El Laco

Geography

El Laco is part of the Cordón de Puntas Negras sector of the Central Volcanic Zone,^:681,682 directly south of that volcanic chain. 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 occurred, which was then buried by Tertiary rhyolites.^:681,682 Two major volcanic lineaments cross in the El Laco area. El Hueso volcano to the north is 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).

The city of Antofagasta is located 320 kilometres (200 mi) west of El Laco.^:681 Other close towns are Calama and San Pedro de Atacama. The international road 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.

Geology

The El Laco volcanic complex is formed by about seven minor stratovolcanoes and lava domes. 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.^: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).

The main summit, Pico Laco, has an altitude of 5,325 metres (17,470 ft), although a maximum height of 5,472 metres (17,953 ft) has also been reported. 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.

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.

Ages of 5.3 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. 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.^: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. Further, andesitic volcanism in neighbouring volcanoes has blanketed El Laco.^:684 Reports exist of continuing fumarolic activity and hot springs 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 4,600–5,200 metres (15,100–17,100 ft). The volcano is mainly known for these flows. 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.^:684 The iron-containing rocks include lava flows, ash, and lapilli, as well as ore breccias. 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 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. 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 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 Nortes (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. 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 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, 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 0.5–2-metre (1 ft 8 in–6 ft 7 in) aureole separates the magnetite rocks from the host rocks. The magnetite lava flows are 50 metres (160 ft) thick, the pyroclastics 30 metres (98 ft) and 20 metres (66 ft) respectively.^:684–685 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 dating. The lavas contain veins likely generated by hydrothermal activity.

Origin

Temperatures estimated for the erupted rocks cover a wide range, with some exceeding 800 °C (1,470 °F). These rocks are of enigmatic origin, which may be geothermal or magmatic, with the presence of lava bombs of magnetite lava supporting the magmatic origin theory. Other viewpoints consider the texture and chemical composition of the rocks as evidence that metasomatism of andesitic rocks formed the magnetite "lavas". The role of a post-magmatic fluid phase, which was inferred from inclusions in crystals, has also been suggested. Some magnetite was oxidized to hematite,^:681 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.^:688–689 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. 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 30 metres (98 ft) of rock. In 2009, these mineral reserves were mined by Cia Minera del Pacifico S.A. It is estimated that the deposit contains one billion tons of ore, consisting of 50% iron.^: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. 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.

Comparable deposits

The Kiruna magnetites in Sweden resemble the El Laco ones in terms of manganese and vanadium content, and their titanium content is comparably low. Other deposits of volcanic iron ore are the "Chilean iron belt", the Tertiary Cerro el Mercado deposit in Mexico, the Eocambrian Bafq district in Iran, 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, 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 class of volcanic rocks. The andesites contain plagioclase clinopyroxene, orthopyroxene, and phenocrysts of magnetite.^:681–682 Magnetite, and in lesser measure hematite, are the most abundant iron minerals;^:685 anhydrite, diopside, goethite, limonite, maghemite, pyrite,^:685 and diadochite are also found. 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. High oxygen-18 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 of the central lava dome and iron-bearing deposits has generated alunite, anatase, chlorite, copper veinlets, gypsum, illite, jarosite, labradorite, quartz, rutile, sanidine, smectite, and sulfur. Some of these minerals forms veins inside the rock.^:684 Silification is prominent and has formed cristobalite and tridymite. Elemental sulfur is also found. 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.

Environment

The vegetation in the area is primarily low bushland. Short-tailed chinchillas can be found at El Laco.

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 (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%.