Space-Time Relations Between Mineralization and Argentinian Andes D

Home , Farallon Negro (volcano), Fumarole mineral

Proceedings 24* New Zealand Geothermal Workshop 2002

SPACE-TIME RELATIONS BETWEEN MINERALIZATION AND ARGENTINIAN ANDES D. G. c.

'Instituto de Geologia y Recursos Minerales, SEGEMAR - CONICET, Buenos Aires, 2Geological Survey of Canada, Vancouver BC, Canada

SUMMARY - New regional geological compilation and field work, including petrological and geochronological data, facilitated the reconstruction of the volcanic history of the southern end of Central Volcanic Zone in Argentina (between Cenozoic arc migration and broadening extended volcanism over several geological provinces such as Cordillera Frontal, Precordillera, Famatina and Pampean Ranges. Four major volcanic events built different volcanic arcs in this Andean segment: 1) a Paleogene magmatic arc, 2) an Oligocene-Early Miocene volcanic arc (the Maricunga Belt), 3) a Late Miocene volcanic arc which migrated and broadened to the east, and 4) a Pliocene-Quaternaryvolcanic arc that is today represented by fumarolic activity on Nevado's Ojos del Salado volcano. The volcanism developed above a thickened crust and was accompanied by the emplacement of important copper-bearing porphyries and development of epithermal precious metal deposits.

1. INTRODUCTION processes (Allmendinger et al., 1997). On the basis of geochemistry and geophysical data, Kay The Andean mountain chain encompasses the and Mpodozis (2002) used magmatism as a tool in record of a classic magmatic welt from subduction deciphering the Neogene tectonic evolution. of oceanic crust under a continent. The Central The Andean segment between is Volcanic Zone (CVZ) of the Andes, an active increasingly important in terms of economic volcanic arc (between developed along mineralization associated with Cenozoic 1500 km of continental margin between Southern volcanism. Peru and Northwestern Argentina. It is flanked to The Maricunga Belt is an important gold district the north and south by zones of sub horizontal of the Central Andes, 20 km wide and about 200 subduction (flat-slab zone) lacking active- km long in a N-S direction, starting in Chile and volcanism (Barazangi and Isacks, 1976) and is continuing into Argentina south of It bound by the Altiplano-Puna high plateau contains important epithermal deposits of gold and (average elevation of m above see level). silver (La Coipa, Arqueros, The Argentinean Andes mountain range between Marte and Cerro Casale), which are associated 27" and S (Figure 1) includes the northern end with central volcanic structures, dome complexes of the flat-slab zone and the southern end of the and hypabyssal intrusions. Ages range from 24 to CVZ. From Argentina to the Chilean border four 12 Ma (Sillitoe et al., 1991; Mpodozis et al., morpho-structural units converge: Sierras 1995). On the western border of the present Pampeanas, Famatina, Precordillera and Andean segment some exploration targets have Cordillera Frontal, (Figure 1). This is also a zone been identified Valle Ancho, Laguna Verde, of transition between the Altiplano-Puna Plateau Salado, Tres Quebradas) (Rubiolo, 1999) and to the north and the basement cored Sierras farther east, in the Sierras Pampeanas, important Pampeanas to the south. Cu-Au-bearing porphyries and epithermal deposits The subduction angle of the Nazca plate changes are found. from E, (under the Puna), to sub-horizontal Near the Chilean border there are mining south of S in this area (Cahill and Isacks, exploration targets Laguna Verde, Valle 1992). Neogene andesitic volcanism also Ancho, Rio Salado) with similar characteristicsto disappears to the south. In the flat-slab the epithermal Au-Ag Maricunga Belt deposits. In subduction zone there is eastwards migration of the eastern extremes of this segment, important the volcanic arc and deformation in Argentina and Cu-bearing porphyries and hydrothermal deposits Chile (Mpodozis et al., 1996; 1997). This region have been identified Farallon Negro, Bajo La also hosts the earth's highest stratovolcanoes Aumbrera, Agua etc.). Many Cu-Ag-Au composed of thick sequences of andesitic to mineral deposits are associated with Cenozoic dacitic lavas and pyroclastic flows intruded by volcanic rocks, and have some specific dacitic to rhyolitic domes. petrochemical characteristics. The Cenozoic volcanism is characterized by eruption through thick continental crust, in places 2. METHODOLOGY AND OBJECTIVES over 70 thick (Beck et 1996). The extraordinary crustal thickening is essentially a In order to understand the space-time relationship Neogene phenomena, attributable to structural between volcanism, tectonism and metallogeny, it

73 SIE R RAS

Figure 1: Location of the study area in the Andes 3.2 Magmatic Stage 2 between 27" and (Argentina).

