ANDEAN
METALLOGENESIS: A SYNOPTICAL REVIEW AND INTERPRETATION (*)
Departamento de Minas, Universidad de
(*) 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 Andes
(10º S to 35º S), where the belt has developed a thick continental crust, as a
consequence of a higher degree of orogenic evolution during the
Mesozoic-Cenozoic span. This relationship has a parallel in the
magmatic-metallogenetical evolution of the island arcs, where the number of
different types of ore deposits and the magnitude of the larger ones followed
to the development of a dioritic-tonalitic crust. A possible explanation to this
analogous behaviour may be related to the growing opportunities for
interactions between magmas, solid materials and fluids from different layers
(from the asthenosphere to the crustal sedimentary strata) provided by the
increasing complexity of the orogen.
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 Andes offers a most interesting frame for describing the metallogenical
development of the belt and searching for the reasons that explain the origin
and geological evolution of their mineral belts.
The Andean belt is a complex
orogenic system, that has its maximum wide (near
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 Caribbean plates, and one of oceanic-continental nature, the
Antartic plate. Only the Cocos, Nazca, and Antartic plates present active
subduction (the relative motion of the Caribbean plate being of transcurrent
type). The seismic activity affects all the Andean belt, but the Benioff zone
under the Continent exhibits important differences in definition and angle of
dip, attaining a maximum depth (some
The continental crust has different
thickness along the belt, attaining a maximum of
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 (Colombia) and Atacama
(Chile), have a history that began, at least, during the Early Mesozoic. The
transversal faults linked to differential displacements of the continental
plate, have also played an important role in the distribution of some ore
deposits, e.g., the Chaucha porphyry copper, Ecuador (Goossens and Hollister,
1973).
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
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 Africa that began
during the Jurassic, did not imply radical changes in the Andean belt
evolution, which remained basically a "magmatic belt" (Zeil, 1979).
As suggested by Coney (1970) and other authors, this evolution may be described
in terms of the superimposition of magmatic arcs over the edge of the
Continent. This description, which is valid for the central Andean segment,
should also include episodes of accretion to the Continent of oceanic
magmatic-sedimentary prisms at the northern and southern parts of the belt.
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
Chile, Aberg et al., 1984), attained during their evolution several
characteristics of the marginal basins.
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 Northern Andes, whereas
calc-alkaline magmatism predominated along the principal magmatic arc of the
belt, and alkaline magmatism appeared in small amounts, as intrusive and
extrusive bodies in the back arc region. Also, the presence of shoshonitic
rocks has been established both in the Jurassic-Early Cretaceous magmatic arc
in Central Chile (Levi et al., 1988) and in the Tertiary back-arc region of
northwest Argentina (Sasso and Clark, 1998).
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 Southern Andes, and which are
generally associated to sub-volcanic igneous rocks.
The post-Paleozoic metallic
provinces appear as 50 to
The Andes are one of the richest
orogenic belts in terms of metallic ores and several of the Andean countries
are among the top ten of the world, either in production or in geological
reserves of antimony, barium, berilium, bismuth, boron, copper, indium, iodine,
lead, molybdenum, nitrates, platinum, rhenium, selenium, silver, tellurium,
tin, tungsten and zinc (Petersen, 1977). Chile, alone, has about a quarter of
the world’s copper reserves and close to one third of those of molybdenum. The
tin-silver province of south Bolivia is well known by their
"fabulous" deposits, as Cerro Rico, Potosí, which produced some
60.000 t of silver, in addition to high-grade tin ores. Equally famous are the
polymetallic and copper provinces of Perú. Besides, the last 30 years have been
generous in terms of the discovery of new "world class" ore deposits,
such us El Indio and Escondida in Chile, and the Andes are still considered a
first class target, atracting about 15% of the world’s investment in mineral
exploration. On the other hand, important scientific studies have been
dedicated to those types of ore deposits that are well represented in the
Andean belt. This is the case of porphyry copper, epithermal Au-Ag deposits,
the Sn-Ag sub-volcanic deposits in Bolivia and the zoned polymetallic deposits
of Perú. Also, some industrial minerals of the Andes present special interest,
as in the case of the evaporitic deposits of Chile, Bolivia and Argentina, that
contain huge amounts of potassium, lithium, iodine, nitrate and borates.
