Circular segment with a diameter of 0.75 kilometer corresponding to an emission dyke of magnetitite lava from Laco Norte, which reaches 0.8 kilometers in length and a variable width between 20 and 80 meters. It has a concave curved shape towards the north and is surrounded by hydrothermally altered andesites. View towards the east (by José Antonio Naranjo).
Geological Period
Pleistocene
Main geological interest
Volcanology
Location
Antofagasta Region, Chile
23°49’37”S, 067°29’29”W
Circular segment with a diameter of 0.75 kilometer corresponding to an emission dyke of magnetitite lava from Laco Norte, which reaches 0.8 kilometers in length and a variable width between 20 and 80 meters. It has a concave curved shape towards the north and is surrounded by hydrothermally altered andesites. View towards the east (by José Antonio Naranjo).
Unique magnetite lavas due to their preservation quality, surface structures and other features of iron eruptions.
In small proportions as microcrystals called “accessories”, magnetite is a common mineral in silicate lavas. However, the concentration, proportion and volume of eruptive products, lavas and pyroclasts, of magnetite in a volcano is a really rare phenomenon on Earth. On the other hand, the quality of preservation and environmental and structural volcanic conditions of the forms and products of magmatic iron within the El Laco Volcanic Complex represent a case of extraordinary quality and provides new understandings for geology in general and iron geology in particular.
Stalaphytes of ca. 18 centimeters length and 2.5-3 centimeters width of frozen magnetitite in cavities developed in the northern free wall of the Laco Norte magnetitite dyke (by José Antonio Naranjo).
- Geological description
The geology of El Laco Volcanic Complex (ELVC) includes the exogenous andesitic dome Pico Laco (5,310 meters a.s.l.), and nine subsidiary magnetitite eruptive centers at altitudes between 4,600 and 5,100 meters a.s.l. Although the accessibility conditions are through a paved route, due to the altitude at which it is located, you can easily get altitude sickness.
The ELVC in the Central Andes (23°48’S, 67°30’W; 5,300 meters a.s.l.) is formed by andesitic domes, up to 10-meter-thick andesite lava flows and sphene porphyritic bodies. K-Ar dating gives ages from 0.3 to 5.3 Ma. ELVC presents an extensive zone of hydrothermal alteration with differential erosion and subsequent alluvial deposits. Similar to carbonatites (Ol Doinyo Lengai, Tanzania), El Laco contains unique iron oxide eruptive products including lavas emitted from fissures (0.3 to 0.7 kilometer long and 10 to 30 meters wide), pyroclastic fall material as bombs, lapilli and ash, and pyroclastic density currents. Among the latter, remarkable surge deposits are common. All these eruptive products contain clinkery, aa, and smooth surface textures, like pahoehoe stalactites. The rough appearance of the former is due to the growth of small octahedral magnetite crystals and dictaxitic-type porosity, which contrasts with the bright surfaces and smooth-walled vesicular porosity of the latter. These products resulted from the eruption of an iron-rich lava flow with a high volatile content, together with a strong degassing process that contributed to the rapid consolidation, the great variation in viscosity and the formation of the observed textures.
- Scientific research and tradition
Since the earlies 1960’s, it has been considered the most spectacular case of iron lava volcanism in the world.
Geological sketch map of the El Laco Volcanic Complex showing the different subsidiary magnetitite volcanoes (Naranjo et al., 2010).
- Reference
Alva-Valdivia, L.M. et al. (2003) ‘Rock-Magnetic and Oxide Microscopic Studies of the El Laco Iron Ore Deposits, Chilean Andes, and Implications for Magnetic Anomaly Modeling’, International Geology Review, 45(6), pp. 533–547. Available at: https://doi.org/10.2747/0020-6814.45.6.533.
Broman, C. et al. (1999) ‘Fluid inclusions in magnetite-apatite ore from a cooling magmatic system at El Laco, Chile’, GFF, 121(3), pp. 253–267. Available at: https://doi.org/10.1080/11035899901213253.
Guijón, R., Henríquez, F. and Naranjo, J.A. (2011) ‘Geological, Geographical and Legal Considerations for the Conservation of Unique Iron Oxide and Sulphur Flows at El Laco and Lastarria Volcanic Complexes, Central Andes, Northern Chile’, Geoheritage, 3(4), pp. 299–315. Available at: https://doi.org/10.1007/s12371-011-0045-x.
Henriquez, F. and Martin, R.F. (1978) ‘Crystal-growth textures in magnetite flows and feeder dykes, El Laco, Chile’, The Canadian Mineralogist, 16(4), pp. 581–589.
Naranjo Soza, J.A., Henríquez Barrientos, F. and Nystrom, J.O. (2010) ‘Subvolcanic contact metasomatism at El Laco Volcanic Complex, Central Andes’, Andean geology: Formerly Revista geológica de Chile, 37(1), pp. 110–120.
Park, C.F. (1961) ‘A magnetite “flow” in northern Chile’, Economic Geology, 56(2), pp. 431–436. Available at: https://doi.org/10.2113/gsecongeo.56.2.431.
- Author(s)
José Antonio Naranjo.
Servicio Nacional de Geología y Minería, Chile.
Fernando Henríquez.
Departamento de Ingeniería en Minas, Universidad de Santiago de Chile.
Jan Olov Nyström.
Swedish Museum of Natural History, Stockholm, Sweden.
Richard Naslund.
Department of Geological Sciences, SUNY, Binghamton, USA.
