El Laco were generated by glaciation both on El Laco and Puntas Negras.

Surface exposure dating Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion hist ...

has indicated ages of 226 and 287 ka for some ice-affected lavas and argon-argon dating produced a youngest age of about 120,000 years. Further, andesitic volcanism in neighbouring volcanoes has blanketed El Laco. Reports exist of continuing

fumarolic A fumarole (or fumerole) is a vent in the surface of the Earth or another rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

activity and

hot spring A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal activity, geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...

s with the deposition of clay and other minerals.

Iron-rich deposits

On the flank of the volcano,

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 ...

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

deposits are found at altitudes of . The volcano is mainly known for these flows, but such material is also found in the form of

tephra Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...

. The deposits lie on top of flat lava flows of andesitic composition, concentrically around Pico Laco. They are named Laco Norte, Laco Sur, San Vicente Alto, San Vicente Bajo, and Rodados Negros. The deposits consist of dykes, hydrothermal deposits, lava flows, pyroclastics, and subvolcanic structures and were erupted from parasitic vents and fissures. The magnetite is classified as porphyry-like. Apatite is present as an accessory mineral in the lavas and is abundant in the intrusions. Iron-rich zones also formed in tuffs and lavas. Magnetite in the subvolcanic bodies exists in more massive crystals. The iron-containing rocks include lava flows, ash, and

lapilli Lapilli (: lapillus) is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' is Latin for "little stones". By definition lapilli range from in dia ...

, as well as ore

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

s formed presumably when the volcano collapsed. The El Laco magnetite lava flows have drawn scientific attention for decades and are unique in the world; they were deemed a

IUGS The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to global cooperation in the field of geology. As of 2023, it represents more than 1 million geoscientists around the world. About Fo ...

Geological Heritage Site in 2024. They formed during active subduction.

Individual deposits

Of these deposits, Laco Norte is the largest and was probably separated from neighbouring Laquito by erosion. It is thick and covers a surface area of . It was erupted from feeder dykes on its southern and eastern end and forms a table-shaped body on a spur, in the shape of a

mesa A mesa is an isolated, flat-topped elevation, ridge, or hill, bounded from all sides by steep escarpments and standing distinctly above a surrounding plain. Mesas consist of flat-lying soft sedimentary rocks, such as shales, capped by a ...

. At Laco Norte, which may be a

lava lake Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified (sometim ...

, a structure of five layers is found: a basal andesite, ore in pyroclastic form, magnetite lava, pyroclastics which contain ore, and andesite at the top. Laco Sur has a similar morphology and dimensions of ; it has been mined. San Vicente Alto is a

spatter cone Volcanic cones are among the simplest volcano, volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone (geometry), cone with a central crater. Volcanic cones are of different types, depen ...

or lava flow on the upper parts of the volcano (), and San Vicente Bajo is probably a lava dome (). Laquito ( long and wide) and Rodados Negros () appear to be dykes, while Cristales Grandes ( long and up to wide) is more likely a vein and generally shows signs of hydrothermal formation. A magnetic layer of rock spreads north from the volcano, and a large magnetite body has been modelled beneath Pasos Blancos.

Structure and appearance

The magnetite lavas are primarily aa lava, but other surface features are also found, including

pahoehoe Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or unde ...

features. Columnar morphologies are found on the magnetite, implying that they cooled quickly. There is only one other place in the world where columnar magnetite has been found –

Kiirunavaara Kiirunavaara (Northern Sami: ''Gironvárri'', Meänkieli: ''Kierunavaara'') is a mountain situated in Kiruna Municipality in Norrbotten County, Sweden. It contains one of the largest and richest bodies of iron ore in the world. History The pres ...

, in

Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

. Centimeter-sized vesicles to meter-sized tubes coated on the inside by euhedral magnetite were formed in the lava by escaping volcanic gas. Both before and after the magnetite lavas, layers of magnetite-containing pyroclastics were erupted. A aureole separates the magnetite rocks from the host rocks. The magnetite lava flows are thick, the pyroclastics and respectively. The pyroclastic-like deposits are porous and fragile and show traces of stratification. The pyroclastics at Laco Sur contain spherules of magnetite as well as some blebs of destinezite. An age of 2.1±0.1 million years has been found for ore by fission track dating. The lavas contain veins likely generated by hydrothermal activity.

