Porco caldera is a

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...

which contains a major deposit of lead, silver, tin, and zinc ore. This caldera was formed by a volcanic eruption 12 million years ago in the Bolivian Cordillera Oriental. Subsequent to the caldera collapse, volcanic intrusions occurred as many as 3 million years after the caldera formation. Ore deposits form veins within the caldera infill. These ores were mined for silver as early as the Inca period; more recently, Porco is the site of zinc mining.

Geography and geomorphology

Porco lies in southwestern Bolivia in the Cordillera Oriental, close to the towns of Agua de Castilla and Porco. The jagged, wind-swept landscape lies at an elevation of . Porco consists of a

with a diameter of . The caldera formed along a ring fault and was enlarged by subsequent

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...

s of its oversteepened margins into a bowl-shaped depression. The rims of the caldera are particularly conspicuous on its northern side and form a recognizable

scarp Scarp may refer to: Landforms and geology * Cliff, a significant vertical, or near vertical, rock exposure * Escarpment, a steep slope or long rock that occurs from erosion or faulting and separates two relatively level areas of differing elevatio ...

on the northeastern side. The caldera contains two hills in its southern part, high Cerro Apo Porco and high Huayna Porco. Both of these hills are intrusive

stocks Stocks are feet restraining devices that were used as a form of corporal punishment and public humiliation. The use of stocks is seen as early as Ancient Greece, where they are described as being in use in Solon's law code. The law describing ...

; other smaller intrusions are found elsewhere in the caldera and some may be exposed feeder dikes of now eroded

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 on ...

s. The

basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...

of the caldera is formed by phyllite rocks of Ordovician age, which in turn are overlaid by the Cretaceous Toro Toro

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

, which consists of sandstone. Porco is found at the southern margin of the Los Frailes volcanic field, a large ignimbrite field that extends from Potosi in the east as much as to the Altiplano in the west. This ignimbrite field was erupted between 6–9 million years ago from a number of

vents Vent or vents may refer to: Science and technology Biology *Vent, the cloaca region of an animal * Vent DNA polymerase, a thermostable DNA polymerase Geology * Hydrothermal vent, a fissure in a planet's surface from which geothermally heated wa ...

and consists of dacite and

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

ash-flow tuffs. Porco was at first considered to be part of the Los Frailes field, but in 1964 it was found to be a distinct volcanic system. The Porco tuff is of dacitic composition. It contains abundant crystals and phenocrysts of

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 alumino ...

, plagioclase, quartz, and sanidine. Close to the Apo Porco and Huayna Porco hills, the tuff has been altered to varying degrees, forming

propylite Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

and sericite. A number of individual veins of ore rock cross-cut the Porco tuff and have names such as Cerro Milagro vein system, Larga vein, Panfilo vein, and San Antonio vein. Ore minerals include acanthite, arsenopyrite,

cassiterite Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem. Cassiterite was the chief tin ore throughout ancient history and remains t ...

, cerussite,

galena Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver. Galena is one of the most abundant and widely distributed sulfide minerals. It cryst ...

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydroxid ...

, pyragyrite, pyrite, semseyite,

sphalerite Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in Sedimentary exhalative deposits, sedimen ...

stannite Stannite is a mineral, a sulfide of copper, iron, and tin, in the category of thiostannates. Background The chemical formula Cu2 Fe Sn S4. Zinc commonly occurs with the iron and trace germanium may be present. Stannite is used as an ore of tin, ...

, and stephanite. The climate is cold and dry, with temperatures below occurring in ten months. Precipitation is low, amounting to about .

Eruption history

Twelve million years ago, the Porco Tuff was erupted from the volcano, causing the formation of the Porco caldera. This welded tuff accumulated to thicknesses of more than inside the caldera and extended outside of the caldera, where it crops out in patches outside of its western margin. Material from the pre-caldera basement consisting of Cretaceous sandstone breccia was embedded in the tuff on the southern and eastern side of the caldera. Porco is also considered to be the source of the Cebadillas ignimbrite. The Huayna Porco hill formed about the same time as the caldera, 12.1 ± 0.4 million years ago, while Apo Porco is more recent, of 8.6 ± 0.3 million years. The formation of these hills coincided with hydrothermal alteration of the Porco tuff contained in the caldera; the 3 million year gap between the Porco tuff and the Apo Porco intrusion suggests that the intrusion was a distinct magmatic event, probably part of the Los Frailes volcanism.

Mining

Silver mining at Porco occurred already during pre-Columbian times. For example, the silver in the Coricancha temple of Cuzco originated at Porco. In fact, Porco was the major silver source for the Inca, and consequently their government heavily intervened in the region. Even after the Inca, Porco was a major source of silver for the Spanish. Later additional ores of lead, tin, and zinc were mined at Porco as well, and the mine became Bolivia's largest source of zinc. , of ore reserves existed at Porco, this ore containing 1.5% lead, silver, and 15% zinc. Gold also occurs at Porco, but not in quantities that would justify mining it.

References

Sources

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External links

* * {{Commons category-inline, Porco (caldera) Calderas of Bolivia Mining in Bolivia Miocene calderas