Andean Metallogenesis: A Synoptical Review and Interpretation (*)

Jorge Oyarzún M




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 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 Colombia, while only one cordillera exists in south Ecuador. The present configuration of the belt is relatively recent. Thus, the Bolivian Altiplano was a subsident zone until its Pliocene uplift. The valley or longitudinal depressions present a rapid subsidence in some sectors (e.g., Colombia, south Chile), where the accumulated Plio-Quaternary sedimentary and volcanic materials attain up to 5-10 km in thickness.

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 350 km) at the central part of the Andes. The oceanic trench also attains a maximum depth (some 8 km) between lats. 22º and 25º S, where its run paralell to the coast, some 100 km westward. Considering the relatively close heights of the Principal Cordillera, this part of the Andean belt presents the major topographic contrast of the Earth.

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 Himalayas. In exchange, the thickness of the continental crust is small or null under a part of the coastal region of Colombia and Ecuador. Also, the continental crust varies in thickness transversally to the belt, first increasing, then diminishing, until it become stable at a figure of 30-35 km under the continental shield.

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 30 km in diameter, Francis and Baker, 1978). Some of the Andean volcanoes have supplied important clues for understanding the genesis of the ore deposits, as in the cases of San Fernando, Ecuador (Goossens, 1972a), and El Laco, Chile (Park, 1961).

Although some authors, as Aubouin et al. (1973) and Zeil (1979), sustain the existence of fundamental differences between the Paleozoic and post Paleozoic geologic development of the Andean belt, these differences depend on the Andean segment and the period considered. Neither the episodes of marginal basin development nor the stages of strong horizontal compressive tectonics are exclusive traits of the Paleozoic evolution. On the other hand, important sedimentary Paleozoic basins are characterized by vertical tectonics. Also, calc-alkaline magmatism, so typical of the Mesozoic-Cenozoic Andean belts, is equally abundant during the Paleozoic, and attains a peak during the Permian. Thus, the Parmian-Triassic transition occurs in geological continuity. Finally, Paleozoic and post-Paleozoic tectonic directions are similar and the Paleozoic metallogenesis includes the same metals deposited in Mesozoic and Cenozoic times, although the areal distribution of the metallic belts is different. Porphyry copper deposits, a main trait of the Cenozoic Andean metallogenesis, were formed in the Andean domain at least since the Carboniferous (Sillitoe, 1977).

Nevertheless, some of the characteristics traits of the Andean belt, e.g., the generation of large amounts of calc-alkaline magmatism, became heightened in Mesozoic and Cenozoic times, whereas other, like the accretion of oceanic prisms, lessened their relative importance. The separation of South America from 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 300 km wide belts, elongated parallel to the Andes. Between lats. 14º S and 30º S, where three or all the four provinces are present, the iron belt appears close to the Pacific coast, followed by the copper, polymetallic and tin belts (the last one, some 500 km from the coast). Although these major provinces are defined by the predominance of one or two principal metals, they contain ore deposits of different ages, typologies and paragenesis. Both, the copper and the polymetallic provinces are present along the major part of the Andes, although some segments are extremely rich and others have scarce or null presence of mineral deposits. In exchange, the tin and the iron provinces are more restrained along the Andean belt but present an homogeneous distribution of the ore deposits.

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 100 M.t., containing 30% Fe (Oyarzun et al., 1984).

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 300 M.t. ore, containing 40% Fe (Angelelli et al., 1970).

The oolithic iron deposits of Colombia are part of a 650 km long NNE belt, between Lagunillas (Venezuela) and Sabanalarga (Colombia), that presents at least four zones of mineralized outcrops. The principal zone is that of Paz del Río (6º11’ N / 72º43’ W), where the oolithic iron formation is 0.5 to 8 m thick and crops out along 57 km in N30ºW direction, with a maximum wide of 8 km. The mineral ores contain 42-47% Fe and 0.8-1.2% P. Paz del Río, as other districts of the belt, was formed at the Upper Eocene, during a marine transgression, as a part of a sequence of sandstones and shales deposited in the beach-lagoon transition zone. The iron ore has an aluminous composition and chert content is low. Some 70 km south from Paz del Río, crops out the Sabanalarga formation, that present similar oolithic iron ores (Angulo, 1978).

