Technical University of Clausthal
Institute of Mineralogy and Mineral Resources

Adolph-Roemer-Str. 2A
38678 Clausthal-Zellerfeld
GERMANY

 

Andean ore deposits

Summer field trip 2000 (21 July to 7 August 2000)

 

If you want to hunt for elephants you have to go to elephant country. The Central Andes are one of the most spectacular metal provinces on Earth. Since the Spanish conquistadora, they are the scene of a turbulent history deeply marked by their enormous mining resources. A large part of the present-day global exploration and mine development investments is directed towards this region.  

The Central Andes provide a textbook example of an active continental margin with long-lasting subduction of the Pacific lithosphere under the South American continent. The interaction between geodynamics, geomorphology, climate, magmatism, and hydrothermal activity has produced a wide spectrum of ore deposits which reaches from copper-gold, copper-molybdenum, gold-silver and tin-silver porphyries and magnetite skarn to epithermal precious and base-metal systems and on to supergene salt, lithium, boron and nitrate deposits.  

The field trip allowed to visit a number of major and giant ore deposits such as Llallagua (largest hard-rock tin deposit of the world), Cerro Rico de Potosi (largest silver deposit and important tin porphyry system), San Cristobal (one of the largest silver mine development projects), Salar de Uyuni (largest salt lake on Earth, and largest lithium resource), Chuquicamata (largest copper porphyry system), Collahuasi (1.7 billion USD copper porphyry project under development), and salitres of the Atacama desert (nitrate mining ghost towns from the 19th century saltpetre boom).

The Central Andes also provide extreme morphology and climate contrasts, and are home of a number of advanced historic civilizations of which relics are still alive in large parts of the Bolivian highlands.

 

  1st day
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  5th day
  6th day
  7th day
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10th day
11th day
12th day
13th day
14th day
15th day
16th day

  Arrival La Paz
  Chacaltaya
  La Joya
  Llallagua
  Llallagua - Potosi
  Cerro Rico de Potosi
  Potosi - Uyuni
  San Cristobal
  Salar de Uyuni
  Chuquicamata
  Calama - Collahuasi
  Collahuasi
  Iquique - Arica
  Arica - La Paz
  Lake Titicaca
  Departure La Paz

The field trip started in La Paz. The first field target was the Cerro Chacaltaya and, surprisingly, all participants made it to the summit (5400 m asl). Cerro Chacaltaya is made of Silurian clastic metasediments and hosts a small Triassic granite porphyry intrusion with a large hydrothermal halo and pervasive tourmaline alteration. Part of this halo are the Kellhuani manto tin deposits, and the Milluni tin-zinc vein system.


On the way from La Paz to Oruro we visited the Inti Raymi gold deposit, also known as Khori Khollo or La Joya. The 14 Ma-old Cerro Khori Khollo dacite stock with pervasive phyllic alteration hosts the porphyry-style gold mineralization. The Cerro Khori Khollo main pit is 180 m deep with 25 Mt @ 1.95 g/t Au + 8.25 g/t Ag (cutoff 0.95 g/t Au) of sulfide ore. Gold recovery is 60-75 % (closed-system cyanide leaching). The neighboring Cerro Llallagua ore deposit is currently in development, with a bio-oxidation plant for its highly refractory sulfide ore.

 

After an overnight stop in Oruro we went on to Huanuni and Llallagua. Llallagua, also known as Siglo XX, has produced about 0.5-1 Mt Sn from vein and porphyry mineralization with a bulk grade of around 0.35 % Sn. The hydrothermal system is centred on the 1.0 x 1.7 km large Salvadora rhyodacite stock which is emplaced within Lower Paleozoic clastic sedimentary rocks. Pervasive alteration is mainly of quartz-sericite and quartz-tourmaline style, including tourmaline breccias. 

 

The 350 km from Llallagua to Potosi make a full day of travel, passing through the Lower Paleozoic clastic sedimentary rocks of the Eastern Cordillera with inliers of Cretaceous red beds, and the Tertiary ignimbrite plateaus of the Meseta de los Frailes.  

