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Chiaradia, M., and Fontboté, L., 2003. Magmatic and tectonic controls on the formation of Tertiary porphyry and epithermal deposits of Ecuador. In D. Eliopoulos et al. eds., Mineral Exploration and Sustainable Development, Millipress, in press.

Magmatic and tectonic controls on the formation
of Tertiary porphyry and epithermal deposits of Ecuador

Massimo Chiaradia
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K.

Lluís Fontboté
Section des Sciences de la Terre, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland

Keywords: Ecuador, Tertiary, porphyry-Cu, epithermal, magmatism

ABSTRACT: Tertiary porphyry and epithermal deposits of Ecuador are associated with Eocene-Late Miocene (ELM) magmas that evolved through plagioclase fractionation and assimilation of radiogenic basement rocks in parental magma chambers situated at high crustal levels. Late Miocene-Recent (LMR) magmatic rocks, in contrast, have evolved from magmas that underwent plagioclase-free fractionation and assimilated residual garnet-bearing mafic rocks at deep crustal levels and no mineralization is apparently associated with them. Metallogenic implications of the association of magmatic-related deposits with ELM rather than with LMR magmatism are discussed in the frame of the Tertiary geotectonic evolution of Ecuador.

1. INTRODUCTION

Epithermal and porphyry-Cu deposits are genetically associated with shallow level intrusions. Their formation results from the interplay of magmatic, tectonic and hydrothermal processes. In this study we present geochemical data of Eocene to Late Miocene (ELM: 55-6 Ma) magmatic rocks that are spatially and temporally associated with the historically mined Au-bearing porphyry and epithermal deposits of Ecuador. We show that these “fertile” magmas are calc-alkaline and have evolved through AFC (assimilation and fractional crystallization) processes at high crustal levels. We also present geochemical data on Late Miocene-Recent (LMR: 7-0 Ma) magmatic rocks of Ecuador, which are apparently not associated with significant magmatic-related ore deposits, and show striking geochemical differences when compared to the “fertile” ELM rocks. Metallogenic implications of the association of magmatic-related deposits with ELM rather than with LMR magmatism are discussed.

2. GEOTECTONIC SETTING

Ecuador consists of several terranes (Piñon, Macuchi, Chaucha, Tahuin, Alao, Loja, Salado) that were formed during the Triassic separation of the North and South American plates and the subsequent subduction of the Farallon/Nazca plate under the South American continent (Fig. 1). These terranes have been accreted onto the Amazon craton from Early Cretaceous to Eocene. After complete accretion of the Ecuadorian crust, the Tertiary continental arc magmatism resulted from the subduction of the Farallon/Nazca plate.

Figure 1: Geotectonic map of Ecuador (modified from Litherland et al. 1994)
with location of the investigated samples and porphyry/epithermal deposits.

Splitting of the Farallon plate into Nazca and Cocos at ~25 Ma caused orthogonalization and flattening of the subduction zone (Daly 1989). Subductions of the inferred Inca plateau at 10-12 Ma (Gutscher et al. 1999a) and of the Carnegie ridge possibly since 8 Ma (Gutscher et al. 1999b) caused a further subduction flattening with consequent arc widening and the onset of widespread volcanism during Pliocene in central and northern Ecuador. As a consequence of this geotectonic evolution, the Northern Andes have been subjected to a transpressional regime from Late Oligocene to Miocene (27-5 Ma), whereas they have been mainly characterized by a compressional regime from Pliocene to Recent (5-0 Ma) (Noblet et al. 1996).

Figure 2. Sr/Y vs. Eu/Gd diagram of the investigated samples
showing the different evolutionary trends of the ELM and LMR groups.