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 200 M.t. (60% Fe) with some 2000 M.t. ore for the whole belt. The paragenesis of this Kiruna-type ore deposits includes low Ti-magnetite, actinolite and apatite as main species, as well as minor scapolite and a late sulfide phase (pyrite, minor chalcopyrite, etc.). Both, replacement and fracture filling, are observed, but replacement is dominant in the larger deposits. The mineralized complexes are formed by volcanic and sub-volcanic andesitic rocks, intruded by dioritic bodies, probably comagmatic with the andesitic rocks (Oyarzún and Frutos, 1984). The iron-bearing magmatic complexes presents horizontal sections that were originally circular (Boquerón Chañar) or ellipsoidal (El Romeral) but were later modified by strike-slip faulting. In general, faulting is intensive and extensive in the iron belt, and dynamic schists of this origin are frequent. At a regional scale, the alignement of the major deposits coincides with a pre-Cretaceous crustal weakness line that controled the western border of the Neocomian basin (the present Atacama Fault Zone).

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 2000 M.t. (60% Fe) by Petersen and Vidal (1996). Among the principal deposits are Huancabamba, Colquemarca, Livitaca and Tintaya, the last one considered as a copper deposit.

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 100 km wide, that also contain other types of ore deposits. However, if we considerer only those porphyry copper that have been selected for high tonnage mining operations, the field is geographically restrained to the sector between 10º S and 35º S and to those deposits of Tertiary age. They alone account for about a 25 to 30% of the world’s reserves and current production of both copper and molybdenum. As shown in Fig. 9, this sector is in close coincidence with the Andean segment that presents a thicker continental crust. The larger porphyry copper deposits of the segment, like Chuquicamata and El Teniente, attain ore reserves (before mining) up to 50 M.t. metallic Cu (Oyarzún and Frutos, 1980).

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 150 km long narrow band in SW Perú (Figs. 3 y 9). Also, six major Eocene-Oligocene deposits, including Chuquicamata, El Abra and Collahuasi, are distributed along a 125 km N-S line, between M.M. and Quebrada Blanca, following the important Domeyko fault system (Fig. 10). Other important cluster is that of the Los Bronces-Río Blanco and El Teniente (130 km south), the three deposits of Pliocene age.

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 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 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 1-10 M.t. ore range (Camus, 1985). Some typical deposits of this group are Buena Esperanza, Carolina de Michilla, Talcuna and Lo Aguirre (Fig. 2).

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 4 km soth from Chuquicamata, the source of the Cu mineralization, contained some 3-4 M.t. metallic Cu (before mining). Similar figures (1.2 to 3.5 M.t. metallic Cu) are given by Munchmayer (1996) for Damiana, in the western slope of Cerro Indio Muerto (El Salvador porphyry copper district). In general, exotic Cu deposits formed by lateral migration of supergene solutions from porphyry copper deposits. Their mineralogy (chrysocolla, atacamite, Cu-wad) was controlled by Eh-pH conditions of ground water, and their shape by the former topography around the porphyry. According to Munchmayer (1996), the exotic mineralization episodes mainly occured during the Lower Miocene.

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, La Joya (Long et al., 1992) has shown an important potencial as a gold and silver deposit with reserves before mining of 10 M.t. oxide ore, containing 1.65 g/t Au and 20 g/t Ag The ores are present in fractures of an intrusive dacitic body and were deposited at high temperatures (300º-550º C) from highly saline fluids. They are distributed in four hills: Kori Kollo, Llallagua, Quiviri and La Joya.

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 1970. A rapid increase began in 1980 (6 t), passing to 12 t in 1981 and to 18 t in 1982, as a result of the discovery and development of the high grade enargitic gold deposit of El Indio, the first of a series of discoveries in the Tertiary volcanic belt of the Andes between 26º S and 31º S (Cuadra and Dunkerley, 1991; Sillitoe, 1991). In 1996, a production of 53 t was attained.