Origin

Temperatures estimated for the erupted rocks cover a wide range, with some exceeding . These rocks are of enigmatic origin, which may be magmatic-hydrothermal or magmatic, with the presence of

lava bomb A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscosity, viscous fragments of lava during an eruption. Because volcanic bombs cool a ...

s of magnetite lava and other evidence supporting the magmatic origin theory. According to the magmatic genetic hypothesis magma would have separated from a precursor magma due to spontaneous unmixing of an Fe-rich melt from an andesitic parent magma in a shallow magma reservoir beneath the volcano. Other viewpoints consider the texture and chemical composition of the rocks as evidence that

metasomatism Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in the chemical com ...

or hydrothermal replacement of andesitic rocks formed the magnetite "lavas". A third hypothesis envisages the crystallization of magnetite within a silicic magma and its subsequent extraction through flotation attached to volatile bubbles. The role of a post-magmatic fluid phase, which was inferred from inclusions in crystals, or of a combination of magmatic and hydrothermal mechanisms has also been suggested. Some magnetite was oxidized to hematite, probably under the influence of rainwater as indicated by isotope analysis. Only a minor amount of hematite is primary. Isotope data indicate that the formation of this magnetite magma was accompanied by the segregation of plagioclase. This plagioclase may have generated the rhyodacite lava dome. An iron-phosphate-rich magma generated the magnetite lava flows after release of volatile substances. The magma was probably bordering on forming a two-phase melt containing nelsonite and

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

. A favourable tectonic context associated with the compression of the magma chamber and the presence of faults helped with the eruption of the magnetite. The magma formation probably occurred in a magma chamber. During the cooling of the magma, the ores formed. This process was probably not directed by water-rich phases, and the segregation occurred at a shallow depth. High phosphorus and volatile content may have lowered the melting point of the magma and facilitated its eruption, as well as overcoming density-based constraints on the eruption of iron-rich magmas; the magma would later have forcefully degassed within the volcano. Suggestions that

anatexis Anatexis (via Latin from Greek roots meaning "to melt down") is the partial melting of rocks. Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term " partial melting" refers to the p ...

of iron-rich sediments generated the iron-rich magmas appear implausible. The ultimate origin of the El Laco iron may be subducted metal-containing sediment.

Human history and exploitation

These iron oxide deposits were found in 1958 and first described in 1961. Mining in Laco Sur removed about two million tons of magnetite between the 1970s and 1990s, leaving an open pit exposing of rock. In 2009, these mineral reserves were mined by Cia Minera del Pacifico S.A. It is estimated that the deposit contains 733.9 million tons of ore, consisting of 50% iron. The geological interest in these kinds of mineral deposits is enhanced by their frequent association with other minerals, as has been noted at Olympic Dam,

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. Other magnetite-apatite ore deposits in the Andes are Incahuasi (10.3±0.8 mya), south of El Laco, and Magnetita Pedernales (Tertiary), about south-southwest of Laco.

Comparable deposits

The Kiruna magnetites in Sweden resemble the El Laco ones in terms of

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

and

vanadium Vanadium is a chemical element; it has Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

content, and their

titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

content is comparably low. The deposits at Kiruna and El Laco have been classified as "Iron oxide-apatite" ore deposits, and El Laco is the best preserved and youngest such deposit on Earth. Other deposits of volcanic iron ore are Nenana in Alaska, the

Chilean Iron Belt The Chilean Iron Belt is a geological province rich in iron ore deposits in northern Chile. It extends as a north-south beld along the western part of the Chilean regions of Coquimbo and Atacama, chiefly between the cities of La Serena and Talt ...

, the Tertiary Cerro el Mercado deposit in

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

, the Eocambrian Bafq district in

Iran Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the nort ...

, and the

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

Kiruna field in Sweden. Of these, Sierra Bandera in the Chilean iron belt may be another example of surface volcanic iron ore rather than subvolcanic ore as is commonly assumed of these deposits.

Petrology

The main rocks of the volcano are andesite and

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

, which contain

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

and

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 ( ...

as well as blebs containing

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

. The iron-containing rocks are a less important component. Other components are

diopside Diopside is a monoclinic pyroxene mineral with composition . It forms complete solid solution series with hedenbergite () and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull ...

pegmatite 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 c ...

s. The whole rock falls into the

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 ...

class of volcanic rocks, similar to these erupted by the neighbouring volcanoes

Lascar A lascar was a sailor or militiaman from the Indian subcontinent, Southeast Asia, the Arab world, British Somaliland or other lands east of the Cape of Good Hope who was employed on European ships from the 16th century until the mid-20th centur ...

and

Llullaillaco Llullaillaco () is a dormant stratovolcano on the border between Argentina (Salta Province) and Chile (Antofagasta Region). It lies in the Puna de Atacama, a region of tall volcanic peaks on a high plateau close to the Atacama Desert, one of ...

