University of Clausthal
Institute of Mineralogy and Mineral Resources
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.
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.
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
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
The field trip ended
with a visit of Tiahuanacu (pre-Inca civilization) and a trout lunch
at the Lake Titicaca.
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