Andean
Metallogenesis: A Synoptical Review and Interpretation (*)
(*) In: CORDANI, U.G. / MILANI, E.J. / THOMAS
FILHO, A. / CAMPOS, D.A. TECTONIC EVOLUTION OF SOUTH AMERICA, P. 725-753 / RIO
DE JANEIRO, 2000
Abstract- The paper presents an introductory view of the Andean
belt and their mineral deposits, followed by a general description of each of
the principal Andean metallic provinces: the iron, copper, gold-silver,
pollymetallic and tin belts. Finally, the segmentation, zoning and
metallogenetic evolution of the Andean belt is described and discussed.
Although a major part of the Andean ore deposits are
related to magmatic activity, and calc-alkaline magmas are dominant, at least
the larger deposits of the belt are related to short-lived disruptions in the
normal tectonic regime and in the mechanisms of magma generation and
emplacement. Both changes in rate and angle of convergence of the tectonic
plates are key factors for explaining such disruptions though the deep
structure of the continental lithospheric plate seems also important.
Most of the larger ore deposits of the Andean belt
have a Tertiary age and are in the central part of the
Resumen- La presente contribución entrega una visión
introductoria de la cadena andina y sus yacimientos minerales, seguida por una
descripción general de cada una de sus principales provincias metálicas: las
fajas ferríferas, cupríferas, de metales preciosos, polimetálica y estañífera.
Finalmente, se describen y discuten la segmentación, la zonificación metálica
transversal y la evolución metalogenética de la cadena andina.
Aunque una parte principal de los yacimientos metalíferos
andinos se relaciona directa o indirectamente a la actividad magmática y el
magmatismo calcoalcalino ha sido dominante, al menos los principales
yacimientos del orógeno se relacionan con trastornos del régimen tectónico y de
los mecanismos de generación y emplazamiento de magmas. Tales trastornos han
sido producidos por rápidos cambios en la velocidad de convergencia de las
placas tectónicas oceánica y continental, así como por modificaciones del
ángulo de convergencia, aunque probablemente también la geometría de la corteza
continental profunda ha tenido un rol significativo.
La mayoría de los grandes yacimientos metalíferos andinos
tiene edad terciaria y se encuentra en la parte central del orógeno (10º S a
35º S) donde su corteza continental es más profunda. Ello se interpreta en
términos del mayor grado de evolución orogénica de ese segmento andino durante
el lapso Mesozoico-Cenozoico. La relación antes señalada tiene un paralelo en
la evolución magmática-metalogénica de los arcos de islas, donde tanto la
producción de yacimientos de distinta tipología como la magnitud que ellos
alcanzan crecen junto con el desarrollo de una corteza diorítico-tonalítica.
Una posible explicación de esta analogía radica en las mayores oportunidades de
interacción entre magmas, materiales sólidos y fluidos (desde la astenósfera
hasta los niveles sedimentarios corticales) que ofrece la creciente complejidad
del orógeno.
Introduction: The Andean Belt and its Mineral Deposits
In geological terms, the Andean belt has a particular
importance as a model for the evolution of magmatic arcs developed over close
to the continental crust, on an active, plate consuming, convergence border.
Although the magnetic anomalies of the oceanic floor permit to follow the
convergence history of the margin only as far back as the Cretaceous, there are
geological evidence of plate tectonic activity in the Andean domain during
Paleozoic times. In consequence, the geological evolution of the
The Andean belt is a complex orogenic system, that has
its maximum wide (near 800 km) around 18º S and comprehends several
cordilleras, sierras, plateaux, basins and valleys. Three well defined
different cordilleras and one sierra are distinguished in
The present Andean cordilleras lift up over the
western and north-western border of the South American tectonic plate and face
four other tectonic plates, three of them of oceanic type: The Nazca, Cocos and
The continental crust has different thickness along
the belt, attaining a maximum of 70 km under the Principal Cordillera, between
14º S and 22º S, a figure close to that of the continental crust under the
The presence of major longitudinal and transversal
faults is an important trait of the Andean geology. The first ones have
controled the vertical displacement of the longitudinal tectonic blocks, as
well as the magmatic emplacement and the distribution of ore deposits. Several
of these faults, as those of Romeral (
The Andean belt presents hundreds of strato volcanoes
and many of them are important heights of the Belt. They are distributed in
three main active segments: 5º N - 2º S (andesitic-basaltic), 16º S - 28º S
(andesitic) and 37º S - 46º S (andesitic-basaltic). Only five strato-volcanoes
are known in a more southern position (48º S - 56º S), and their composition
are andesitic. The principal volcanic segment: 16º S - 28º S, also presents
around 150.000 Km2 of Miocene-Pliocene rhyo-dacitic ignimbrites;
some of the flows being linked to very large calderas (up to 30 km in diameter,
Francis and Baker, 1978). Some of the Andean volcanoes have supplied important
clues for understanding the genesis of the ore deposits, as in the cases of San
Fernando, Ecuador (Goossens, 1972a), and El Laco, Chile (Park, 1961).
Although some authors, as Aubouin et al. (1973) and
Zeil (1979), sustain the existence of fundamental differences between the
Paleozoic and post Paleozoic geologic development of the Andean belt, these
differences depend on the Andean segment and the period considered. Neither the
episodes of marginal basin development nor the stages of strong horizontal
compressive tectonics are exclusive traits of the Paleozoic evolution. On the
other hand, important sedimentary Paleozoic basins are characterized by
vertical tectonics. Also, calc-alkaline magmatism, so typical of the
Mesozoic-Cenozoic Andean belts, is equally abundant during the Paleozoic, and
attains a peak during the Permian. Thus, the Parmian-Triassic transition occurs
in geological continuity. Finally, Paleozoic and post-Paleozoic tectonic
directions are similar and the Paleozoic metallogenesis includes the same
metals deposited in Mesozoic and Cenozoic times, although the areal
distribution of the metallic belts is different. Porphyry copper deposits, a
main trait of the Cenozoic Andean metallogenesis, were formed in the Andean
domain at least since the Carboniferous (Sillitoe, 1977).