The present exposition will now
describe the different metallic provinces of the Andes, including a special section
on precious metal deposits and will be completed with a discussion of the main
factors involved in the metallogenetical interpretation of the belt.
Metallic Provinces in the Andes
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 (Chile); oolithic iron dposits
in northwest Argentina and Colombia; Kiruna-type deposits in the coastal ranges
of north Chile and Perú and skarn type Fe-Cu deposits of the Andahuaylas-Yauri
zone in Perú. The magnetite deposit of El Laco volcanic structure in north
Chile is included in the third group, but will be considered separately,
because of the specials characteristics of the district.
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
The oolithic iron deposits are found
in northwest Argentina, where they have a Lower Silurian age, and in Colombia,
where they are Upper Eocene in age. The Argentinean deposits are in coastal
marine facies at the eastern border of a central craton. The ores are oolithic
and the iron beds, deposited during a marine transgression, contain chamosite
(partly altered to hematite) as the principal iron mineral (Bossi and
Viramonte, 1975). Though the productive formation (Zapla) crops out along
hundreds of kilometers in a north-south direction, the principal deposits occur
between 24º S and 25º S, close to 65º W. They are those of Zapla, Río Iruya and
Unchimé. Their total pre-mining reserves are about
The oolithic iron deposits of
Colombia are part of a
The Kiruna-type iron deposits of
north Chile are distributed along a narrow N to NNE belt on the Coastal Range
between 25º S and 31º S. The axis of this belt is in close coincidence with
that of the Neocomian magmatic arc. The principal districts (e.g., El
Algarrobo-Penoso: 28º47’ S; El Romeral: 29º43’ S), are situated between 27º S and
30º S and their reserves (before mining) are about
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 Chile. The principal deposit is
Marcona, made up of stratiform ore lenses, hosted in carbobnatic and
volcanoclastic rocks. According to Atkin et al. (1985), their origin is related
to replacement by hydrothermal fluids from Middle Jurassic subvolcanic
intrusive rocks.
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 Montreuil
(1972) they contain the highest potential ore reserves in Perú, estimated at
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 Chile, close to the border with Argentina (Fig.
1). Pyroxene andesites are dominant in the volcanic flow, but a central
subvolcanic intrusive has a dacitic composition. El Laco iron deposit contains
several hundreds M.t. of iron ore but has not been extensively mined. In
exchange, the peculiar characteristics of the deposits have been the matter of
several studies, as well as the origin of controversies regarding the genesis
of the deposits (Park, 1961; Frutos and Oyarzún, 1975; Frutos et al., 1990;
Nystrom and Henríquez, 1994; Larson and Oreskes, 1994).
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 Andes
will be considered.
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 El
Salvador deposit (Gustafson and Hunt, 1975), have been studied in great detail,
becoming classic examples of their type. Also, the distribution of the deposits
along and across the Andean belt and the facts that they belong to a wide
chronological span, present different erosion levels and were emplaced in a
variety of host rocks, under distincts tectonic conditions, have allowed the
construction of a number of genetical models (e.g., concerning porphyry copper
deposits and plate tectonics: Sillitoe, 1972, and the tops and bottom of
porphyry systems: Sillitoe, 1973). On the other side, the abundance of
important deposits and studies about them, make difficult to present a
synoptical view. For that purpose, the publication by Camus et al, eds., (1996)
is strongly recomended, as well as the paper by Sillitoe, (1992).
Sillitoe (1988), considers six
epochs of porphyry copper mineralization in the Chilean-Argentinean sector of
the Andes, from Late Carboniferous-Early Permian to Middle Miocene-Early
Pliocene, and also six epochs, from Jurassic to Middle Miocene-Early Pliocene,
for the Perú to Colombia Andean sector. Each of these epochs is represented by
longitudinal belts up to
Most porphyry copper deposits in the
Andes are related to dacitic-granodioritic porphyric stocks, emplaced in
volcanic rocks or in intrusive complexes. Although the stocks generally belong
to the calc-alkaline series, shoshonitic or high-K cal-alkaline rocks have been
identified at the Farallon Negro district (Sasso and Clark, 1998). Sr isotopic
ratios of the porphyric stocks are low and point out to a deep seated origin.