ELM rocks, with which Tertiary Au-bearing porphyry and epithermal deposits are associated, crop out in the southwestern part of Ecuador, which is characterized by the presence of crustal-scale, arc normal E-W to NE-SW trending faults and sutures of the Huancabamba deflection, representing the transition between the Central and Northern Andes (Fig. 1). The LMR rocks, with which no major porphyry/epithermal ore deposits are associated, crop out mostly in northern Ecuador and east of the ELM rocks in southern Ecuador, in a crustal domain characterized by the presence of arc-parallel NNE-trending suture zones and by the absence of arc normal structures (Fig. 1).

3. RESULTS AND DISCUSSION

3.1 Magmatic-related ore deposits

The major districts of porphyry (e.g., Chaucha, Gaby, Fierro Urcu, San Gerardo, El Torneado, Los Linderos, Laguar,) and epithermal (e.g., Portovelo-Zaruma, Bella Rica, Tres Chorreras, Gigantones, La Tigrera, La Playa, Peggy, Gañarin, El Mozo) deposits of Ecuador are concentrated in the southern part of the country and are preferentially located in proximity of terrane sutures where also Oligo-Miocene (ELM) intrusive rocks are emplaced (Fig. 1).

Geochronological data on ore and alteration minerals of these deposits and/or on the associated magmatic rocks indicate that mineralization took place between 10 and 30 Ma. Therefore, the geographic and temporal distributions suggest that these deposits are genetically related with ELM and not with LMR magmatism.

Figure 3. Pb and Sr isotope ratios vs. Eu/Gd diagrams of the investigated samples
showing the different evolutionary trends of the ELM and LMR groups

Lead isotope compositions of the major Oligo-Miocene deposits of Ecuador largely fall within the compositional field of the Tertiary magmatic rocks suggesting a magmatic derivation of lead and by inference of the other economic metals in these deposits.

3.2 Magmatic rocks

Among the analyzed samples, the least evolved ELM and LMR rocks have similar low radiogenic lead and strontium isotope compositions as well as REE signatures suggesting a common source for their parent magmas (Figs. 2 and 3), possibly an enriched MORB-type mantle. In contrast, radiogenic isotopes as well as trace and rare earth element geochemistry indicate striking different evolutions between ELM and LMR rocks (Figs. 2, 3, 4). The “fertile” ELM rocks are calc-alkaline and have consistently low Sr/Y and decreasing Eu/Gd with fractionation indices (Fig. 2), suggesting an evolution through plagioclase-dominated fractional crystallization. Covariations of lead and strontium isotopes with the Eu/Gd ratio (Fig. 3) suggest that the parental magmas of “fertile” ELM magmatic rocks have assimilated Pb- and Sr-radiogenic basement rocks while evolving through plagioclase-dominated fractionation (AFC process). Because plagioclase is stable at pressures <0.5-0.7 GPa in hydrous tholeiitic/andesitic magmas, ELM rocks must have evolved in parental magma chambers situated at high crustal levels (<20 km). A prolonged evolution of parental magmas at high crustal levels seems to be necessary to form magmas conducive to porphyry-Cu and related deposits (Tosdal & Richards 2001) and this may explain the “fertile” nature of ELM magmas.

Figure 4. Chondrite-normalized plots of magmatic rocks of the ELM and LMR groups.

In contrast with ELM rocks, LMR rocks, which are apparently not associated with mineralization, have consistently higher Sr/Y (>20-180: Fig. 2), lower Yb (<1.5: Fig. 4) and increasing Eu/Gd ratio with fractionation indices (Fig. 2). Lead and strontium isotopes correlate with the Eu/Gd ratio in an opposite way to that of ELM rocks (Fig. 3). Overall, LMR rocks have adakite-type geochemical signatures. Trace and rare earth elements as well lead and strontium isotopes preclude that the adakite-type LMR rocks of Ecuador investigated in the present study derive from slab melting.

Figure 5. Geotectonic models for the generation of the ELM (top) and LMR (bottom) magmas
in Ecuador and metallogenic implications inferred from Tosdal and Richards (2001).