. The andesites contain

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

clinopyroxene 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 (Fe ...

orthopyroxene 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 (Fe( ...

, and

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s of magnetite. Magnetite, and in lesser measure hematite, are the most abundant iron minerals;

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...

maghemite Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same formula as hematite, but the same spinel ferrite structure as magnetite () and is also ferrimagnetic. It is sometimes spelled as "maghaemite". ''Maghemite'' ...

pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

scapolite The scapolites (, "rod", and , "stone") are a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate. The two endmembers are meionite () and marialite (). Silvialite i ...

and diadochite are also found. Erupted magma was probably gas-rich, as the magnetite lavas would otherwise have melting points of over . The lavas lost most of their

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

and phosphorus after their eruption. High

oxygen-18 Oxygen-18 (, Ω) is a natural, stable isotope of oxygen and one of the environmental isotopes. is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). Generally, in the radiopharm ...

amounts in the Laco magmas indicate either crustal contamination or isotopic effects during

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

. Some atmospheric water influence has been inferred from isotope data as well.

Hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a Rock (geology), rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in t ...

of the central lava dome and iron-bearing deposits has generated

alunite Alunite is a hydroxylated aluminium potassium sulfate mineral, formula potassium, Kaluminium, Al3(sulfur, Soxygen, O4)2(Ohydrogen, H)6. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum ...

anatase Anatase is a metastable mineral form of titanium dioxide (TiO2) with a Tetragonal crystal system, tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other Pol ...

bassanite Bassanite is a calcium sulfate mineral with formula CaSO4· H2O or 2CaSO4·H2O. In other words it has half a water molecule per CaSO4 unit, hence its synonym ''calcium sulfate hemihydrate''. Bassanite was first described in 1910 for an occurren ...

chlorite The chlorite ion, or chlorine dioxide anion, is the halite (oxyanion), halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as s ...

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 ...

veinlets,

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

illite Illite, also called hydromica or hydromuscovite, is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandw ...

jarosite Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct dur ...

kaolinite Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina () ...

labradorite Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller). Labradorite is an intermediate to calcic member of the plagioclase series. It ...

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 ...

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal sys ...

smectite A smectite (; ; ) is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secon ...

, sulfur and

tridymite Tridymite is a high-temperature polymorphism (materials science), polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is sili ...

. Some of these minerals forms veins inside the rock. Silification is prominent and has formed

cristobalite Cristobalite ( ) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, Si O2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members o ...

and

. Elemental sulfur is also found. Vast regions of the volcano have been altered hydrothermally at temperatures of , giving the rock a clear appearance. Minor exhalation deposits are also found in the form of

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

s that sometimes conserve conduits. Red-coloured alteration halos occur in andesites adjacent to iron deposits, probably due to iron input. It is possible that the formation of the iron-rich magma was accompanied by the production of large amounts of

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

fluids which then triggered the hydrothermal alteration.

Environment

The vegetation in the area is primarily low

bushland In Australia, bushland is a blanket term for land which supports remnant natural area, remnant vegetation or land which is disturbed but still retains a predominance of the original floristics and structure. Human survival in bushland has a wh ...

Short-tailed chinchilla The short-tailed chinchilla (''Chinchilla chinchilla)'' is a small rodent part of the Chinchillidae family and is classified as an endangered species by the IUCN. Originating in South America, the chinchilla is part of the genus ''Chinchilla'', w ...

s can be found at El Laco, one of the few occurrences of this

threatened species A threatened species is any species (including animals, plants and fungi) which is vulnerable to extinction in the near future. Species that are threatened are sometimes characterised by the population dynamics measure of ''critical depensatio ...

in Chile. El Laco has a classical cold mountain climate at the line between the dry Altiplano with summer precipitation and the hyper-arid Atacama Desert climate. A nearby weather station () at altitude showed an average temperature of in 1991, with strong short-term variability. The majority of precipitation falls during southern hemisphere summer; winter snowfall has been recorded. Air humidity recorded in 1991 was 10–30%.

References

External links

El Laco field excursion
{{Authority control Iron ore mines in Chile Pleistocene stratovolcanoes Pliocene stratovolcanoes Miocene stratovolcanoes Volcanoes of Antofagasta Region Mines in Antofagasta Region