BOOK: METALLIFEROUS ORE DEPOSITS OF BOLIVIA (Second Edition)Bolivia covers an area of 1,098,581 km2, which is highly endowed with resources of silver, tin, gold, lead, zinc, copper, bismuth, antimony, tungsten, platinum, palladium, tantalum, chromium, nickel, cadmium and indium. Diverse deposits that host these resources are mainly distributed throughout the Andean Orogen and areas underlain by Precambrian basement rocks, which together comprise about 60% of Bolivia (Arce-Burgoa, 2009; Fig.1).
1.0 Synthesis of the Geology of Bolivia
The geologic framework of Bolivia can be divided into six physiographic and tectonic provinces. From east to west, these provinces are the Bolivian Precambrian, the Chaco-Beni Plains, the Subandean, the Eastern Cordillera, the Altiplano, the Western Cordillera. The latter four provinces comprise the Andean Orogen in Bolivia (Fig. 2).
1.1 The Bolivian Precambrian Shield
The Bolivian Precambrian Shield (or Bolivian Precambrian), located in easternmost Bolivia, forms part of the Brazilian Central Shield or Cratón de Guaporé, and it is comprised of Proterozoic crystalline rocks, which may be covered by Tertiary laterites and Quaternary alluvial basins.
1.2 The Chaco-Beni Plains
The Chaco-Beni Plains are located in the central part of the country and cover 40% of Bolivia. They are comprised of alluvial sediments, all of which are below 250 m.a.s.l. in elevation. They overlie Tertiary red beds that are more than 6 km in thickness, which themselves cover the Precambrian crystalline, Paleozoic and Mesozoic rocks.
1.3 The Bolivian Andean Orogen
The Bolivian part to the Andean Orogen covers approximately 42% of Bolivia and includes the Subandean zone, the Eastern Cordillera, the Altiplano and the Western Cordillera. These are a series of mountain chains, isolated mountain ranges, and plains, which generally cross the country from north to south.
1.3.1 The Subandean Zone.
The Subandean Zone comprises a folded longitudinal and thrust belt that forms an active foreland basin. It is characterized by north-south trending, narrow mountain ranges with elevations between 500 and 2.000 m.a.s.l. Lithologies consist of Paleozoic siliciclastic marine sedimentary rocks and Mesozoic and Tertiary continental sedimentary rocks.
1.3.2 The Eastern Cordillera.
The mountainous Eastern Cordillera is comprised of polydeformed Ordovician to Recent sequences of shales, siltstones, limestones, sandstones, slates, and quartzites. These mainly clastic and metamorphic rocks fill a marine basin with an approximate area of 280,000 km2 .
1.3.3 The Altiplano.
The Altiplano is a continental foreland basin, approximately 850 km-long and with an average width of 130 km (approx. 110.000 km2), located at elevations between 3,600 and 4.100 m.a.s.l. Geomorphologically, the province consists of an extensive flat plain that is interrupted by isolated mountain ranges.
1.3.4 The Western Cordillera.
The Western Cordillera consists of a volcanic mountain chain that is 750 km in length and 40 km in average width. Late Jurassic and Early Cretaceous lava flows and pyroclastic rocks, and sequences of marine sandstones and siltstones dominate the Cordillera. Lesser Late Cretaceous continental sediments were deposited above the marine rocks and at the same time, large granitoid plutons were emplaced along the coast of Peru and Chile (Ritcher et al., 1992).
2.0 Structural Geology and Stratigraphy of Bolivia
2.1 Structural Geology
The Proterozoic structural evolution of the Bolivia is closely related to deformational events during the San Ignacio, Sunsás, and Brasiliano Orogenies. The main structures are shear zones that may be more than 2 km in width, and also faults, folds, anticlines, synclines and saddle reefs that are associated with the mineral occurrences in the Bolivian Precambrian (Arce-Burgoa, 2007).
The main Phanerozoic, deformation began in the Devonian and continued until the Triassic (Hercynian Phase; Bard et al., 1974 and Martinez, 1980), and was characterized by local uplifting and folding (Sempere, 1995). Subsequent mid to upper Oligocene, intense deformations was contemporaneous with formation of the Andes. The main tectono-magmatic events took place mainly between 27 and 19 Ma and 11 and 5 Ma. Other minor deformation occurred at 42 Ma, from 17-15 Ma, and at 2 Ma. Late Oligocene to Recent deformation during Andean orogenesis in Bolivia including thrusting to the east and the northeast of allochthons of Paleozoic shales.
2.2 Stratigraphy (Table 1)
Table 1 Division of the Bolivian stratigraphy (modif. from Sempere, 1995).
Most rocks older than Middle Triassic are of marine origin, unlike those of the Middle Triassic to Recent, which were mainly deposited in a continental environment.
The magmatic evolution of Bolivia began in the Mesoproterozoic-Neoproterozoic, and it is closely related to the San Ignacio, Sunsás and Brasiliana Orogenies that range between 1800 and 570 Ma. During the Middle and Late Ordovician, magmatism took place only locally in the Eastern Cordillera. Permian to Late Eocene magmatism is concentrated north of Lake Titicaca, from Illampu peak to the Zongo sector northeast of La Paz. Granitoids are products of both extensional and subduction-related tectonism(Tist, 1985; Schneider, 1990; Arce-Burgoa et. al, 2000).During the Cretaceous, magmatism in the northern part of the Eastern Cordillera included emplacement of the Huato granodiorite and of the Charazani Igneous Complex. Younger rhyolite to rhyodacite lavas, pyroclastic rocks, and rhyolite domes in the same complex have ages of 56.8 ± 1.5 Ma and 47.1 ± 1.0 Ma; (Argandoña, 2006). Extensive Oligocene magmatism took place in the southern central part of Bolivia (Kari Kari area). Late Oligocene alkaline volcanism occurred in the Altiplano and Western Cordillera. Middle Miocene magmatic arc development took place throughout the Western Cordillera, Altiplano, and Eastern Cordillera. During the Early Miocene, the formation of the resurgent Kari Kari caldera was completed. Extensive rhyolite domes correspond to a magmatic event from 4-1 Ma and finally the Nuevo Mundo Province is < 1 Ma (Schneider, 1987).