Nevertheless, some of the characteristics traits of
the Andean belt, e.g., the generation of large amounts of calc-alkaline
magmatism, became heightened in Mesozoic and Cenozoic times, whereas other,
like the accretion of oceanic prisms, lessened their relative importance. The
separation of South America from
Ensialic basin development was also important during
the Mesozoic and during the Cenozoic Andean evolution. However, some of these
Mesozoic basins (e.g., the Neocomian basin in central
The Andean belt exhibits the imprints of several
important compressive episodes. However, their intensity was different along
the belt. Besides, strong folding was attained only on the miogeosynclinal
facies between the western volcanics and the eastern continental terrains.
Mesozoic Andean magmatism includes tholeiitic,
calc-alkaline and alkaline series. Tholeiitic series are characteristic of the
accreted oceanic prisms of the
Regarding the older ore deposits in the Andean domain,
the only ones that have a possible Precambrian age are some Ni and Cr ores in
ultrabasic rocks of the Eastern Cordillera of Perú, as well as some Ni-Cr
deposits in ultrabasic rocks, Cu-Fe deposits in amphibolites and W deposits in
granulites of the Pampean Ranges of Argentina, which have minor economic
importance (Di Marco and Mutti, 1996; Stoll, 1975).
Though Paleozoic and post-Paleozoic Andean ore deposits
contain basically the same metals, there are some differences regarding the
type of deposits (e.g., there are not post-Paleozoic BIF’s). However, the main
difference concerns the huge amounts of ores formed after the Paleozoic,
specially in the Central and
The post-Paleozoic metallic provinces appear as 50 to
300 km wide belts, elongated parallel to the
The
The present exposition will now describe the different
metallic provinces of the
Metallic Provinces
in the
The iron belt
The iron ore deposits of the Andean domain (Fig. 1) may be grouped in four types: BIF type deposits of
the Nahuelbuta belt (
The BIF-type iron ores of Nahuelbuta are emplaced in
high-pressure metamorphic rocks (pelitic schists, cherts and greenschists) that
have a Lower Carboniferous metamorphic age and belong to an accreted terrain
(Aguirre et al., 1972). The oceanic volcano-sedimentary prisms contains, in
addition to the magnetite ores, some chromite podiform deposits and also some
pyritic Cu-Zn massive sulfide bodies. The principal iron mineralization, that
is interbeded with micaschists, crops out in three main areas, situated between
38º05’ S and 38º30 ‘ S, close to 73º15’ W. Ore reserves are about 100 M.t.,
containing 30% Fe (Oyarzun et al., 1984).
The oolithic iron deposits are found in northwest
The oolithic iron deposits of
The Kiruna-type iron deposits of north
Hydrothermal alteration is widespread and complex.
However, actinolite, partly altered to chlorite, is dominant, followed by
silicification and rock bleaching. Isotopic (K-Ar) dating of the iron deposits
are between 128 Ma (Boquerón Chañar, Zentilli, 1974) and 110 Ma (Los Colorados,
Pichón, 1981, and El Romeral, Munizaga et al., 1985). Several age
determinations at El Algarrobo (Montecinos, 1983) are also in the 128-111 Ma
span, which is coincident with the climax of the mafic magmatism, but also with
the passage from the "Mariana" to the "Chilean" style of
oceanic plate subduction (Sillitoe, 1991).
The iron belt also include smaller iron vein-type
deposits as well as a few iron skarns, like Bandurrias, and some
chalcopyrite-magnetite skarn ores, like San Cristobal, that have been mined for
their copper content.
Concerning the origin of the main iron ore deposits of
the belt, pneumatolytic-hidrothermal fluids were considered as a satisfactory
depositional mechanism by Ruiz et al. (1965), Bookstrom (1977), Oyarzún and
Frutos (1984) and other authors, although there are differences concerning the
source of the fluids. However, Nystrom and Henríquez (1994) and Travisany et
al. (1995), have recently proposed that these deposits were formed at a
magmatic stage and later overprinted by hydrothermal fluids.
