ANDEAN
METALLOGENESIS: A SYNOPTICAL REVIEW AND INTERPRETATION (*)
Departamento de Minas, Universidad de La Serena, Casilla 554, La
Serena, Chile
(*) 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 Palaeozoic 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 controlled 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 Palaeozoic evolution. On the other hand, important sedimentary
Palaeozoic basins are characterized by vertical tectonics. Also, calc-alkaline
magmatism, so typical of the Mesozoic-Cenozoic Andean belts, is equally
abundant during the Palaeozoic, and attains a peak during the Permian. Thus,
the Permian-Triassic transition occurs in geological continuity. Finally,
Palaeozoic and post-Palaeozoic tectonic directions are similar and the
Palaeozoic 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
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 Palaeozoic and
post-Palaeozoic 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 Palaeozoic, especially 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 principal 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, which 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
Favourable 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
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
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, La Coipa, La Pepa,
Nevada/Pascua and El Indio-Tambo). 2- Low
sulfidation, epithermal (Faride,
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 the entire Andean belt, although their principal
deposits are located in the Peruvian segment, which 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
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
favourable 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 8 km south
from Cerro de Pasco is hosted by the Tertiary La Calera series, formed by
clastic sediments and carbonatic rocks, with frequent chert and tuffitic intercalations.
Although this deposit has been traditionally classified as a hydrothermal
replacement (Mc Kinstry, 1936), Lehne (1990) proposes a syngenetic origin,
considering bedding and other sedimentary features of the ores. The thickness
of the ores beds is normally less than 2 m and they are separated from each
other by shale beds.
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 Palaeozoic metamorphic rocks (phyllites and marbles of marine
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 Palaeozoic singenetic mineralization, Schneider and Toloza
(1990) argue that all ore deposits of the district (which 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 500 km from the western border of the Continent,
and so far, no tin ores have been found along the Chilean territory. Besides,
the tin province is located in the central part of the Andean belt, where
the present continental crust attain the maximum
thickness (Figs. 8, 9 and 11).
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 Palaeozoic
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 greissen 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 greissen-type alteration, within small batholiths
(e.g., Yani, Sorata) or in the contact metamorphic zone imprinted by the batholiths
in the Palaeozoic 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 20 M.y.
old intrusive rocks that crops out between lats. 16º30’ S and 19º50’ S, and
are associated to several important deposits, such as Catavi and Llallagua.
However, the richest Sn-Ag deposits (Cerro Rico, Chorolque etc., Fig 8), are related
to a second, younger (17-12 Ma) group of sub-volcanic bodies that crop out
between lat. 19º S and north Argentina. The association of these acidic intrusions
to ignimbritic materials is frequent. Thus, the Potosí district is associated
to a large ignimbritic source: The Karikari resurgent caldera. Grant et al.
(1980) distinguished two types of tin deposits in this belt, that they denominated
"porphyric" and "non porphyric".
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 80 M.t. of disseminated ore grading 0,3% Sn, which 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 within 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 contribution 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 does 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, which has
been almost permanent in the belt, the matallogenetic activity seem rather
discontinuous and related to significant tectonic disruptions that abruptly
displaced the magmatic belts. Therefore, favourable 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 extrusive magmas has been much variable, the volcanism being favoured
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 consistently explained in terms of the interactions
of the continental and oceanic lithospheric plates. Among the main consequences
of this interaction are the continuous production of calc-alkaline magmas,
the accretion to the continent of oceanic prisms, the development of back-arc
basins, the occurrence of several orogenetic episodes, the formation of mega-fault
zones and the generation of ore deposits.
Post-Palaeozoic accretion
of oceanic prisms occurred during Tertiary times in the
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, 1953 in Maksaev and Zentilli, 1988).
However, this time the process followed an important compressive pulse (the
Inca compression). During a short span (10 Ma) in the Eocene-Oligocene boundary,
several of the most important porphyry copper deposits of the
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 metallogenetical 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 heterogeneities
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 are rather uncertain in terms
of cause-effect. Thus, the tin province may be, in part, a consequence of
the thicker continental crust between boundaries 5 and 8, that could have favoured 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 favourable
for the crop out of porphyry copper deposits (Petersen, 1970). Also, different
erosion levels in the tin belt of
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 favourable to the abundant deposition of carbonatic
sediments, a factor considering favourable 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
attention. 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 metallic 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 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
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
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 laboured 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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