Also, their Pb isotopic ratios have a narrow range. Thus, in the case of
Chuquicamata and El Salvador, Pb isotopic ratios are similar to those of the
Southern Volcanic zone of the Andes, whose magmas are not affected by crustal
contamination (Zentilli et al., 1988). Besides, there are a number of evidences
suggesting that the magmas responsible for the porphyric systems, rapidly rise through
the crust, allowing a small to null degree of contamination (Maksaev and
Zentilli, 1988). In general, the emplacement-alteration-mineralization process
can be generalizad as "a subvolcanic magmatic development of a metal-rich
magma, where residuals fluids mixed with meteoric waters during the late stage
of its cooling" (Ambrus, 1978). Although a majority of the deposits fits
well in the Lowell and Guilbert (1970) model, the phyllic zone is rather absent
in some of them, such as El Abra or El Teniente. The last one, that has been
recently reinterpreted in terms of the intrusive emplacement of a high-K, ore
bearing, mafic magma (Skewes and Arévalo, 1997), fits better in the dioritic
model proposed by Hollister (1974).
Porphyry copper deposits present both
spacial and chrological clusters in the Andean belt. Thus, the Arequipa
lineament includes four important Paleocene deposits (Cerro Verde, Cuajone,
Quellaveco and Toquepala) along a
As pointed out before, many
important porphyry copper deposits in the Andes are in or close to large fault
zones (Fig. 10). However, although this
structural control is evident for those deposits of the stockwork-type, such as
Chuquicamata, this is not the case for porphyry deposits of the breccia
pipe-type, like Los Bronces-Río Blanco or El Teniente (Camus, 1975).
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
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 Andes.
However, Petersen and Vidal (1996) remark that the number of large and high
grade enargitic deposits is an unusual trait of the Peruvian metallogeny. The
Peruvian deposits are well zoned from Cu and Au in the center to Zn and Pb in
the margins. Among the principal enargitic deposits in Perú are Quiruvilca,
Cerro de Pasco, Colquijirca, Huarón, Morococha, Yauricocha and Julcani (Figs. 2
and 5). The rich vein gold
deposit of El Indio, Chile (Fig. 4) also belongs
to the enargitic type. According to Sillitoe (1983), enargite-bearing massive
sulfide deposits may represent the upper levels of porphyry copper systems.
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 Chile by Candelaria
(Fig. 2). Both of them are related to the Lower Cretaceous
basin and present mineralogical analogies with regard to the Kiruna-type iron
deposits of Perú and Chile.
Breccia pipe ore deposits are
widespread in the Andes. Although many of them are related to porphyry copper
systems, other appear as independent mineralizations and exhibit much variety
in diameter of the pipe as well as in the number of deposits in a given
district. The mineralogy of the deposits is generally cupriferous (with Au) or
polymetallic. A detailed description of Cu-bearing tourmaline breccia pipes in
Chile was produced by Sillitoe and Sawkins (1971).
Manto-type copper deposits are
typically found in volcano-sedimentary formations of Mesozoic age in north and
central Chile (Espinoza et al., 1996). The deposits are stratiform or
stratabound but frequently also include veins, ores in breccias, stockworks
etc, that are probably co-genetic (Vivallo and Henríquez, 1998). Their
paragenesis is rather simple and includes chalcocite, bornite, chalcopyrite,
pyrite and hematite, the Cu/Fe ratio decreasing outward from the Cu-rich cores.
The stratiform Cu mineralization, that also contains some g/t Ag, was deposited
in the groundmass and vesicles of lava flows or in the matrix of pyroclastic
rocks. The associated hydrothermal alteration is propylitic and includes
albite, chlorite and calcite. Mineralization occured in the epithermal or low
mesothermal range. These deposit have magnitudes up to hundred M.t. ore,
containing 1-2% Cu (El Soldado), but normally are in the 1-
Massive sulfide deposits are not
abundant in the Andean belt, although the accreted oceanic prisms of the
Northern Andes offer favorable environments for Cyprus-type deposits, and a few
are known in western Coombia (Ortiz, 1990). Also, an important Fe-Cu-Zn
volcanogenic massive sulfide deposit, Tambo Grande, is located in the NW corner
of Perú, at 5º S, close to the border with Perú. In Chile, the manto-type Cu
deposits at Punta del Cobre (Fig. 2) and the
polymetallic skarn of El Toqui, at 45º S have been interpreted as massive
sulfide deposits by Camus (1985) and by Wellmer et al. (1983) repectively.