The iron deposits of the coastalt belt of Perú (Soler
et al, 1986; Cardozo and Cedillo, 1990) are similar in mineralogy to the
Cretaceous deposits of north
The iron-copper skarns deposits of the
Andahuaylas-Yauri zone in Perú are located along a WNW trending belt between
13º30’ S - 14º30’ S and 71º39’ W - 73º39’ W. The deposits are associated to
quartz monzonite stocks dated at 34-33 Ma, that intrude carbonatic sediments
dated as Albian-Turonian (Noble et al, 1984; Soler et al, 1986). The ores
include magnetite with some native gold as early minerals, and chalcopyrite as
a later sulfide phase. According to Bellido and
The El Laco Kiruna-type iron ore deposits, are made up
of several flow-like and subvolcanic intrusive magnetite bodies with the same
mineralogy, that also includes minor apatite. These bodies crop out across a
surface of 1,8 km2 around a Pliocene volcanic center of north
The copper province
Copper deposits are present from the northern to the
southern ends of the Andean belt, and their ages cover the Upper Paleozoic to
Pleistocene span. The deposits belong to a variety of types, among them
porphyry copper, enargitic vein and replacement, skarn, breccia pipe,
manto-type, massive sulfide, exotic etc. In those deposits, copper is
associated to a number of metals, like Mo, Fe, Au, Ag, Zn and Pb. In the
following paragraphs, the principals traits for each deposit type in the
Porphyry copper deposits are also present along the
whole andean belt (Fig. 9), where
they attain world’s marks, both in tonnage and grade. Besides, some of them, as
Sillitoe (1988), considers six epochs of porphyry
copper mineralization in the Chilean-Argentinean sector of the
Most porphyry copper deposits in the
Porphyry copper deposits present both spacial and
chrological clusters in the Andean belt. Thus, the
As pointed out before, many important porphyry copper
deposits in the
The Andean porphyry copper deposits have Mo contents
that range between 0.01% and 0.1% and this metal follows copper in economic
importance. Given the large tonnages of porphyries like Chuquicamata and El
Teniente, they also rank among the major Mo deposits of the world (Ambrus,
1978). In exchange, gold content are rather low, with the important exception
of the Farallon Negro district in Argentina, where Bajo de la Alumbrera attains
780 M.t. ore, containing 0.52% Cu and 0.67 g/t Au (Sasso and Clark, 1998).
Although the enargitic vein and replacement Cu +/- Au,
Ag, Zn, Pb deposits are better represented in Perú, they are also common in
other zones of the Tertiary volcanic belts of the
The Peruvian territory is also richely endowed in Cu
+/- Fe, Au, Zn deposits related to calcic skarns, partly as a consequence of
the broad distribution of Mesozoic back-arc carbonatic rocks, that host
Tertiary monzonitic granitoids (Fig. 7). As
mentioned before, some skarns deposits of the Andahuylas-Yauri zone are also
important for their magnetite content. Among the major skarn deposits in Perú,
stand out Antamina, Cobriza, Ferrobamba and Tintaya (Petersen and Vidal, 1996).
A second type of skarn, the amphibolitic Cu +/- Fe skarns deposits (Vidal et
al, 1990) is represented in Perú by Raul-Condestable and in
Breccia pipe ore deposits are widespread in the
Manto-type copper deposits are typically found in
volcano-sedimentary formations of Mesozoic age in north and central
Massive sulfide deposits are not abundant in the
Andean belt, although the accreted oceanic prisms of the
Favorable climatic and tectonic conditions for the
formation of exotic Cu deposits, existed in the Andes of south Perú and north
Chile between 12º S and 27º S (Munchmayer, 1996). In
Copper vein deposits are widespread, in the Andean belt
and it is difficult to present a synthesis of this subject. However, it is
important to state that Cu mining in the
Gold and silver metallic belts
Gold and silver were main lures for the Spanish
conquerors in the Andean countries, and their hidden deposits, together with
those of copper, are today the first target for the mining exploration
companies.
In the northern Andes,
Gold mining began in Colonial times in
A general view of gold deposits in Perú was presented
by Noble and Vidal (1994). This country has a long and important history as a
gold and silver producer, that began in pre-Hispanic times. Noble and Vidal
(1994), classify the Peruvian gold deposits (Fig. 5) in the following groups: 1-
Quartz veins of Paleozoic and Mesozoic age: a) Pataz-Buldibuyo belt
(Pataz, Parcoy, etc.); b) Santo Domingo-Ananea region (Ananea, Santo Domingo,
etc.); c) Nazca-Ocoña belt (Calpa, Ishihuinca). 2-
Gold bearing systems of Cenozoic age: a)Au-bearing porphyry and
skarn deposits (Michiquillay, Tintaya, etc.); b) Sedimentary rock-hosted gold
(Yauricocha, Utupara, etc.); Polymetallic and precious metal deposits,
subdivided in: -Polymetallic systems (Quiruvila, Sayapullo, etc). -Epithermal
deposits of the adularia-sericite type Ag-Au vein systems (Cailloma, Arcata,
etc.) and of high-level, acid-sulfate systems (Yanacona, Ccarhuaraso, etc.). At
julcani, the acid-sulfate stage was developed between two stages of
adularia-sericite alteration. 3- Bulk mineable
ores (Yanacocha, Hualgayoc). 4-
Quaternary placer deposits.
Although Perú ranks third in present gold production
among the Andean countries (after Chile and Colombia), this situation should
soon be changed, due to a number of important mining projects, such as the
Pierina mine by Barrick, near Ancash, programmed for a production of 22 t
Au/year (equivalent to total gold production of Perú in 1993).
Silver is also an abundant metal in many hydrothermal
deposits in the volcanic rocks of the Western Cordillera of Perú, appearing in
independent primary (argentite, proustite, etc.) or secondary (native Ag,
acantite, etc.) minerals, as well as in inclusions of silver minerals or soild
solutions in galena and Cu sulfominerals (tetrahedrite, etc.). In exchange, Ag
is commonly found only in solid solutions or inclusions in galena and
sulfominerals in the deposits hosted by sedimentery rocks in the western and
eastern cordilleras (Bellido and Montreuil, 1972). Among the principal Ag-rich
deposits are Quiruvilca (polymetallic; Ag/Au = 100) and the ephithermal
deposits of San Juan de Lucanas: Ag/Au = 160; María Luz-Huachacolpa district:
Ag/Au = 450 and Julcani: Ag/Au = 65 (Noble and Vidal, 1994).