 

The Cerro Rico de Potosi (4790 m asl) is mined for silver since about 500 years and has produced in between 30,000 to 60,000 t Ag which makes it the historically largest silver deposit in the world. Potosi was the largest town of both Americas in the late 16th/early 17th century. The Cerro Rico is a 14 Ma-old dacitic to rhyodacitic dome within clastic Ordovician country rocks and displays a hydrothermal zonation pattern from an advanced argillic silica cap on the top to quartz-sericite down to quartz-tourmaline alteration with tin mineralization in its deepest parts.

 

The San Cristobal silver prospect is set to become one of the biggest silver mines in the near future. It is situated a few hours drive southwest of Uyuni, within a partly eroded 8 Ma-old andesitic to dacitic volcano. Mineralization is both syngenetic (volcano infill) and epigenetic (stockworks, breccia zones), with dominantly quartz-illite alteration. The system is not yet fully explored and has currently proven and probable reserves of 240 Mt @ 2 oz Ag + 1.67 % Zn + 0.58 % Pb. Another 100 Mt of ore are indicated. We were lucky to have Larry Buchanan as a guide to this prospect, a dedicated explorationist who spent many years on the San Cristobal system.

 

The Salar de Uyuni is the largest salt lake on Earth (10,000 km2), and is located at 3650 m asl within the central depression of the Altiplano. The Altiplano is characterized by strong Pleistocene clima fluctuations in between arid and humid. Periodic large-scale flooding and evaporation produced widespread calcareous lake sediments (stromatolitic limestone) and salt deposits up to more than 100 m thick. The southern part of the Salar de Uyuni hosts very large resources of lithium- and boron-rich brines.  

 

The Bolivian-Chilean border station of Ollague lies within the recent volcanic arc and is surrounded by active volcanoes. Very young tectonic block movements are indicated by tilted salt lake sediments. The road track crosses the extensive volcanic arc with widespread ignimbrite plateaus and slowly descends towards the desert town of Calama (2600 m asl) and the Chuquicamata mining district.

 

The Chuquicamata copper porphyry system is part of a 14 km long NS trending zone which also hosts the Radomiro Tomic, MM (Mensa Mina), and the exotic copper oxide ore deposit of Mina Sur. The total copper resource of this local trend is >11 Gt @ 0.76 % (cutoff 0.2 % Cu). The individual porphyry systems are controlled by a regional NS trending strike slip fault (West Fissure system) which extends over more than 1000 km and also hosts the Escondida and El Salvador systems to the south, and the Collahuasi and Quebrada Blanca systems to the north. The age of this porphyry belt is 41-31 Ma.  

The Chuquicamata open pit is 2 x 3 km wide and 810 m deep. The ore tonnage mined during the last 85 years is 1.5 Gt @ 1.5 % Cu + 0.07 % Mo; reserves are 1.3 Gt @ 0.6-0.7 % Cu, including 500 Mt @ 1 % Cu, of which part will be mined in an underground operation down to 1100 m depth. The current production rate is 160,000 t ore (1.1-1.2 % Cu + 200-300 g/t Mo) plus 400,000 t waste per day.

 

 

The modern Collahuasi mine comprises two 33 Ma-old copper porphyry systems, the Uyina and Rosario quartz monzonite porphyry stocks. Total resources are about 3 Gt @ 0.82 % Cu (cutoff 0.4 % Cu). The Ujina open pit produces currently with a grade of 1.6 % Cu, after stripping of 135 Mt of barren overburden. Rosario is in development. The mine is in spectacular landscape at 4,500-5,000 m asl within the active volcanic arc. Total investment for this mining project was 1.76 billion USD (joint venture of Falconbridge, Minorco, Mitsui, and private Chilean investors).

 

The road from Arica at the Pacific coast to La Paz (4,000 m asl) provides a cross-section through the succession of magmatic arcs of the western Andes, from the relatively low-lying Jurassic coastal belt to the strongly uplifted (mainly during the last 10 Ma) extensive Tertiary ignimbrite sequences into the present-day volcanic arc at the Chile-Bolivia border.

 

 

The field trip ended with a visit of Tiahuanacu (pre-Inca civilization) and a trout lunch at the Lake Titicaca.

 

Contact for additional information:

Professor Bernd Lehmann
Phone  +49-5323-722776
E-mail:  Lehmann@min.tu-clausthal.de