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 Chile, twelve
deposits of this type are yet known. The larger one, Exótica deposited on a
wide paleochannel, 2 to
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 Andes began with this
type of deposit. In north Chile, favorable climatic and tectonic conditions
produced a high degree of supergene enrichment in Cu +/- Au vein deposits,
allowing the development of a highly profitable mining activity during the 19th
century.
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, Colombia has
been an important gold producer, the first of the world in Colonian times.
Although the gold production of this country is mainly obtained from placer and
vein-type deposits, there are also several lode gold deposits, such as those of
California, Segovia, Frontino and Marmato (Fig. 4), some of
them related to porphyry copper systems, like California and Marmato (Sillitoe
et al., 1982). In exchange, there are not important silver deposits in
Colombia, and this metal is a sub-product of gold mining. It is interesting to
remember that platinum was first discovered in Colombian placer deposits and
that this country was the only platinum producer in the world till 1819
(Angulo, 1978).
Gold mining began in Colonial times
in Ecuador with the famous Portovelo deposit (Fig. 4) and with
many small Au-Ag veins and placer gold deposits. According to Gemutz et al.
(1992), gold deposits and prospects in Ecuador belong to the epithermal vein
(Portovelo, Pilzhum, and Molleturo), skarn type (Nambija and Pachicutza),
stockwork-vein (Chinapitza), intrusive breccia (Gaby) and porphyry copper
(Fierro Urco) types, besides the placer deposits. Their age is Jurassic for a
few deposits (Nambija, Chinapitza), but most of them are Tertiary in age. As in
Colombian deposits, silver is subordinated to gold in most of the Ecuadorian
precious-metal deposits.
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 Bolivia, are best known for their Sn-Ag veins as well as for the Sb
vein deposits. However, they are also related to polymetallic veins and
stockworks in the boundary zone with the Altiplano region. Among the
polymetallic districts,
Although there are important Au-Ag
deposits in Chile, most of them linked to sub-volcanic magmatic activity of
Miocene age in the high Andes, in a large number of deposits Au is rather
related to Cu and Fe. Besides, there are many important Ag deposits of the
"Guanajuato" type that are almost devoid of Au contents.
Gold production in Chile attained a
peak in 1938 with some 11 t Au, mostly coming from placer deposits, then
gradually descended to 2-3 t/year between 1955 and
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, San Cristóbal, El Guanaco,
Chile was an important silver
producer in the 19th century (300 t in 1873, 15% of total world
production). Among the principal silver districts are those of Huantajaya,
Caracoles, Tres Puntas, Chañarcillo and Agua Amarga (Fig. 6). They are
epithermal, low sulfidation vein-type deposits, hosted by stratified rocks
belonging to the volcanic-sedimentary transitional facies of the Jurassic and
Cretaceous back-arc marine basins. Silver mineralization includes a variety of
sulfide species (argentite, proustite, pyrargirite, etc.), and supergene
processes are responsible for the deposition of secondary minerals (native Ag,
cerargyrite etc.) in very rich oxidation zones (Ruiz et al., 1965).
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 Jujuy province. After
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 Sierra
Nevada, Venezuela (Carlson, 1977) or El Aguilar in NW Argentina, most of the
deposits are Mesozoic or Cenozoic in age.
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
Although Mesozoic and Cenozoic
polymetallic deposits are present in the Northern Andes (Colombia and Perú),
most of these vein-type deposits has been mined for silver. Also, polymetallic
deposits are poorly represented in the Chilean territory, except for the
Patagonean Cordillera, between 46º00’ S and 47º20’ S. Thus, the
clastic-carbonatic rocks interfingered with andesitic volcanics of the Jurassic
and Lower Cretaceous back-arc basin, mainly host epithermal silver veins or
skarn-type Cu or Fe deposits. In exchange, a rich polymetallic province
developed in the Peruvian territory, that may be partly explained by the widespread
distributiion of Mesozoic marine sediments, including abundant carbonatic
facies (Fig. 7).
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 Mississippi Valley deposits.