The Miocene sub-volcanic deposits of the central and
southern part of the Cordillera Real, west from the Altiplano region of
Although there are important Au-Ag deposits in
Gold production in
Chilean hydrothermal gold deposits are Jurassic to
Upper Miocene in age and their mineralizations are in hydrothermal breccias,
veins, stockworks and disseminations (Sillitoe, 1991). Although most of the Au
+/- Cu deposits correspond to Mesozoic pluton-related veins, only two
districts: Los Mantos de Punitaqui and El Bronce (Fig. 5) had Au content over 10 t. The rest of the deposits
over 10 t Au were classified by Sillitoe (1991) in four types: 1-High sulfidation, epithermal (Choquelimpie, Guanaco, El Hueso,
Of those deposits containing more than 10 t Au listed
before, only six deposits have Ag/Au ratios over 10 (Choquelimpie, Faride,
A review of precious and base metal deposits in
Argentina by Gemuts et al. (1996) mentions the Paramillos, (Mendoza) silver
deposit and the Gualilán gold deposit as the older mines in Argentina (Gualilán
dates from the 17th century). Modern exploration pre-1960 was
centered in high-grade precious and base metal deposits such as Mina Angela
(Ag-Pb-Zn-Au vein), Farallón Negro (Mn-Ag-Au vein) and El Aguilar, a sedex
massive sulfide deposit in the
The polymetallic province
The polymetallic province (Fig. 11) is present along all the Andean belt, although their
principal deposits are located in the Peruvian segment, wich also present thick
and widespread carbonatic sedimentary strata. Besides, though Paleozoic deposits
are known, some of them important like the Zn-Pb-Cu deposit of Los Bailadores,
in
El Aguilar (23º13’ S / 65º42’ W), a Pb-Zn-Ag sedex
deposit in Ordovician quartzites, represents the largest Paleozoic Pb-Zn
concentration in South America (Sureda and Martin, 1990), with some 30 M.t. ore
(12% Pb+Zn; 100 g/t Ag). The fact that a Cretaceus plutonic intrusion thermally
modified the original deposit and some skarn-type ore bodies were formed,
obscured the genesis of the deposit, now well established as a sedex
mineralization. Other Pb-Zn-Ba ores in Ordovician clastics sediments are those
of Pumahuasi (22º17’ S / 65º33’ W). They are part of a belt that continues for
some 500 km north, to the
Although Mesozoic and Cenozoic polymetallic deposits
are present in the Northern Andes (
During the Upper Triassic, the sea advanced from the
north, and reached 13º S (Audebaud et al., 1973), covering the Pucará basin
domain, a NW trending band between 76º W-77º W at 9º S and 72º W-74º W at 14º
S, where clastic and carbonatic sediments were deposited. Westward, the basin
also received andesitic lavas. The marine sedimentation continued during the
Lias, when the basin was divided in two sectors (north and south). These
sectors were united in the Dogger and separated again during the Malm by a
major NW trending positive block. During the Malm and the Lower Cretaceous,
marine sedimentation continued -in association to andesitic volcanics- only in
the southwestern basin. However, a new marine transgression during the Albian
-the sea coming this time from the south- covered the zone of the present
western and Eastern cordilleras of Perú, and the sea remained there until the
Upper Cretaceous (Senonian). Thus, paleogeographic conditions were favorable
for the deposit of carbonatic rocks on the Peruvian territory. In exchange,
contemporary basins on the Bolivian territory received only clastics sediments,
except for some carbonates of Campanian-Maastrichtian age (Pareja et al.,
1978).
Rich stratiform polymetallic deposits, with very high
Zn grades, are found in the sedimentary rocks of the Triassic--Liassic platform
of the Pucará basin (Amstutz and Fontboté, 1987; Cardozo and Cedillo, 1990).
They are, in part, of the Mississippi Valley type, such as San Vicente, located
in the eastern facies of the basin, and Shalipayko, in the western part, which
also includes some deposits that present volcanic influence, e.g., Carahuacra,
San Vicente, that has been the larger Zn producer of Perú is in sedimentary
rocks of tidal flats, lagoon and carbonatic reef facies. The Cercapuquio Pb-Zn
stratiform deposit in central Perú (Cedillo, 1990), hosted by lagoonal
sediments of Upper Jurassic age, also exhibits strong semilarities to
About 80 stratabound Zn-Pb (Ag-Cu) ore deposits and
prospects are known in the Valanginian to Aptian Santa Formation, deposited in
an ephemeral basin (Cardozo and Cedillo, 1990). Among the principal deposits
are Huanzala (Fig. 7) and El Extraño
(9º09’ S / 78º05’ W). Several traits of these ore deposits indicate a
syn-diagenetic origin, e.g., the presence of rhytmites involving the ore minerals
(Samaniego, 1980). However, there are also evidences of hydrothermal activity
and contact metamorphism affected the deposits.
The stratabound ore deposits of the Casma Formation
(Middle Albian) are rich in sphalerite and barite and have minor Cu, Pb and Ag
contents. The principal deposits of this group, Leonila Graciela (Vidal, 1987),
in 11º51’ S / 76º37’ W, is hosted by altered volcano-sedimentary rocks.
Lead-zinc (silver) stratabound deposits are hosted by
Upper Cretaceous carbonate rocks in Hualgayoc (Fig. 7), Western Cordillera of northern Perú (Cardoso and
Cedillo, 1990). Many of the deposits are in the Chulec Formation (e.g.
In northwest
The major enargitic stratabound Cu-Pb-Zn-Ag deposit of
Colquijirca (Fig. 7) some
Most of the hydrothermal polymetallic deposits in Perú
(Soler et al., 1986; Cardozo and Cedillo, 1990) are associated to subvolcanic
intrusive of Miocene age in the northern and central part of the country.
Although it is possible that some of the deposits considered as Miocene, such
as Uchucchacua are Late Eocene-Early Oligocene in age (Soler and Bonhomme,
1988, cited by Cardozo and Cedillo, 1990), the Miocene remains as a principal
metallogenical period for this and other types of ore deposits. Cardozo and
Cedillo (1990) classify the hydrothermal polymetallic deposits of Miocene age
in five groups: 1- Complex deposits, including
both replacement and veins. They are normally zoned and rich in
Cu-As sulfosalts. Cerro de Pasco, Huarón, Morococha etc, are included in this
group. 2- Skarn bodies, some of them
associated with veins, like
The Miocene belt of polymetallic ore deposits in
A further southward extension of the Miocene
polymetallic belt is represented by Pb-Zn-Ag (Cu, Bi) veins in northwest
In the Patagonian Cordillera of Argentina and
At least in the case of the Chilean polymetallic
deposits of the Patagonian Cordillera, it is possible that they belong to
different ages of mineralization although these ages remain uncertain. Thus,
Pb-Zn-Ag-(Cu) deposits occur between 46º00’ S and 47º20’ S, hosted by Paleozoic
metamorphic rocks (phyllites and marbles of marin origin) intruded by
post-Paleozoic granitoids (Ruiz and Peebles, 1988; Schneider and Toloza, 1990).
The main deposit, Mina Silva (46º33’ S / 72º24’ W) is made up of high grade
Pb-Zn (Ag) ores, with minor copper contents, that form lenticular bodies hosted
by metamorphic limestone. Although Ruiz and Peebles (1988) interpreted the
deposit as a Paleozoic singenetic mineralization. Schneider and Toloza (1990)
argue that all ore deposits of the district (wich also include stratabound and
not-stratabound deposits in Jurassic rocks) are related to calc-alkaline
magmatism developed in a Mesozoic back-arc setting.
