El Laco is a volcanic complex in the

Antofagasta Region The Antofagasta Region ( es, Región de Antofagasta, ) is one of Chile's sixteen first-order administrative divisions. The second-largest region of Chile in area, it comprises three provinces, Antofagasta, El Loa and Tocopilla. It is bordered t ...

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

. It is directly south of the

Cordón de Puntas Negras Cordón de Puntas Negras is a volcanic chain located east of the Salar de Atacama in Chile's Antofagasta Region. Cordón de Puntas Negras is constructed along the major Calama–Olacapato–El Toro fault and is long. The long volcanic chai ...

volcanic chain. Part of the Central Volcanic Zone of the

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

, it is a group of seven

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

and a

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

. It is about two million years old. The main summit of the volcano is a lava dome 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-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 up ...

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 which 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 tec ...

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 ...

basement that was lifted from the seaground during the Acadian orogeny and is of

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

age. Later,

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

and Cenozoic sedimentation occurred, which was then buried by

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

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. After the Spanish American wars ...

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 volcano. ...

. The international road through

Sico Pass Sico Pass ( es, Paso de Sico) is a mountain pass on the border between Argentina and Chile. The pass is located on the main divide of the Andes. Administratively, it separates the province of Salta in Argentina and the region of Antofagasta in Chi ...

connecting Salta in

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

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

Atacama Desert The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in th ...

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 stratovolcanoes 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 in the geologic time scale that extends from 5.333 million 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 predo ...

s formed a stratovolcano. During the Pliocene, ash and pyroclastic 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 ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...

, 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. 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 margin ...

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

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 activity and

hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by c ...

s with the deposition of clay and other minerals.

Iron-rich deposits

On the flank of the volcano, apatite, hematite, and magnetite 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 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, they re ...

. 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, and San Vicente Bajo. 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, as well as ore

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

s. The El Laco magnetite lava flows are unique in the world and 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, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a ...

. At Laco Norte, 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 lava flow on the upper parts of the volcano (), and San Vicente Bajo is probably a lava dome (). Laquito ( long and wide) and Rodados Nortes () 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 und ...

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. Large tubes coated on the inside by magnetite were formed in the lava by escaping 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. An age of 2.1±0.1 million years has been found for ore by

fission track Ion tracks are damage-trails created by swift heavy ions penetrating through solids, which may be sufficiently-contiguous for chemical etching in a variety of crystalline, glassy, and/or polymeric solids. They are associated with cylindrical d ...

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 geothermal 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 viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...

s of magnetite lava and other evidence supporting the magmatic origin theory. The magma would have separated from a precursor magma due to immiscibility effects. 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 the replacement of one rock by another of different mineralogical and chemical co ...

of andesitic rocks formed the magnetite "lavas". A third hypothesis envisages the crystallization of magnetite within a silicic magma and its subsequent extraction through 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. 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 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. 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. 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 with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

and vanadium content, and their

titanium Titanium is a chemical element with the Symbol (chemistry), 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, resista ...

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 the Chilean Iron Belt, the Tertiary Cerro el Mercado deposit in

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

, the Eocambrian Bafq district in

Iran Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...

, and the Proterozoic 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 and pyroxene as well as blebs containing iron oxide. The iron-containing rocks are a less important component. 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 m ...

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 land east of the Cape of Good Hope, who was employed on European ships from the 16th century until the middle of the ...

and

Llullaillaco Llullaillaco () is a dormant stratovolcano at the border of 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 dri ...

. The andesites contain

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 pro ...

clinopyroxene The pyroxenes (commonly abbreviated to ''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 calcium (Ca), sodium (Na), iron (Fe I ...

, orthopyroxene, and

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

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 ...

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 ...

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,

maghemite Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same spinel ferrite structure as magnetite and is also ferrimagnetic. It is sometimes spelled as "maghaemite". ''Maghemite'' can be considered as an Fe(II)-deficie ...

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 giv ...

scapolite The scapolites (Gr. σκάπος, rod, and λίθος, 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 marialit ...

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 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 radiopharmaceu ...

amounts in the Laco magmas indicate either crustal contamination or isotopic effects during fractional crystallization. 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 by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

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

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

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 occurrenc ...

, chlorite,

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 ...

veinlets,

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate 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, blackboard or sidewalk chalk, and drywal ...

illite Illite 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 sandwich of silica tetrahedron (T) – alumina ...

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 du ...

, kaolinite,

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 atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

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 visib ...

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 (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...

, sulfur and

tridymite Tridymite is a high-temperature 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 Si O2. Tridymite was first describe ...

. 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, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the ...

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. Short-tailed chinchillas can be found at El Laco, one of the few occurrences of this

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

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