During the Middle Miocene (17-11 Ma) the was fundamental to prepare an updated regional volcanic arc continued migrating to the east, geologic map on scale This work where it is dominated by intermediate volcanic compiled new and existing information on the rocks (Mmv) and pyroclastic felsic rocks (Mmt). regional distribution of eruptive centres and A second mineralization episode in the Maricunga magmatic rocks, petrology, geochemistry of major Belt is linked to the emplacement of 14-12 Ma and trace elements (Figures 3 and 4), and mineral fault-controlled "gold porphyries" Mina and whole rock and isotopic dates Marte; Sillitoe et al., near the end of the (Figure 2). These new regional studies were done second volcanic stage. This mineralization is (in part under the Multinational Andean Project), found only in subvolcanic domes or porphyritic from Sierras Pampeanas to the Chilean border rocks, and erosional remnants of pyroclastic flows between 27" and 28" S. free of hydrothermal alteration.

3. CENOZOIC MAGMATIC HISTORY 3.3 Magmatic Stage 3

The Cenozoic magmatic history began with the In the Late Miocene (11-5 Ma) the volcanic arc development of a 'Paleogene volcanic arc in rapidly migrated and broadened to the east. Chilean territory (40-28 Ma). Neogene volcanism Predominantly andesitic and dacitic rocks (Msv) started about 26 Ma, and migrated to the east from were erupted accompanying by dacitic pyroclastic 17 to 12 Ma. At this time, the flat subduction flows (Mst). Farther east, copper porphyries and zone developed in the northern Puna (Kay et al., epithermal gold mineralization (9 - 6 Ma; 1999). The distribution of the igneous units in and Clark, 1988) occurred in the Pampean Ranges space and time is shown in Figure 2. in the Farallon Negro volcanic field Mina La Alumbrera, Agua Rica, not on the map). 3.1 Magmatic Stage 1 Hyaloclastite and till material interbedded with dacitic-andesitic lavas from the Cordillera Frontal During the Oligocene-Lower Miocene (27-17 Ma) (7.59 0.08 Ma, dated by Rubiolo et a volcanic arc formed the Maricunga Belt 2000) show evidence of interaction between (Mpodozis et al., about 200 km long by 20 felsic magma and glacial ice in a region today km wide. This volcanic belt was weakly dominated by strongly arid conditions. This developed in Argentina between and is suggests a humid cold climate during the Late represented by intrusive rocks and porphyries Miocene for the Central Andes at the S., as and volcanic and volcano-sedimentary opposed to the extreme dryness that dominates at rocks In addition, a series of the region at this moment. Also Neogene intramontane basins with continental red-bed sedimentary deposits to the east (in the Pampean deposits developed The Maricunga belt Ranges), contemporary with those volcanic rocks, contains important epithermal Au-Ag and show evidence of a climate more humid than the porphyry Cu-Au deposits associated with central present one. Strong has taken place volcanic structures, dome complexes and since the Pliocene in association with uplift of the hypabyssal intrusions. Ages range from 23-21 Ma Pampean Ranges (sierras de Aconquija and (Sillitoe et al., 1991; Mpodozis et al., 1995; Ambato) in the east. Zappettini et al., 2001).

74 28 00' Km

CORDILLERA FRONTAL SIERRAS

QUATERNARY PLIOCENE

LATE

EARLY OLIGOCENE EOCENE

PALEOCENE LATE CRETACEOUS

LATE

0 W

EARLY

Figure 2

Figure 2:Regional overview of Andean geology between and in Argentina. Pre-Cenozoic units: Peis: Low to medium grade metamorphic rocks; Piv: Marine sedimentary rocks with interbedded volcanic sequences, locally mafic and ultramafic rocks; Pig: Intrusive rocks Ma); Intrusive rocks (420-200 Ma); Volcanic and volcano-sedimentaryrocks. Cenozoic volcanic and volcano-sedimentary units: Volcanic and volcano-sedimentary rocks (80-55 Ma); Intermediate volcanic rocks Ma); Magmatic stage Intrusive rocks and porphyries (27-17 Ma); OMiv: Volcanic and volcano-sedimentary rocks (27-17 Ma); Magmatic stage Intermediate volcanic rocks (17-11 Ma); Mmt: Pyroclastic rocks (17-11 Ma); Msv: volcanic rocks (1 1-5 Ma); Mst: Pyroclastic rocks (11-5 Ma); Magmatic stage Intermediate to mafic volcanic rocks. Include debris flows Ma); Magmatic stage Domes and domes complexes Ma), pyroclastic rocks Ma). Sedimentary rocks: Prs: Marine and continental sedimentary rocks; Continental sedimentary rocks; Continental sedimentary rocks with evaporite intercalations; Continental sedimentary rocks with pyroclastic intercalations.