The other important district of this belt is El Toqui,
at 45º00’ S / 71º58’ W, described by Wellmer et al. (1983) and Wellmer and
Reeve (1990). The district, which covers some 25 km2, contains
several bodies in an Early Cretaceous formation made up of silicic volcanic
rocks and clastic and carbonatic marine sediments, intruded by quartz-bearing
porphyries. The basal volcanic unit is cross-cut by Zn-Pb-Ag veins and is
overlaid by andesitic-rhyolitic flows and clastic-carbonatic sediments, that
host the statiform sulfide ore bodies. They are localizad in three
stratigraphic levels, at the interfingered zones of carbonatic rocks with black
shales or pyroclastic horizons, and contain Zn-Pb-Cu or just Zn as principal
economic metals, while Ag is recovered as a sub-product. The larger ore body,
The tin province
Of the different Andean metallic provinces, the tin
belt presents the higher degree of definition and specification. Thus, all the
major deposits are in the Bolivian territory, along a NW to NS belt, up to
Although the principal deposits of the tin metallic
province have a Tertiary or Lower Mesozoic age and are located in the
Cordillera Real of Bolivia, tin deposits of Paleozoic ages are known in the
Argentinean territory. Also, it is possible that some minor tin deposits in the
Caraballa Cordillera of Perú, close to the Bolivian border, be related to
Permian granitoids (Clark et al., 1983).
The Argentinean Paleozoic tin deposits occur in two
areas of the Pampean Ranges (Fig. 12). Those
of the northern area are vein or greisen type; their age is Cambrian to
Silurian and their ores include cassiterite, wolframite and sulfide minerals.
The deposits of the southern area are pegmatitic and have a Cambrian to
Ordovician age (Malvicine, 1975). Their interest is more scientific than
strictly economic.
The tin belt of
The host rocks for both the igneous bodies and the tin
deposits of the whole belt are Paleozoic clastic metasedimentary rocks, that
are the products of a detritic sedimentation that began as early as the
Cambrian, in a shallow but persistent intercratonic marine basin (Zeil, 1979)
and continued till the Middle Devonian, when conditions changed from marine to
continental, but the subsidence of the basin -and the sedimentation- persisted
up to the Mesozoic. The outcrops of these monotonous series of shales and sandstones
-10 to 20 km thick- make up a major part of the present
Two types of tin deposits of Upper Triassic-Lower
Jurassic age are known. The more abundant correspond to Sn-W veins associated
to greisen-type alteration, within small batholiths (e.g., Yani, Sorata) or in
the contact metamorphic zone imprinted by the batholiths in the Paleozoic
sedimentary host rocks. The age of the batholiths emplacement is in the 257 to
150 M.a. span (Grant et al., 1980). Among the principal districts are those of
Sayaquira, Caracoles and Araca. None of them attains the magnitude of the
Tertiary Sn-Ag deposits.
The other type of Upper Triassic-Lower Jurassic tin
deposits, which is found along a NW band, north of 19º S, present stratabound
control of the ores. Although this type of tin deposit is not economical under
present tin price conditions, its origin (syngenetic or epigenetic deposit of
the ores) poses an interesting problem (Schneider and Lehmann, 1977). As stated
by Lehmann (1985, 1990), the host rocks for the stratabound tin deposits are
Lower Paleozoic metasedimentary rocks, wich are intruded by granites and
granodiorites.
Kellhuani, one of the three principal stratabound-type
tin deposits (Lehmann, 1985; 1990) is located some 15 km north of
The Tertiary tin deposits (Sillitoe et al., 1975; Grant
et al., 1976, 1980; Francis et al., 1981) are related to sub-volcanic intrusive
bodies, partly brecciated, at a high emplacement level, that cross-cut the
Paleozoic clastic formations. Grant et al. (1979), distinguished two
chronological groups. The first is formed by 26 to
The first group include such important deposits as
Llallagua, Cerro Rico and Chorolque. Although their principal economic
mineralization is vein-type, they also contain, as a whole, some 80 M.t. of
disseminated ore grading 0,3% Sn, wich is still far from attaining economic
interest, but represents an important reserve for the future. Five principal
geological-mineralogical traits are common to the deposits of this groups:
1-The mineralization is centered on small (1-2 km2) porphyric
stocks, emplaced under or whitin volcanic pipes. 2- Several pulses of intrusion
and breccification are observed. Some stocks are converted to breccia pipes. 3-
The stocks and their host rocks have suffered and intense and penetrative
feldspar-destructive hydrothermal alteration, in which sericite and tourmaline
predominate. 4- The mineralization is very complex. The main sulfides that
accompany the cassiterite are pyrite, stannite, chalcopyrite, sphalerite and
arsenopyrite. 5- The disseminated mineralization is earlier than the high-grade
vein-type one. The radiometric dating by Sr isotopy have yielded Miocene ages
like 20 Ma at Llallagua, 15-14 Ma at Cerro Rico and 17-12 Ma at Chorolque
(Grant et al., 1980).
The magmas related to tin mineralization usually have
a much differentiated petrological evolution (Lehmann, 1990). Although some
magmas related to the Bolivian tin porphyries are evolved, like at Karikari,
Potosí, where peraluminous, high initial Sr isotopic ratios (0.707-0.716)
magmas, evolved from andesite to toscanite (Grant et al., 1980), in general,
tin porphyries are associated with only moderately fractioned subvolcanic rocks
of rhyodacitic composition. However, the recent paper by Dietrich et al. (1999)
provided analytical evidence (melt inclusions data) for the origin of the
Bolivian tin porphyry magmas by mixing of high evolved silicic melts
-containing quartz phenocryts- with andesitic to basaltic melt fractions, in an
upper crustal reservoir. We will back again to this section on Andean magmas.