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. Carolina, Porica), as well as
in the Pulluicana Formation (e.g. Yanacancha, Quijote). Although mined since
Spanish Colonial times for their silver ores, the deposits of the Hualgayoc
district were later mined for their polymetallic ores (Zn, Pb, Cu, Ag) beneath
the oxidation and supergene enrichment zones. As pointed out by Canchaya
(1990), the origin of the stratabound deposits of the district remains obscure,
in spite of the large number of geological studies already performed.
In northwest Argentina, there is a
number of polymetallic (Cu, Pb, Zn) stratabound sulfide ore deposits in
carbonatic rocks of Late Cretaceous-Early Tertiary age (Sureda et al., 1986).
The deposits are dispersed along a
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 Santander and Milpo-Atacocha. 3- Veins, hosted by Mesozoic sedimentary rocks and
Oligocene-Miocene volcanics, e.g., Colqui, Casapalca, etc. 4- Irregular bodies, skarns, veins and disseminations related to the Cordillera Blanca batholith. This
group includes the polymetallic skarns of Magistral, Antamina and Contonga, as well
as the polymetallic veins with silver and tungsten of Pusajirca.
The Miocene belt of polymetallic ore
deposits in Bolivia is located west of the Sn-Ag province and represents a
sothward extension of the Peruvian Miocene belt. Its geological frame (Miocene
sub-volcanic intrusives hosted by Paleozoic clastic rocks) is similar to that
of the tin belt. Among their principal deposits are Laurani, San Andreas,
Berenguela, Carangas, Negrillos and Garcí Mendoza. Laurani, a main one, is a
zoned deposit, associated to an andesitic-dacitic complex, cross cut by a
rhyolitic stock and by dykes, directly related to the mineralization (Ahlfeld,
1967; Routhier, 1980).
A further southward extension of the
Miocene polymetallic belt is represented by Pb-Zn-Ag (Cu, Bi) veins in
northwest Argentina (Salta and Jujuy provinces). The major districts, Pan de
Azúcar (22º43’ S / 66º06’ W),
In the Patagonian Cordillera of
Argentina and Chile, between 46º and 52º S, there are numerous polymetallic
deposits hosted by Paleozoic, Mesozoic and Cenozoic rocks of different types.
Márquez (1988) describes a general zoning pattern, with Mo, W in or around
central granitic intrusive rocks and Pb-Zn, Cu, Au and Ag in the periphery.
According to this author, the granitic rocks responsible for the mineralization
are Miocene in age.
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,
San Antonio, overlays a quartz-bearing porphyric sill, partially altered and
mineralized. Some cross-cutting mineralization feeders, and basal hydrothermal
alteration and mineralization, have been recognized in the district. Wellmer
and Reeve (1990) interpreted the genesis of El Toqui district deposits in terms
of massive sulfides mineralization in the submarine volcanic environment of an
aborted back-arc system, in the Jurassic-Cretaceous time boundary.
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 Bolivia (Turneaure,
1971), may be divided in two segments. North from 18º S, the belt trends NW and
most of the deposits have an Upper Triassic-Lower Jurassic age. In exchange, in
the southern segment -as well as in the central part of the belt- Miocene Sn-Ag
deposits are dominant. While the ore deposits of Lower Mesozoic age are related
to granitic rocks, those of Miocene age are associates to acidic subvolcanic
bodies. The strong hydrothermal alteration associated to both types of
deposits, difficults the determination of the original composition of the
mineralizing igneous rocks. However, their high potassic, peraluminous
character, is recognized, as well as the likely participation of crustal
material in the generation of their magmas. This participation is coherent with
the larger distance of the tin province to the possible situation of the
paleo-subduction zones (during the Triassic-Jurassic and the Miocene,
respectively). It is interesting to note that the southern part of the tin belt
coincides with a rich Sb sub-province (Bolivia was the third World’s Sb
producer and has some 200 deposits of this metal, Routhier, 1980). Eastward of
the tin province, there are several polymetallic deposits (mainly rich in Ag).
This fact rise the question of whether the southern part of the tin province is
located eastward or rather superimposed to the larger polymetallic one.
