El Laco

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 / 23.841556°S 67.490167°W / -23.841556; -67.490167[1]
Geography
El Laco Location of El Laco

El Laco is a

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.

Geography

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]

Geology

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

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]

Individual deposits

Of these deposits, Laco Norte is the largest and was probably separated from neighbouring Laquito by erosion. It is 60–90 metres (200–300 ft) thick and covers a surface area of 1,000 by 1,500 metres (3,300 ft × 4,900 ft). 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.^[19] 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.^[16] Laco Sur has a similar morphology and dimensions of 30–70 by 600 by 750 metres (98 ft–230 ft × 1,969 ft × 2,461 ft); it has been mined. San Vicente Alto is a lava flow on the upper parts of the volcano (30 by 320 by 480 metres (98 ft × 1,050 ft × 1,575 ft)), and San Vicente Bajo is probably a lava dome (250 by 390 metres (820 ft × 1,280 ft)). Laquito (150 metres (490 ft) long and 50 metres (160 ft) wide) and Rodados Negros (500 by 600 metres (1,600 ft × 2,000 ft)) appear to be dykes, while Cristales Grandes (80–100 metres (260–330 ft) long and up to 30 metres (98 ft) wide) is more likely a vein and generally shows signs of hydrothermal formation.^[19] A magnetic layer of rock spreads north from the volcano,^[25] and a large magnetite body has been modelled beneath Pasos Blancos.^[12]

Structure and appearance

The magnetite lavas are primarily

Some magnetite was oxidized to hematite,[3]^: 681 probably under the influence of rainwater as indicated by isotope analysis. Only a minor amount of hematite is primary.^[19][30] 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.^{[3]: 688–689 [24][38]} The magma formation probably occurred in a magma chamber.^[12] During the cooling of the magma, the ores formed.^[25] This process was probably not directed by water-rich phases, and the segregation occurred at a shallow depth.^[6] High phosphorus and volatile content may have lowered the melting point of the magma and facilitated its eruption,^[22] as well as overcoming density-based constraints on the eruption of iron-rich magmas;^[39] the magma would later have forcefully degassed within the volcano.^[21] Suggestions that anatexis of iron-rich sediments generated the iron-rich magmas appear implausible.^[22] The ultimate origin of the El Laco iron may be subducted metal-containing sediment.^[40]

Human history and exploitation

These iron oxide deposits were found in 1958 and first described in 1961.^[41][42] Mining in Laco Sur removed about two million tons of magnetite between the 1970s and 1990s, leaving an open pit exposing 30 metres (98 ft) of rock.^[1] In 2009, these mineral reserves were mined by Cia Minera del Pacifico S.A.^[43] It is estimated that the deposit contains 733.9 million tons of ore,^[7] consisting of 50% iron.^[3]: 684 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.^[39] Other magnetite-apatite ore deposits in the Andes are Incahuasi (10.3±0.8 mya), 26 kilometres (16 mi) south of El Laco, and Magnetita Pedernales (Tertiary), about 300 kilometres (190 mi) south-southwest of Laco.^[25]

Comparable deposits

The

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 / 23.750°S 67.333°W / -23.750; -67.333) 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]

References