In the group of "non-porphyric" deposits are
included vein-type Sn mineralizations, hosted in Paleozoic clastic rocks that
are not related to outcroping intrusive bodies (except dykes). Among them are
the Colquiri (fluorite-sphalerite-cassiterite); Huanuni,
Tin-silver veins in northwest
Andean Metallogenesis
Andean magmas and ore deposits
Magmatic rocks are dominant in the Andean belt and
most ore deposits are directly or indirectly associated to magmatic activity. A
major part of the extrusive and intrusive rocks of Paleozoic to Cenozoic age
belong to the calc-alkaline series, although tholeiitic rocks are present in
the accreted oceanic prisms of the northern Andes, and both shoshonitic and
alkaline rocks are associated to the calc-alkaline series. Except for the
tholeiitic rocks, the chemical and isotopic composition of Andean igneous rocks
suggest that their magmas originated from common though variable sources and
mechanisms. This point is illustrated by the strong similarities in chemical
and isotopic composition of rocks from such differents setting and age as the
Paleozoic granitoids of the Cordillera Frontal in Argentina (87Sr/86Sr
(i) = 0.7053 - 0.7070; Caminos et al., 1979) and the Plio-Quaternary andesites
of the Central Andes (87Sr/86Sr (i) = 0.7051 - 0.7077;
Pichler and Zeil, 1972; Mc Nutt et al., 1975). The general model (López-Escobar
et al., 1977, 1979, 1995; Thorpe and Francis, 1979) considers that the Andean
magmas originate in the Upper Mantle zone between the subducted oceanic plate
and the continental crust. The model also considers the participation of melts
and fluids from the upper layers of the subducting plate, as a trigger
mechanism for partial melting in the mantle, a contibution that has been sustained
by Be-10 isotopy (Morris et al, 1985). The final composition of Andean magmas
are then explained in term of different contribution from the oceanic plate,
variable degrees of partial melting of mantle materials, different fractional
crystallization processes during the rise of magmas and possible contamination
in their passage through the continental crust. An alternative source proposed
for Andean magmas generated in zones with a thick continental crust, are the
lower crustal levels (e.g., Pichler and Zeil, 1972; Mc Kee et al., 1994). The
participation of mantle melts interacting with crust derived melts in deep
reservoir, has also been considered and sustained by Sr isotopy (e.g., Deruelle
and Moorbath, 1993, for lavas from the south-central Andes).
The incorporation of crustal -igneous and sedimentary-
materials to the magmas during its passage through the crust is well
established as a mechanism for emplacement of the Coastal Batholith of Perú
(described in the important book by Pitcher et al., eds., 1985, and considered
as a model for batholith emplacement in the
However, it is possible that crustal materials
contribute to the magma enrichment in LIL-type (e.g., K, Rb, Ba) and
incompatible (e.g., Cu, Mo, Pb) elements, by partial assimilation of crustal
materials. Thus, normal high-K and shoshonitic, intermediate to mafic, Mesozoic
volcanics rocks in central-north Chile, differ only by their K, Rb and Ba
content, non LIL-elements remaining almost constant (Oyarzún et al., 1993).
In consequence, several sources are possible to
contribute metals and metaloids to the Andean ore deposits related to magmatic
processes, and the isotopic data are relevant to assess their relative
importance.
Two elements are most relevant in terms of their
isotopic ratios to evaluate possible ore sources. They are the Pb isotopic
ratios for the metals and the S isotopic ratios for the metaloids. However, Pb
has a strong tendency to accumulate in the crust and the interpretation of their
isotopic ratios in term of sources for the ores do not necessarily apply to
other metals like Cu, Zn or
There are numerous studies on Pb isotopic ratios in
Andean igneous rocks and ore deposits. In general, they conclude that different
sources participate in variable degrees according to the tectonic settings of
the rocks and the ore deposits. Thus, Puig (1988, 1990) points out to the
relatively narrow range of Pb isotopic ratios in Andean ore deposits,
interpreted by this author in terms of reservoir mixing processes during the
Andean evolution. However, he also established some relationship between the Pb
isotopic ratios and the tectonic setting of the deposits. Thus, polymetallic
ores in volcano-sedimentary rocks of the tectonically extensional Lower
Cretaceous basin in
Regarding to 32S/34S isotopy,
the different studies are coincident in terms of the magmatic origin of sulphur
in most of the sulfide metallic deposits of the Andean belt. In the case of
porphyry copper systems, d34S in sulfide minerals is very close to
the meteoritic standard (e.g.; -3 o/oo at
Though the close relationship between magmas and
Andean ore deposits is well established, many aspects of this relation remain
poorly understood or are just begining to clarify. In the following paragraphs,
some of this aspects will be briefly considered.
Porphyry copper deposits are the best studied deposits
in the Andean belt and possibly in the world. They have low 87Sr/86Sr
(i) ratios, very low d34S indexes and, at least those of the
Eocene-Oligocene span in northern
Several studies (e.g., Baldwin and Pearce, 1982;
López-Escobar and Vergara, 1982) have intended to find some significant
relation between the chemical composition of low altered intrusive rocks
associated to porphyry copper deposits and their "productivity" in
terms of porphyric mineralization. However, no significant difference was found
regarding "non-productive" contemporary intrusive rocks. The only
exception was some smaller content of Y and Mn observed by Baldwin and Pearce
(1982) in the "productive" porphyries of the
However, the possibility that porphyry copper systems
were not related to normal calc-alkaline batholiths but rather to
magnetite-rich, mafic bodies of batholithic magnitude, was recently rise by
Behn and Camus (1997). These authors considered the presence of large ENE and
NWN magnetic anomalies that exhibit spacial coincidence with Eocene-Oligocene
porphyry copper deposits between 18º S and 27º S, in terms of mafic magmatic
reservoirs from which porphyry copper systems were possibly derived.