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
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
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
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 groupe 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
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, Santa Fe and Morocala (cassiterite)
and Tasna (cassiterite, with Bi and Cu in the sulfide phase) deposits (Grant et
al., 1980)
Tin-silver veins in northwest
Argentina (Sureda et al., 1986) represent the southward extension of the
Bolivian tin belt. The major deposit, Pirquitas (22º44’ S / 66º27’ W) is hosted
by strongly folded, clastic Paleozoic rocks. Its paragenesis includes high Tº
(pyrrhotite, cassiterite, arsenopyrite etc) and low Tº (sphalerite, galena,
sulfominerals etc) phases, both crystallized at shallow sub-volcanics levels.
The average grade of the deposit is 1.1% Sn and 500 g/t Ag.
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 Andes).
Although this process involves the continuos (since 102 Ma to 60 Ma)
"canibalistic" assimilation of stratified rocks, the fact that they
were mainly volcanics, with similar chemical and isotopic composition to the
batholith’s magmas, implies that no sensible compositional change occured.
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 Mo. Besides, where the country
rocks are volcanic or volcano-clastic with a similar age and composition to
that of the intrusive ones, Pb isotopic ratios are not usefull to discriminate
between the metal provided by the magma from the metal scavenged from the
country rocks by hydrothermal or metamorphic fluids. This situation is fairly
common for metallic ore deposits in the Andean belt Mesozoic and Cenozoic
rocks.
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 Chile, are less radiogenic than those found in similar
Jurassic rocks. These results are consistent with the conclusion of Fontboté et
al. (1990) for stratabound deposits in the Andes: those related to mafic or
intermediate rocks have Pb isotopic ratios pointing to a mantle source, while
those deposits related to felsic igneous rocks or to sediments present isotopic
ratios according to an "orogenic" (recycled lower and upper crust) or
to an upper crustal source (San Vicente). They also remarks that Pb isotopes of
the ores are more radiogenic in those deposits located eastward. Petersen et
al., (1993), enlarging the previous study by Macfarlane et al. (1990), proposed
four Pb isotopic provinces for the central Andes, from W to E: Coastal region
of Perú and northern Chile, High Andes (Perú, Chile, Bolivia, Argentina),
Eastern Andes (Perú, Bolivia, Argentina) and eastern foothills of the Andes. A
deep source is suggested for the former two provinces, a shale-bed source for
Eastern Andes ore deposits and a craton-source for those of the eastern
foothills of the Andes.
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 El Salvador,
Field and Gustafson, 1976; -1.4 o/oo at Chuquicamata,
-2.1o/oo at Río Blanco and -3.1 o/oo
at El Teniente, Sasaki et al., 1984). This is also the case for sulfides in magnetite
ore deposits (e.g., minor pyrite at El Laco, Vivallo et al., 1994). Concerning
stratabound sulfide deposits, those emplaced in volcanic, volcanoclastic or
coarse detritict sedimentary rocks have d34S close to the meteoritic
standard. In exchange, those hosted in sedimentary rocks including black shales
generally have d34S in the -10 to -40 o/oo,
suggesting the effect of bacterial activity over sulfate ions of magmatic
origin (Spiro and Puig, 1988). An important exception is the San Vicente
"Mississippi Valley" Zn-Pb deposit in Perú, that presents positive
and homogeneous d34S values between +6.9 o/oo
and +13 o/oo, which are interpreted in terms of bacterial
reduction of 34S-enriched sedimentary sulfate (Gorzawski, 1990).
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 Chile,
have Pb isotopic ratios that are much narrower than that of all other types of
ore deposits or the intrusive and volcanic rocks of all ages in the present
Central Volcanic Zone (Zentilli et al., 1988). Generally accepted models (e.g.,
Sillitoe, 1973) situate their porphyric intrusives over the cupola of calc-alkaline
batholites. However, as pointted by Maksaev and Zentilli (1988), the
Eocene-Oligocene porphyries, the last important magmatic activity recorded in
the Domeyko range, before a 50 to
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 El Salvador
district (north Chile).
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 Taylor (1976) diagram. It is interesting the fact that
high-K or shoshonitic magmas have been established at the Farallón Negro
complex (Sasso and Clark, 1998), related to porphyric Cu (Au) mineralization.
