El Laco is a volcanic complex in the Antofagasta Region of Chile. It is directly south of the Cordón de Puntas Negras volcanic chain. Part of the Central Volcanic Zone of the Andes, it is a group of seven stratovolcanoes and a caldera. 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 5,325 metres (17,470 ft) or 5,472 metres (17,953 ft) high. The edifice has been affected by glaciation, and some reports indicate that it is still fumarolically active.

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El Laco
The conical summit to the right is named Pico El Laco. The black zones to the left of it are Cerro Laco Sur and Cerro Laco Norte. The image shows only about half of the volcanic complex.
Highest point
Elevation	5,325 m (17,470 ft)
Coordinates	23°50′29.6″S 67°29′24.6″W[1]
Geography
El Laco Location of El Laco

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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 with a volume of about 30 cubic kilometres (7.2 cu mi);^[2] 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.

El Laco is part of the Cordón de Puntas Negras sector of the Central Volcanic Zone,^{[3]: 681,682} directly south of that volcanic chain.^[4] It sits atop a quartzite and sandstone basement that was lifted from the seaground during the Acadian orogeny and is of Ordovician age. Later, Mesozoic and Cenozoic sedimentation of the Salta Group occurred, which was then buried by Tertiary rhyolites.^{[5]: 515 [3]: 681,682} Two major volcanic lineaments cross in the El Laco area,^[6] including the so-called Calama-Olacapato-El Toro lineament.^[7] El Hueso volcano to the north is 5,028 metres (16,496 ft)^[8]-5,029 metres (16,499 ft) high and has a basement diameter of 2.5 kilometres (1.6 mi). It has a crater with a diameter of 1 kilometre (0.62 mi).^[9]

The city of Antofagasta is located 320 kilometres (200 mi) west of El Laco.^[3]: 681 Other close towns are Calama and San Pedro de Atacama. The international road through Sico Pass^[10] connecting Salta in Argentina with Calama in Chile runs close to El Laco. A number of tourist sites are found in the Atacama Desert adjacent to El Laco, and the dry climate also makes the area suitable for astronomy facilities.^[11]

The El Laco volcanic complex is formed by about seven minor stratovolcanoes and lava domes.^[12] The complex started its activity in the Miocene-Pliocene, when porphyric andesites formed a stratovolcano. During the Pliocene, ash and pyroclastic eruptions formed a caldera with a diameter of 4–5 kilometres (2.5–3.1 mi), which also contains a central lava dome that formed 6.5 million years ago. Finally, probably during the Pleistocene, 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.^{[3]: 682–684} The volcanic complex is located an altitude of 4,300–5,470 metres (14,110–17,950 ft) and covers a surface area of 7 by 5 kilometres (4.3 mi × 3.1 mi)^[13] with a minimum volume of 14 cubic kilometres (3.4 cu mi) of volcanic rock.^[14]

The main summit, Pico Laco^[11] or Pitón El Laco,^[14] has an altitude of 5,325 metres (17,470 ft),^[11] although a maximum height of 5,472 metres (17,953 ft) has also been reported.^[15] Pico Laco is an andesitic lava dome with a height of 400 metres (1,300 ft) above the surrounding terrain. The dome, with dimensions of 1.5 by 1 kilometre (0.93 mi × 0.62 mi), has two summits: the higher eastern one and a 5,166-metre (16,949 ft) western summit,^[9] and has also been described as a volcanic plug.^[14] Pico Laco has been described as a resurgent volcano within a caldera, that is surrounded by secondary vents.^[7]

Other summits include the northwestern Hueso Chico, a cone with a height of 120 metres (390 ft) above its surroundings and a crater 250 metres (820 ft) wide. This cone is of dacitic composition. "Volcano 5009" is heavily eroded, and its core of lava and hyaloclastite has been exposed. It has a diameter of 2.5 kilometres (1.6 mi). Eruptive activity here probably coincided with glacier activity during the Pliocene, as evidenced by moraines in the area.^[9]

