Morococala is a

volcanic field A volcanic field is an area of Earth's Earth's crust, crust that is prone to localized volcano, volcanic activity. The type and number of volcanoes required to be called a "field" is not well-defined. Volcanic fields usually consist of clusters ...

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

, in the Department Oruro. It is formed by

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

s and associated volcanic features. It is part of the chain of

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s that extends over the Cordillera Real and the Cordillera Occidental and are the sites of mining activity.

Geography and geomorphology

It lies in the Cordillera Occidental. The city of

Oruro Oruro (Hispanicized spelling) or Uru Uru is a city in Bolivia with a population of 264,683 (2012 calculation), about halfway between La Paz and Sucre in the Altiplano, approximately above sea level. It is Bolivia's fifth-largest city by populat ...

lies northwest of Morococala, and the town of

Llallagua Llallagua (in hispanicized spelling) or Llallawa ( Aymara for a monstrous potato (like two potatoes) or animal, Quechua for the god of seed-time during the Inca period) is a town in the Potosí Department in Bolivia. It is the seat of the Llal ...

is just south. Morococala consists of a high plateau, and is a field consisting of

s and

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

s, which were deposited on top of the basement. These form sequences with thicknesses on average less than . This field covers a surface area of . The presence of two calderas has been inferred, one at Tankha Tankha in the northern part of the field and one at Condoriri in the southern part of the field. The Tankha Tankha caldera has a

resurgent dome In geology, a resurgent dome is a dome formed by swelling or rising of a caldera floor due to movement in the magma chamber beneath it. Unlike a lava dome, a resurgent dome is not formed by the extrusion of highly viscous lava onto the surface, ...

which has erupted domes and lava flows. Other volcanic landforms in the field are

lava dome In volcanology, a lava dome is a circular, mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions ...

s. Mines are located at San Pablo, Morococala and Japo. Morococala is part of the so-called

Bolivian tin belt The Bolivian tin belt () is a mineral-rich region in the Cordillera Oriental of Bolivia. Being a metallogenetic province the Bolivian tin belt is rich in tin, tungsten, silver and base metals. The Bolivian tin belt follows the same bend as the ...

, a string of

s of

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Peru Peru, officially the Republic of Peru, is a country in western South America. It is bordered in the north by Ecuador and Colombia, in the east by Brazil, in the southeast by Bolivia, in the south by Chile, and in the south and west by the Pac ...

over Bolivia to

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

. These may or may not be associated with surface volcanic features but contain many mineral deposits. Some areas are associated with Late Miocene ignimbrites, such as Morococala and Los Frailes Plateau. A number of mineralization areas exist at Morococala in the sediment layers, many of them in the sediment layers and associated with volcanic structures.

Composition

Tuffs are gray to white and typically rich in crystals in a mostly devitrified matrix. The rocks of Morococala are dominantly

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. ...

and

rhyodacite Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid ...

. There is a north-south gradient with the larger northern part of the field being latitic and the smaller southern part rhyolitic.

Phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s include

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. In addition, the peraluminous minerals

andalusite Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétherie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the sp ...

cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present, and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-tempera ...

and

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

. Hydrothermal alteration has occurred on these rocks.

Basement

Various

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

and

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

sedimentary formations exist in the area as well. A number of subvolcanic intrusions of

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

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 ...

age are also found there.

Eruptive history

The oldest date obtained on intrusive rocks at San Pablo is 23.3 ± 0.4 million years ago. Hydrothermal alteration occurred later, 20.2 ± 0.35 million years ago. The ignimbrite was deposited much later, 6 million years ago. Three different stages of ignimbritic volcanism have been delineated by argon-argon dating. The first and oldest occurred 8.4 million years ago and formed the rhyolitic tuffs. The second 6.8 million years ago also formed a rhyolitic tuff and originated from the Condoriri caldera. The third 6.4 million years ago originated from the Tankha Tankha caldera.

References

Sources