Besides, the model by Skewes and
Arévalo (1997) is close to the ore-magma concept, which has been applied in
Chile to explain the origin of Kiruna-type iron deposits since 1931, with a
variable degree of acceptance. Although this theory (e.g., Nynstrom and
Henríquez, 1994) has been objected on the basis of mineralogical and physico-chemical
data, it is making a comeback again.
The fact that the Tertiary igneous
rocks related to Sn-Ag mineralization in south Bolivia have a per-aluminous
character and high-Sr (i) isotopic ratios (0.707 - 0.716), suggest a
significant paticipation of the continental crust in their petrogenesis
(Schneider, 1987). However, the recent paper by Dietrich et al. (1999)
presented analytical data also supporting the participation of andesitic to
basaltic melts (mixed with high evolved rhyolitic melts in upper crustal
reservoirs) in the genesis of Tertiary Bolivian tin porphyries. Therefore,
mafic magmas may play a more important part in the genesis of Andean ore
deposits than yet recognized. Also, the mechanism of magma mixing proposed by
Drietich et al. (1999) may be usefull to explain the genesis of other types of
Andean deposits, like the Kiruna-type Cretaceous iron deposits of north Chile,
where evidences for the involvement of both mafic and alkaline, F, Cl rich
magmas exist.
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 (Colombia and
Ecuador), when a Mesozoic mafic igneous-marine sedimentary complex was
incorpored to the western border of the continent. Except for some peridotitic
podiform Cr ores and for some massive sulfide bodies (Ortiz, 1990) this episode
had little direct metallogenetic importance.
Two subduction styles have been
recognized for the tectonic evolution of the central and south central Andes
(Uyeda and Kanamori, 1979): a low-stress Mariana type, for the Jurassic-Lower
Cretaceous span, and the compressive Chilean type of subduction since the Upper
Cretaceous. The passage between both regimes (related to the westward shift of
South America after the break-up of Gondwana), which implies a shallower angle
for the subducting slab, occured for the Chilean segment between 108 and 100 Ma
(Sillitoe, 1991), in close coincidence with the ages of a number of deposits in
the Neocomian back-arc basin domain. This basin, that reached an "aborted
marginal basin" stage in Chile (Levi and Aguirre, 1981) and a straight
marginal character in Perú (Atherton and Webb, 1989), and where attained a
maximum subsidence in the Albian, received several thousands meters of mafic
lavas and marine sediments. About 110 Ma, numerous Kiruna-type Fe deposits and
stratabound and skarn type Cu deposits (several of them rich in magnetite),
were formed in volcanic or sedimentary rocks of the basin. Among them are the
Fe-Cu skarns of Eliana (112 Ma), Monterrosas (110 Ma) and Hierro Acari (109 Ma)
in Perú (Petersen and Vidal, 1996), the Kiruna-type Fe deposits of the coastal
belt in Chile: Los Colorados (110 Ma), El Algarrobo (128-111 Ma; Montecinos,
1983) and several stratabound Cu deposits in central Chile, like El Soldado
(110 Ma) and Lo Aguirre (113 Ma; Munizaga et al., 1988). Shortly after (112-105
Ma) the Andacollo porphyry copper was also emplaced (Sillitoe, 1988). This
coincidence is amazing, considering that extensional conditions still prevailed
in the Huarmey basin of Perú at that time.
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, San Cristobal,
Colqui) may be related to this fault zone (Petersen and Vidal, 1996).
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 metallogenetical zoning and evolution of the Andean belt
Three main subjects will be discussed
in this section: the tectonic segmentation of the Andes, the distribution of
the different metallic provinces and the metallogenetical evolution of the
belt.
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 Andes volcanoes and of the Altiplano-Puna block) and 8 (northern
limit of the Domeyko Cordillera and westward step in the longitudinal belt of
recent volcanoes).
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 Andes (Petersen, 1970; Goossens, 1972b).
This factor may be important at a regional and locale scale, e.g., the deeper
erosion levels of the Peruvian western Andes flank may be favorable for the
crop out of porphyry copper deposits (Petersen, 1970). Also, different erosion
levels in the tin belt of Bolivia expose Triassic to Jurassic Sn-W deposits
related to deeper seated plutonic rocks in the northern part of the belt and
Tertiary Sn-Ag deposits associated to shallow subvolcanic complexes in the
southern segment.