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, San Cristobal, Fachinal). 3- Porphyry-type (Marte, Lobo, Refugio). 4- Distal contact, metasomatic (Andacollo). With the important exception of the Cretaceous Andacollo deposit (Reyes, 1991; Oyarzun et al., 1996), all of them are Tertiary in age. As pointed out by Gemutz et al. (1992) and by other authors, El Indio, Nevada and the Maricunga district (La Coipa, La Pepa, Marte, Lobo, Refugio, etc.) are located over an Andean segment characterized by a flat subduction zone, that also includes the Au-rich porphyries of the Farallón Negro district in Argentina (Sasso and Clark, 1998).

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, La Coipa and Fachinal). La Coipa (Ag/Au = 98), is properly an Ag-Au deposit.

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 1960, a series of porphyry copper prospect were detected and drilled. Several of them are now the basis for an important mining industry in Argentina. Also, after the discovery of El Indio, the Argentinean border of the Andes was rapidly explored and a number of precious metal deposits were discovered (some of them very close to the Chilean ones, e.g., Lama in the vecinity of the Pascua deposit). Besides, a group of goverment geologists discovered the Cerro Vanguardia and El Dorado districts at El Deseado Massif (Santa Cruz province) in southeast Argentina. Both district include epithermal, low sulfidation, Au-Ag and polymetallic veins. They are related to Middle Jurassic acidic magmatic episode, in the pre-rift tectonic conditions associated to the break-up of Gondwana (Giacosa et al., 1988; Echavarría and Etcheverry, 1998).


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 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 Sucre zone in Bolivia (Sureda et al., 1986).

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 150 km narrow N-S band between 24º10’ S / 64º23’ W and 25º15’ S / 65º06’ W. However, both their tonnage and grade are low.

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 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), La Esperanza (24º14’ S / 66º34’ W) and La Concordia (24º10’ S / 66º24’ W), are linked to dacited domes, and their mineralogy includes galena, sphalerite, chalcopyrite, pyrite, tetrahedrite etc. The major deposits of this group have before mining reserves up to 0.26 M.t. ore, containing 5-11% Pb, 1-6% Zn and 200-500 g/t Ag (Sureda et al., 1986).

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 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 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 20 km thick- make up a major part of the present Cordillera Real, where most of the Bolivian tin deposits of all types and ages are found.

Two types of tin deposits of Upper Triassic-Lower Jurassic age are known. The more abundant correspond to Sn-W veins associated to greisen-type alteration, within small batholiths (e.g., Yani, Sorata) or in the contact metamorphic zone imprinted by the batholiths in the Paleozoic sedimentary host rocks. The age of the batholiths emplacement is in the 257 to 150 M.a. span (Grant et al., 1980). Among the principal districts are those of Sayaquira, Caracoles and Araca. None of them attains the magnitude of the Tertiary Sn-Ag deposits.

The other type of Upper Triassic-Lower Jurassic tin deposits, which is found along a NW band, north of 19º S, present stratabound control of the ores. Although this type of tin deposit is not economical under present tin price conditions, its origin (syngenetic or epigenetic deposit of the ores) poses an interesting problem (Schneider and Lehmann, 1977). As stated by Lehmann (1985, 1990), the host rocks for the stratabound tin deposits are Lower Paleozoic metasedimentary rocks, wich are intruded by granites and granodiorites.