Ages of 10.5^[10] 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.^[9] Another date from Pico Laco is 2.0±0.3 mya;^[16] one study suggested four separate episodes of volcano development.^[14] Volcanic activity continued after the emplacement of the magnetite bodies.^[10] 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.^{[17][3]: 684} 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 has indicated ages of 226 and 287 ka for some ice-affected lavas^[12] and argon-argon dating produced a youngest age of about 120,000 years.^[10] Further, andesitic volcanism in neighbouring volcanoes has blanketed El Laco.^[3]: 684 Reports exist of continuing fumarolic activity and hot springs with the deposition of clay and other minerals.^[12]

On the flank of the volcano, apatite, hematite, and magnetite deposits are found^[18] at altitudes of 4,600–5,200 metres (15,100–17,100 ft).^[19] The volcano is mainly known for these flows,^[20] but such material is also found in the form of tephra.^[21] The deposits lie on top of flat lava flows of andesitic composition,^[22] concentrically around Pico Laco.^[13] They are named Laco Norte, Laco Sur, San Vicente Alto, San Vicente Bajo, and Rodados Negros.^[19] The deposits consist of dykes, hydrothermal deposits, lava flows, pyroclastics, and subvolcanic structures and were erupted from parasitic vents and fissures.^[12][17] The magnetite is classified as porphyry-like.^[2] Apatite is present as an accessory mineral in the lavas and is abundant in the intrusions.^[17] Iron-rich zones also formed in tuffs and lavas.^[23] Magnetite in the subvolcanic bodies exists in more massive crystals.^[3]: 684 The iron-containing rocks include lava flows, ash, and lapilli,^[24] as well as ore breccias. The El Laco magnetite lava flows are unique in the world^[11] and formed during active subduction.^[22]

The main rocks of the volcano are andesite and dacite, which contain biotite and pyroxene as well as blebs containing iron oxide. The iron-containing rocks are a less important component.^[24] The whole rock falls into the calc-alkaline class of volcanic rocks,^[36] similar to these erupted by the neighbouring volcanoes Lascar and Llullaillaco.^[14] The andesites contain plagioclase clinopyroxene, orthopyroxene, and phenocrysts of magnetite.^{[3]: 681–682} Magnetite, and in lesser measure hematite, are the most abundant iron minerals;^[3]: 685 anhydrite,^[48] diopside,^[49] goethite, limonite, maghemite, pyrite,^[3]: 685 scapolite^[8] and diadochite are also found.^[24] Erupted magma was probably gas-rich, as the magnetite lavas would otherwise have melting points of over 1,500 °C (2,730 °F). The lavas lost most of their sulfur and phosphorus after their eruption.^[24] High oxygen-18 amounts in the Laco magmas indicate either crustal contamination or isotopic effects during fractional crystallization.^[25] Some atmospheric water influence has been inferred from isotope data as well.^[37]

Hydrothermal alteration of the central lava dome and iron-bearing deposits has generated alunite, anatase, bassanite, chlorite, copper veinlets, gypsum, illite, jarosite, kaolinite, labradorite, quartz, rutile, sanidine, smectite, sulfur and tridymite. Some of these minerals forms veins inside the rock.^{[13][8][3]: 684 [19]} Silification is prominent and has formed cristobalite and tridymite. Elemental sulfur is also found.^[2] Vast regions of the volcano have been altered hydrothermally at temperatures of 200–250 °C (392–482 °F), giving the rock a clear appearance. Minor exhalation deposits are also found in the form of sulfates that sometimes conserve conduits. Red-coloured alteration halos occur in andesites adjacent to iron deposits, probably due to iron input.^[13] It is possible that the formation of the iron-rich magma was accompanied by the production of large amounts of hydrothermal fluids which then triggered the hydrothermal alteration.^[33]

The vegetation in the area is primarily low bushland.^[11] Short-tailed chinchillas can be found at El Laco,^[50] one of the few occurrences of this threatened species in Chile.^[51]

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.^[11] A nearby weather station (23°45′S 67°20′W) at 4,500 metres (14,800 ft) altitude showed an average temperature of 2.3 °C (36.1 °F) 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%.^[52]