4.0 Types of Metalliferous Ore Deposits in Bolivia
In Bolivia, more than 15 types of metallic mineral deposits have been recognized (according the following list), which demonstrates the great variety of potential metallic resources in what could be the most prolific geological and metallogenic environment of the continent, particularly with such favorable environments as the Central Andes and Guaporé Craton (Bolivian Precambrian); Figs 1 and 2.
- “Bolivian-type” polymetallic veins
- Pluton-related polymetallic deposits
- Orogenic Au±Sb deposits
- Epithermal and transitional vein deposits.
- Red-bed copper
- Sedimentary rock-hosted zinc-lead (silver) veins
- Iron oxide-copper-gold (IOCG) deposits.
- Sedimentary exhalative (“SEDEX”) Pb-Zn deposits
- Banded Iron Formation (BIF) hosted iron ores
- Volcanogenic Massive Sulfide (VMS) Cu-Zn-Au (Pb) deposits
- Ultramafic and mafic intrusion-related Ni and PGE deposits
- Mississippi Valley type (MVT) deposits
- Placer gold deposits
Other deposit types in Bolivia (Nb, Ta, TR of Lovozero type; evaporites; precios and semiprecious stones; pegmatites; sulfur). Such a diverse deposits are mainly distributed throughout the Andean Orogen and areas underlain by Precambrian basement rocks, which demonstrate the geological and metallogenic favorability of the Bolivian territory (Arce-Burgoa, 2009).
5.0 General Metallogeny
The metallogeny of Bolivia is closely related to magmatic activity, particularly to the emplacement of intrusive bodies, both plutonic and subvolcanic. The first occur mainly in Precambrian rocks and in the northern part of the Eastern Cordillera, and the latter in the central and southern parts of the Eastern Cordillera. Bolivia contains parts of three important South American metallogenic provinces (Fig. 1):
- The Precambrian Shield (Guaporé Craton),
- The Chaco-Beni Plains (Amazon and Platense Basins), and
- The Central Andes.
5.1 The Precambrian Shield
The three Proterozoic orogenic cycles in Bolivia: San Ignacio (ca. 1800-1280 Ma), Sunsás (ca. 1280-950 Ma), and Brasiliano (ca. 950-570 Ma) are related with the main metallogenic epochs. (Litherland et.al., 1986; Heuchmidt & Miranda, 1995), which young from north to south:
- The Paraguá Craton Au-Mn Belt,
- The Sunsás Polymetallic Belt, and,
- The Mutún-Tucavaca Fe-Mn Belt
5.2 The Chaco Beni Plains
The northern part of the Chaco-Beni Plains is crossed by the great Madera, Madre de Dios, Beni, and Mamoré Rivers and by its numerous tributries. Several of these host significant amounts of placer gold, mainly the Madera and Madre de Dios river basins, and, to a lesser degree, the Madidi, Undumo, Tejeque, and other tributaries of the Beni River. These rivers define what has been termed the Amazon Gold Basin, which enclose important gold placers and paleoplacers (Heuschmidt et al., 2000).
5.3 The Central Andes
The Bolivian Orogen can be divided into four metallogenic belts:
- The Polymetallic Belt of the Altiplano and Western Cordillera
- The Tin Belt
- The Gold-Antimony Belt
- The Lead-Zinc belt
6.0 Metalliferous Resources of Bolivia
Bolivia hosts more than 3,000 metalliferous deposits and occurrences, which are distributed in all its lithotectonic units. Historically, the Bolivian metalliferous production has been mainly polymetallic (of high grades) of Sn, Ag, Zn (+/-Sb, W and Pb), with a clear predominance of Ag-Sn-Zn; mainly from well-known tin and/or polymetallic deposits such as Cerro Rico de Potosí, Llallagua, Huanuni, and Bolivar. With the exception of Llallagua, these other important deposits still contain approximately half of their resources, despite a long period of exploitation.In Bolivia during the last twenty five years, important large tonnages and lower grade targets have been identified for Au, Ag, Zn, Cu, Pb, Pt, PS, Ta, Nb, Ni, Cd, In, Li, and B. These are amenable to "open pit" exploitation at deposits such as Puquio Norte, Don Mario, San Simón, Miguela, and Rincón del Tigre in the Precambrian; Kori Kollo, San Cristóbal, Iroco, and Vinto in the Altiplano; and San Bartolomé, San Bernardino, and Amayapampa-Capacirca in the Eastern Cordillera. In 1990, the first open-pit mine in Bolivia began production at Kori Kollo (Au-Ag). At that time, it became the largest gold producer in South America. All those discoveries incorporate the following reserves: 530 million tonnes with 13 million ounces of Au, 740 million ounces of Ag, 4.1 million tonnes of Zn, 1.4 million tonnes of Pb, and 0.14 million tonnes of Cu.Additionally, there are approximately 250,000 km2 of unexplored (~ 25% of the national territory) and 200.000 km2 of under-explored (~ 20% of the national territory) ground in Bolivia, which are geologically favorable for the occurrence of additional resources of a great variety of metals (Arce-Burgoa, 2007; 2009).