75 3.4 Magmatic Stage 4

From the Pliocene to Quaternary Ma), the volcanic arc contracted and is now active only in the present border area between Argentina and Chile. It is comprised of felsic domes pyroclastic rhyolitic-dacitic flows (PIQt) and dacitic-andesitic volcanic rocks During the Quaternary some rare intermediate and basic 4 lavas were erupted Incahuasi) and Increasing thickness. . explosive activity has occurred. At the present . . . I I I I I time volcanism is represented by fumarolic 2 4 6 10 12 activity on Nevado's Ojos del Salado volcano Px (6864 m). Figure 4: Plot of versus ratios monitoring the behaviour of the slopes of the light 3.5 Chemical composition and heavy REE, respectively. Increasing heavy REE ratios are attributed from pyroxene (Px) to The Cenozoic magmatic units belong to a high-K increasingamounts of amphibole (Hb) and garnet calc-alkaline series and ranges from basaltic (Gt) in the mineral residue of the magmas andesite to rhyolite. The geochemistry of major (symbols are the same as in Figure 3). and REE elements (Figure 3 and 4) can serve as a guide to relative crustal thickness. Increasing ratios mostly reflect pressure-dependant 4. DISCUSSION changes from clinopyroxene to amphibole to garnet in the mineral residue in equilibrium with The Andean segment between S is an evolving magmas (Kay et 1999). interesting region for studying magmatic arc migration and expansion because it: 1) includes development of the Cenozoic magmatic arc across different geological provinces or structural units; b) represents the southern end of the CVZ of the Central Andes, with a transition, towards the south to the zone of flat-slab subduction; and c) includes important epithermal deposits and Cu-bearing porphyries associated with the Cenozoic volcanism. This region also includes Cenozoic and Holocene pyroclastic deposits that are exceptionally well preserved. Some eruptive centres are also potentially active (volcanoes Ojos del Salado, Tres Cruces, Incahuasi, Incapillo, etc). 50 55 60 65 70 75 Petrologic and a regional studies associated the magmatism in this region with changes in the Figure 3: Plot of versus (Peccerillo and subductionangle (Coira and Kay, 1993). Taylor, 1976). The Cenozoic magmatic units These characteristics could reflect changes in the belong to a high-K calc-alkaline series and ranges subduction geometry (Mpodozis et al., 1995; basaltic andesite to rhyolite. Sasso and Clark 1998;Kay et al. 1999). During Late Miocene time, deformation ceased in ratios can serve as a guide to relative the Cordillera Frontal and Altiplano, when the crustal thickness. REE patterns of volcanic rocks tectonically active domain was displaced to the from the first magmatic stage (Omiv) suggest east, upon the initiation of compressive amphibole-bearing lower crustal residues as the deformation at the foreland Cenozoic origin (Figure 4) consistent with a thickened crust foreland basins and a fold and thrust belt over a shallower subduction zone (Kay et al., accompanies the volcanic arc migration towards 1999). This followed by the release of fluid in the east and broadening of the arc in the Neogene conjuction with breakdown of the to reach the Pampean Ranges. This phenomenon bearing crystal assemblages in the crustal melt has been considered a consequence of crystalline zones during subhorizontal shortening and basement thermally weakened by the Late thickening of the crust. During the mineralization Cenozoic magmatism. stage the amphibole breaks down to garnet in a Uplift and deformation of Cordillera Frontal, thickening lower crust in response to increasing Precordillera, Famatina and Sierras Pampeanas pressures. The REE pattern for these volcanic follows the eastward propagation of arc rocks show garnet residual in the source phase magmatism. This implies that uplift (Kay et al., 1999). and deformation are more likely related to thermal weakening and crustal anisotropies than to