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 Bolivia, where most sedimentary series have a clastic
composition, a fact that seems to confirm this hypothesis. However, Mesozoic
carbonatic sediments in Chile host copper or silver deposits, and Pb-Zn ores
are poorly represented (except in the Patagonian Cordillera). In consequence,
the presence of carbonatic-rich sedimentary rocks appear as a contributing
factor, but not a decisive one.
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 Pacific ocean, presents a remarkable
"reflection symmetry", with the copper belts closer to the oceanic
trenches and the tin belts in an interior position. At the Asian margin, this
arrangement comprehends the preferent position of Cu in the islands arcs, and
of Sn (W) at the continental border. This symmetry suggests that the Andean
metallic zoning is a consequence of a general geological mechanism, at least
with respect to their better defined and mutually excluding provinces (the Sn
province is very poor in Cu and there is almost no Sn in the Cu province). The
search for this postulated mechanism, implies the selection of those geological
traits that appear as more significant in terms of regional metallogeny, and
the critical examination of their possible roles. In this perspective, the
hypothesis by Ishihara (1977, 1978) presents a special interest. This author
consider two types of magmatic series: the magnetite (oxidant) magmas and the
ilmenitic (reducing) ones. The fact that an oxidant character of magma is
required for the separation of sulphur as SO2, a necessary step to
permit the later mineralizing activity of this element (Burnham and Ohmoto,
1980) makes magnetite series favorable for sulfide mineralizations. In
exchange, the reducing character of the ilmenitic series (due to a greater
contamination by reducing sedimentary rocks in the upper crustal leves) favors
tin mineralizations (as Sn2+ is not incorporated to petrographic
minerals, which is the case for (Sn4+). The presence (though not
exclusive) of S-type granitoids, belonging to the ilmenitic series of Ishihara,
in the eastern magmatic belts of Bolivia and Argentina (Ishihara, 1977, 1981;
Llambías, pers. com. 1984) is consistent with this model. Also consistent is
the fact that the western magmatics belts containing magnetite and sulfide
mineralization include only I-type granitoids, belonging to the magnetite
series (Ishihara and Ulriksen, 1980). Besides, these relationshps are similar
to those reported by Ishihara (1977, 1978) for eastern Asia. There, the Sn
province in the continental border is associated to ilmenitic granitoids and
the island-arc sulfophile province (Cu, Mo, Pb, Zn) to magmatic rocks of the magnetite
series. The participation of the subducting oceanic slab in the process is
sustained by a precise ratio established in Japan between the convergence speed
rate of the plates and the "productivity" of different arc segment in
terms of volcanic sulphur (Ishihara, 1981).
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 Chile (Levi and Aguirre, 1981). It is possible
that under these circumstances, more complex mechanisms participate, like this
proposed by Márquez et al. (1999) for the Mexican volcanic belt, involving both
an asthenospheric plume and subduction-related process, or the model proposed
by Sasso and Clark (1998) for the Andean segment between 26º30’ S and 30º30’ S,
already mentioned in this review.
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 Andes at 23º49’ S). Certainly, the
possible effect of erosion levels should be considered a contributing factor,
as the Tertiary hypabysal or subvolcanic intrusive rocks are normally eroded at
a level that is favorable both for the exposure and preservation of most types
of hydrothermal deposits.. However, none of the well preserved pre-Tertiary
porphyry copper deposits in the Andes attains the order of magnitude of the
larger Tertiary ones, and the same is true for other types of deposits, like
those of the Bolivian tin belt.
In metallogenetical 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 Paris Sud University in
1985, under the encouraging direction of Prof. R. Brousse. Along this study and
the last 15 years, I have had the opportunity to discuss this matters with
Profs. G.C. Amstutz, B. Lehman, B. Levi and P. Routhier, as well as with many
collegues from the Andean countries, among them Drs. F. Ortiz (Colombia), R.
Carrascal and C. Vidal (Perú), W. Avila (Bolivia), M. Brodtkorb, J. Llambías
and R. Sureda (Argentina), and J. Frutos, Palacios (Chile). I am much indebted
to them, as well to other Andean geologist with whom I have had less personal
contact but, have, as many people, benefited from their important conributions
to the Andean metallogenesis, like Drs. A. Clark, U. Petersen, R. Sillitoe and
F. Camus.
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.
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