Although calc-alkaline magmatism has been assumed as
the source for porphyry copper systems, it is well known that the principal
mineralization is closely associated to potassium metasomatism. Skewes and
Arévalo (1997) have proposed a daring alternative interpretation to their
relationship for the case of El Teniente, where the Cu (Mo) ore is in K-rich
biotitic andesites, that host quartz dioritic and dacitic porphyties. Instead
of the traditional interpretation (that is, the andesites were hydtothermally
altered by the porphyries), they consider that the andesites represent an ore rich,
high-K, intrusive magma. Considering the chemical analysis published by Camus
(1975), these andesites, if interpreted as primary rocks, should be classified
as absarokites (shoshonitic basalt) according to the Peccerillo and
Besides, the model by Skewes and Arévalo (1997) is
close to the ore-magma concept, which has been applied in
The fact that the Tertiary igneous rocks related to
Sn-Ag mineralization in south
Finally, although most of the Andean ore deposits are
associated to magmatic activity, wich has been almost permanent in the belt,
the matallogenetic activity seem rather discontinuos and related to significant
tectonic disruptions that abruptly desplaced the magmatic belts. Therefore, favorable
conditions for mixing of different types of magmas may have occurred during
these disruptive episodes, that will be discussed in the next section.
Andean tectonics and ore deposits
Although magmatic activity provide the direct
source and mechanisms for the generation of ore deposits in the Andean belt,
tectonics controls not only the production and emplacement of magmas, but also
the channels for the ore bearing fluids. Besides, although the association
between plutonic and coeval volcanics rocks is a normal trait of the Andean
magmatism, the ratios between the volumes of intrusive and extrusives magmas
has been much variable, the volcanism being favored during the stretching
stages and the plutonism increasing with the compressive tectonic pulses.
Both the geological and the metallogenetical evolution
of the Andean belt during the Mesozoic-Cenozoic span, can be consistenly
explained in terms of the interactions of the continental and oceanic
lithospheric plates. Among the main consequences of this interaction are the
continuos production of calc-alkaline magmas, the accretion to the continent of
oceanic prisms, the development of back-arc basins, the occurance of several
orogenetic episodes, the formation of mega-fault zones and the generation of ore
deposits.
Post-Paleozoic accretion of oceanic prisms occured
during Tertiary times in the Northern Andes (
Two subduction styles have been recognized for the
tectonic evolution of the central and south central
As pointed out by Sillitoe (1988, 1991), the eastward
shifting of magmatism in the Chilean-Argentinian Andes from the Jurassic to
Miocene times, have produced several N-S ore deposits belts, coincident with
the position of the contemporaneous magmatic belt. They include porphyry copper
deposits since the Albian. Although the eastward shifting has been interpreted
in terms of a flatter angle of the subducting slab, due to an acceleration to
the convergence rate of the tectonic plates, the machanism is not completely
understood. Thus, as stated by Sasso and Clark (1998) for the Middle Miocene
stage: "The arc therefore dis not merely shift eastward (Davidson and
Mpodozis, 1991) but, within the limits of error of the 40Ar/39Ar
dating technique, instantaneously broadened in the Middle Miocene". Other
example of sudden horizontal eastward magmatic and metallogenetic displacement,
is that of the Andahuaylas-Yauri Cu-Fe skarns belt, linked by Noble et al.
(1984) to a change in the subduction geometry due to the Incaica orogeny.
As explained by Scheuber and Reutter (1992), the
stress component normal to the plate boundary produces structures of crustal
shortening or extension, while the component parallel to the plate boundary (in
case of oblique convergence) causes longitudinal wrenching.
Two important fault zones in the north Chilean Andes
are interpreted in terms of oblique subduction. They are the Atacama and the
Domeyko fault zones, to which many high tonnage ore deposits are associated (Fig. 10). The Atacama Foult Zone (AFZ) represent an older
weakness zone of the crust that was reactivated in the Early Cretaceous, as a
consequence of a N20ºE plate convergence, the oceanic Aluk plate coming from
the NNW (Pardo-Casas and Molnar, 1987). The oblique plate convergence generated
regional shearing traduced in dominant sinistral strike-slip movements, up to
several tenths of km (Bonson et al, 1997). During the Lower Cretaceous, magmas
and their derivative fluids, responsible for Kiruna-type Fe and Cu-Fe deposits
like Manto Verde, were focused into dilational sites and fault intersections at
the AFZ (Thiele and Pincheira, 1987; Bonson et al., 1997).
The Domeyko Fault Zone is also interpreted in terms of
an oblique convergence , this time the oceanic plate (Farallón) coming from the
SW with a convergence rate of 12 cm/year. This fault zone is also considered as
an early structure, along which a deep readjustment of the crust occured
(Perry,
An important wrench fault in Perú is the Huara Fault
System (Petersen and Vidal, 1996) that has a N to EN direction and occurs in
the brittle environment of the Coastal Batholith, along a Lima-Cerro de Pasco
course. Several volcanogenic massive sulfide deposits as well as important
polymetallic districts (e.g., Casapalca,
As pointed out by Maksaev and Zentilli (1988), mega
fault zones have complex relationships with both magmas and ore deposits. They
probably represent major weakness zones within the crust, that have some control
on the paths of the rising magmas. However, those magmas also contribute to the
weakness of the zone, affecting the rheological properties of the rocks. In
consequence, the wrenching process due to the parallel stress component
(Scheuber and Reutter, 1992) is enhanced. On the other hand, although most of
the stockwork-type porphyry copper deposits of the Andes (e.g., Chaucha in
Ecuador, Goosens and Hollister, 1973) are related to important faults, other
major deposits, like those of the "Arequipa lineament" (Hollister,
1974) or El Teniente (Camus, 1975), do not present evident structural controls
(although their alignement points to deep seated controls).
Thus, the genesis the major Andean deposits, although
controled by the position of the magmatic arc and favored by structures like
the wrenching faul zones, should be related to deep seated disturbances,
affecting the geometrical and physico-chemical relationships between the
subducting oceanic plate, the asthenosphere and the mantle-crust boundary. This
concept, illustrated e.g., by the Sasso and Clark (1998) model for the Middle
Miocene broading of the magmatic arc and the genesis of porphyry Cu (Au)
deposits in Argentina, may explain why the larger Andean deposits were formed
during such short "pulsative" span as those established for
Kiruna-type deposits in north Chile (Oyarzún and Frutos, 1984) and for porphyry
copper deposits along the whole Andean belt (Sillitoe, 1988).