76 fluctuations in horizontal compressive stress ratios consistent with a more garnet-rich residual (Ramos et al., 2002). Eastward propagation of mineralogy at deep levels of a thicker crust. Near --- magmatism to the foreland since Late Miocene the end of this second magmatic stage a should have played an important role in the mineralization episode linked to the emplacement thermal crustal history. The crust developed of 14-12 Ma "gold porphyries'?at Maricunga Belt brittle-ductile transitions allowing uplift of an old (Sillitoe et al., 1991) occurred. REE pattern of basement-cored block. these magmas indicate also an amphibole-bearing Variations in subduction geometry during residue. Cenozoic time were responsible for a broadening The final mineralization in the Andean segment of the magmatic arc during the Late Miocene and between 27" occurred at 9-6Main the therefore higher thermal gradients. These high Farallon Negro Volcanic Complex to the east gradients favoured softening of the lower crust and Clark, 1998). Sincethe Pliocene, uplift and later, and importantly, thickening and uplift of of the Pampean Ranges (sierras de Aconquija and the basement cored Pampean Ranges to the east. Ambato) in the east (Figure 1)has occurred. The chemical, spatial and temporal distribution of During the Quaternary, some rare intermediate late Oligocene to Recent magmatism in the CVZ and mafic lavas were erupted and explosive can be broadly explained by changes in the dip of activity has occurred in the Cordillera Frontal. At the subducting Nazca Plate and the thickness of the present time volcanism is represented by the lithosphere mantle and crust beneath this fumarolic activity on Nevado's Ojos del Salado segment of the Andes (Coira and Kay, 1993). volcano (6864 m). Temporal changes in lithosphere and crustal thickness were tracked by using REE elements as guides to pressure-sensitive residual minerals and 6. ACKNOWLEDGEMENTS source melting percentages (Kay et al., 1999). As a rough guide in mafic lavas, clinopyroxene is We thank to Bob Anderson (GSC-Vancouver)for dominant at depths of amphibole ca constructive review. This paper has GSC and garnet at km. Kay et al. contribution number 2002136. (1999) present a model in which mineralization is linked to changes in crustal and lithospheric 7. REFERENCES thickness induced by evolving geometry of the subducting Nazca plate. Allmendinger, R.W., Jordan, T.E., Kay S.M. and The mineralization stages are linked to the Isacks B.L. (1997). The evolution of the evolution of fluid phases in conjuction with Altiplano-Plateau of the Central Andes, Annual breakdown of the amphibole-bearing crystal Reviews Earth Planetary Sciences, Vol. 25, 139- assemblages in the crustal melt zones during 174. subhorizontal shortening and thickening of the crust. As a result of amphibole breakdown and Barazangi, M. and Isacks, B. (1976). Spatial mineralization, garnet is stable in a thickening distribution of earthquakes and subduction of the lower crust in response to increasing pressures. Nazca Plate beneath South America. Geology, The REE pattern for volcanic rocks derived 4,686-692. melting these source rocks show garnet residual in the source phase (Kay et al., 1999). Beck, Zandt, G., Myers, S.C., Wallace, T.C., Silver, P.G. and Drake, L. (1996). Crustal- 5. CONCLUSION thickness variations in the central Andes. Geology, Vol. 24,407-410. A recent regional geological compilation and new field work has added petrologic and Cahill, T. and Isacks, B. (1992). Seismicity and geochronologic data, increasing our understanding shape of the subducted.Nazca Plate. Journal of of this region and permitting a more detailed Geophysical Research. B, Solid Earth and Planets, reconstruction of the volcanic history of the 97, 17503-17529. region. The Cenozoic magmatic history began with the Coira, B. and Kay, S.M. (1993). y development of a Paleogene volcanic arc in levantamiento de la Puna, con cambios Chilean territory (40-28 Ma). Neogene volcanism en el de y en el espesor started about 26 Ma, and migrated to the east fkom cortical. Actas del Congreso Geolbgico 17 to 12 Ma. At that time, the flat subduction Argentino y 2" Congreso de de zone developed in the northern Puna (Coira and Hidrocarburos,Vol. 3,308-319. Kay, 1993). REE patterns of the first magmatic stage suggest Kay, S.M. and Mpodozis, C. (2002). Magmatism amphibole-bearing lower crustal residues as the as a probe to the Neogene shallowing of the Nazca origin (Figure 4) consistent with a thickened crust plate beneath the modem Chilean flat-slab. over a shallower subduction zone (Kay et al., Journal of South American Earth Sciences, Vol. 1999). Andesitic units that erupted in the second magmatic stage have higher REE

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