Kellhuani, one of the three principal stratabound-type tin deposits (Lehmann, 1985; 1990) is located some 15 km north of La Paz. About 10 M.t. ore, containing 0.5% Sn, are distributed in the quartzitic units of the Silurian Catavi Formation, while no ores are present in the interlayered black shale strata. The ore have been mined from several quartzitic horizons ("mantos") in the 16 x 4 km, NW trending district. Although the stratabound character of the mineralization (that is both concordant and discordant) favours a syngenetic sedimentary origin, the studies by Lehmann (1985) presented important evidences of the epigenetic-hydrothermal deposition of the ores (which include about 25 mineral species, several of them sulfides and sulfosalts, in addition to cassiterite, the economic mineral). A principal evidence is the zoned character of the geochemical halos and the hydrothermal alteration of the district around the Chacaltaya granite porphyry -dated at 213 +/- 5 Ma- a distribution in which cassiterite occupies a distal position. The fact that sericite alteration associated to cassiterite ore produced a similar age to that of the granitic intrusion, is considered by Lehmann (1990) as a further confirmation for the epigenetic origin of this Upper Triassic district.

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 group include such important deposits as Llallagua, Cerro Rico and Chorolque. Although their principal economic mineralization is vein-type, they also contain, as a whole, some 80 M.t. of disseminated ore grading 0,3% Sn, wich is still far from attaining economic interest, but represents an important reserve for the future. Five principal geological-mineralogical traits are common to the deposits of this groups: 1-The mineralization is centered on small (1-2 km2) porphyric stocks, emplaced under or whitin volcanic pipes. 2- Several pulses of intrusion and breccification are observed. Some stocks are converted to breccia pipes. 3- The stocks and their host rocks have suffered and intense and penetrative feldspar-destructive hydrothermal alteration, in which sericite and tourmaline predominate. 4- The mineralization is very complex. The main sulfides that accompany the cassiterite are pyrite, stannite, chalcopyrite, sphalerite and arsenopyrite. 5- The disseminated mineralization is earlier than the high-grade vein-type one. The radiometric dating by Sr isotopy have yielded Miocene ages like 20 Ma at Llallagua, 15-14 Ma at Cerro Rico and 17-12 Ma at Chorolque (Grant et al., 1980).

The magmas related to tin mineralization usually have a much differentiated petrological evolution (Lehmann, 1990). Although some magmas related to the Bolivian tin porphyries are evolved, like at Karikari, Potosí, where peraluminous, high initial Sr isotopic ratios (0.707-0.716) magmas, evolved from andesite to toscanite (Grant et al., 1980), in general, tin porphyries are associated with only moderately fractioned subvolcanic rocks of rhyodacitic composition. However, the recent paper by Dietrich et al. (1999) provided analytical evidence (melt inclusions data) for the origin of the Bolivian tin porphyry magmas by mixing of high evolved silicic melts -containing quartz phenocryts- with andesitic to basaltic melt fractions, in an upper crustal reservoir. We will back again to this section on Andean magmas.

In the group of "non-porphyric" deposits are included vein-type Sn mineralizations, hosted in Paleozoic clastic rocks that are not related to outcroping intrusive bodies (except dykes). Among them are the Colquiri (fluorite-sphalerite-cassiterite); Huanuni, 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 150 km eastward shift of the magmatic belt, are rather an anomalous trait of calc-alkaline magmatism. The porphyry magmas had to cross a thickened crust, a consequence of the Upper Eocene Incaica compresive stage (a condition in common with the Paleocene porphyries of SW Perú and with the Pliocene porphyries of central Chile). However, they exhibit the lower posible degree of crustal contamination. Therefore, a rapid, diapiric-style of ascent through the crust of deeply generated magmas has been proposed by Maksaev and Zentilli (1988) for the porphyries.. This model is not consistent with the simple relationships of porphyry copper deposits to normal batholiths that the model by Sillitoe (1973) implicates.

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, 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 Andes (Fig. 10) were emplaced along the fault zone, which had a dominant dextral displacement.

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 metallogenic 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 metallogenic terms, an evolved crust implies a higher degree of structural complexity, better opportunities for magma mixing, contributions from sedimentary strata with different chemical compositions etc. Also, a number of geological levels, from the asthenosphere to the sedimentary strata may participate in the generation and differentiation of magmas and in the genesis of the ore deposits resulting of their emplacement and interactions with the host rocks and fluids in the upper levels of the crust.


Acknowledgments-The present contribution has a far background in a doctoral thesis presented at 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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