The metallogenic zoning and evolution of the Andean
belt
Three main subjects will be discussed in this section:
the tectonic segmentation of the
As with many central subjects of Andean
metallogenesis, the implications of the tectonic segmentations of the Andes in
terms of magmatism and ore deposits were first rise by Sillitoe (1974), who
proposed 16 tectonic boundaries between Oº (Carnegie Ridge) and 44º S (Chile
Ridge). Some of these boundaries, which were proposed on the basis of main
structures, seismic and volcanic activity, main morphological units, old
terrain outcrops and the intersections with oceanic ridges, are coincident with
the longitudinal limits of the metallic belts. Thus, the tin belt is restricted
to three segments, enclosed by boundaries 5 (northern limits of the belt of
recent central
The Andean tectonic segmentation is the result of a
number of heterogeneities along the belt, which is made up of old and young
terrains and tectonic blocks. Among the formers is the Precambrian Arequipa
Massif, in SW Perú (Petford and Atherton, 1995), while the Western Cordillera
of Colombia is made up of a Cretaceous oceanic prism accreted to the continent
during Tertiary times. If one considers the heterogeneittes of the continental
crust, the geometry of the continent, the complexities in the oceanic plates
(e.g., the ridges) and the variation in speed and angle of convergence between
the plates (and their consequences in the subduction zone), longitudinal
segmentation is a natural consequence. However, the relationships between
tectonic boundaries and metallic belts is rather uncertain in terms of
cause-effect. Thus, the tin povince may be, in part, a consequence of the
thicker continental crust between boundaries 5 and 8, that could have favored
the magma mixing process proposed by Dietrich et al. (1999). In exchange, the
pause of the iron belt north of boundary 9 may be interpreted in terms of the
higher erosion degree that affect the Lower Cretaceous series, resulting in the
unroofing of the batholithic levels. In general, erosion levels have been
considered an important factor for explaining metallic belts distribution in
the
Besides erosion levels, several other factors have
been considered to explain the longitudinal discontinuities of Andean metallic
provinces (Oyarzún, 1985, 1990). Thus, Mesozoic paleogeographical conditions in
central Perú were favorable to the abundant deposition of carbonatic sediments,
a factor considering favorable for the rich development of the polymetallic
province in this country. In exchange, this province is less developed in
The presence of "metallic domains"
(Routhier, 1980), defined as volumes of the continental crust that are endowed
with a special metalliferous potential during long geological times, is neither
a good explanation for the longitudinal Andean metallic segmentation. In fact,
although Paleozoic and post-Paleozoic Andean metallic provinces are similar in
nature, their different geographical distribution is not consistent with the
concept of metallic domains. Thus, even the Sn-W belts, that have a coherent
"continental" position in all the three geological eras, present,
however, different latitudinal situations.
It is likely that the elusive answer be a combination
of factors, involving plate tectonics, magma mixing, the nature of host rocks,
regional erosion levels etc. For instance, the fact that the Andean segments
between 26º30’ S and 30º30’ S seem anomalously rich in gold, is interpreted by
Sasso and Clark (1998) in terms of an upwelling asthenosphere, a transverse
rupture in the subducting slab and a minimum contamination by shallow crustal
lithologies. Thus, both Cu and Au are considered as directly contributed by the
asthenosphere to the partial melting zone in the overlying lithospheric wedge.
Concerning the transversal zoning of the Andean belt,
the fact that modern volcanic and subvolcanic igneous rocks also present such a
zoning (with alkaline and K-rich magmas at greater distance from the present
oceanic trench, Palacios and Oyarzun, 1975). Although the same factors proposed
to explain the longitudinal segmentation have been considered for the
transversal zoning, plate tectonic has received a major atention. Thus, Sillitoe
(1972) proposed a "geostill" model based on metallic elements
provided by the subducting plate to the melting zone of the lithospheric slab,
and Oyarzún and Frutos (1974) a similar model, but based on the
"anionic" elements, like sulphur and halogens.
The distribution of the Cu and Sn metalliv provinces
at both sides of the
Although the importance of plate tectonics in terms of
Andean metallogenesis is well sustained , it is also certain that the tectonic
and magmatic evolution of some Andean segments include periods when the
subduction process was perturbed or exhibited little activity. This is the
case, e.g., of the Lower Cretaceous basin in Perú (Atherton and Webb, 1989) and
The comparison of the post-Paleozoic metallogenetical
evolution of the Andean belt with that of the island arcs, e.g., the Fidji arc,
reveals interesting similarities, specially in terms of increase in both the
number of different types of ore deposits and the magnitude attained by the
larger ones. For the case of the island arcs, this evolution is parallel to the
development of a dioritic tonalitic crust. Thus, at Fidji (Colley and
Greenbaum, 1980), this crust was developed during the Tertiary, following a
stage of tholeiitic and andesitic volcanism and compressive episode. Not only
the number and magnitude of sulfide deposiits greatly increased, but also the
number of metals involved and the number of types of metallic deposits (from
one: massif sulfides to four, including porphyry copper deposits).
Concerning the Andean belt is amazing the number of
important deposits of Tertiary age, as well as their distribution in or around
the central part of the Andes (10º S to 35º S), where the continental crust
attained its maximum thickness. That is the case for all the metallic
provinces, except for the iron belt (though the important Pliocene magnetite
deposit of El Laco is in the high
In metallogenic terms, an evolved crust implies a
higher degree of structural complexity, better opportunities for magma mixing,
contributions from sedimentary strata with different chemical compositions etc.
Also, a number of geological levels, from the asthenosphere to the sedimentary
strata may participate in the generation and differentiation of magmas and in
the genesis of the ore deposits resulting of their emplacement and interactions
with the host rocks and fluids in the upper levels of the crust.
Acknowledgments-The present contribution has a far background in a
doctoral thesis presented at
I also acknowledge the kind invitation from Dr. C.
Schobbenhaus and from the editors Profs. T. Filho and J. Milani to participate
in this important publication, and to the reviewers who labored to polish the
ideas and the presentation of my manuscript. Finally, my thanks to Angélica for
the drawings that illustrate this paper and to Ricardo, for his help to finish
my manuscript under difficult logistic circumstances.
Volver a Geología y Yacimientos MInerales
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