Rather, they suggest that LMR rocks were derived from the mantle wedge like ELM rocks, but evolved through plagioclase-free fractionation and assimilation of moderately Pb- and Sr-radiogenic garnet-bearing basic lithologies (possibly underplated Jurassic metabasalts of the Alao arc: Fig. 3). The involvement of a source containing residual garnet in the formation of the LMR rocks is required to explain their strong HREE depletion (La/Yb up to 70: Fig. 4). The absence of plagioclase fractionation and the involvement of residual garnet in the formation of LMR rocks suggest that their parental magma chambers were situated at great depth (>35-40 km), most likely at the base of the lithosphere. Evolution of parental magmas in chambers situated at these depths is not favorable to the formation of porphyry-Cu and related deposits (Tosdal & Richards 2001). Additionally, Cu and Au may be sequestered by magmatic sulfides crystallizing at depth (Candela & Piccoli 1998). The residual magmas rising to high crustal levels would thus be Cu- and Au-depleted.

The different evolutions of the ELM and LMR magmas and their metallogenic implications could reflect changing geotectonic regimes through time and space (Fig. 5). ELM rocks were formed during a period of dominant transpression and were emplaced in a crustal domain characterized by the presence of arc-normal structures. The combination of these factors allows a localized passive ascent of magmas to high crustal levels along arc-normal structures or in dilational jogs along arc-parallel structures (Fig. 5). In contrast LMR rocks were formed during a period of major tectonic compression and in a crustal domain characterized by the presence of arc-parallel structures but lacking of arc-normal structures. During compression, arc-parallel structures are compressed and magmas are impeded to rise by buoyancy. Under these conditions LMR magmas ponded at the base of the lithosphere and could rise only episodically perhaps as overpressured magmas (Fig. 5: see also Tosdal & Richards 2001).

4. CONCLUSIONS

Tertiary magmatic-related Au-bearing deposits of Ecuador display a preferential association with Eocene to Late Miocene (ELM) calc-alkaline magmas evolved from parental magma chambers at high crustal levels rather than with Late Miocene-Recent (LMR) magmas evolved in chambers at the base of the lithosphere. This preferential association could ultimately result from a changing subduction style through time and from the interplay between structures of the Ecuadorian crust and subduction styles. The different geochemical signatures (and ages) of ELM and LMR rocks and their bearings on the formation of porphyry-Cu and epithermal deposits could be used as an exploration tool in Ecuador.

ACKNOWLEDGEMENTS

We thank Michael Dungan (University of Geneva, Switzerland) and Bernardo Beate (Escuela Politécnica Nacional, Quito, Ecuador) for stimulating discussions, Agustín Paladines (Universidad Central, Quito, Ecuador) and Jaime Jarrín (Ministerio de Energia y Minas, Quito, Ecuador) for assistance during sample collection, and the Swiss National Science Foundation for funding this study.

REFERENCES

Candela, P.A. & Piccoli, P.M. 1998. Magmatic contributions to hydrothermal ore deposits: an algorithm (MVPart) for calculating the composition of the magmatic volatile phase. Reviews in Economic Geology Vol. 10: 97-108.
Daly, M.C. 1989. Correlation between Nazca/Farallon plate kinematics and forearc basin evolution in Ecuador. Tectonics Vol. 8: 769-790.
Gutscher, M.-A., Olivet, J.-L., Aslanian, D., Eissen, J.-P. & Maury, R. 1999a. The “lost Inca Plateau”: cause of flat subduction beneath Peru? Earth and Planetary Science Letters Vol. 171: 335-341.
Gutscher, M.-A., Malavieille, J., Lallemand, S. & Collot, J.-Y. 1999b. Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision. Earth and Planetary Science Letters Vol. 168: 255-270.
Litherland M., Aspden J.A. & Jemielita R.A. 1994. The metamorphic belts of Ecuador. Overseas Memoir 11. BGS, Keyworth, U.K.: 147 p.
Noblet, C., Lavenu, A. & Marocco, R. 1996. Concept of continuum as opposed to periodic tectonism in the Andes. Tectonophysics Vol. 255: 65-78.
Tosdal, R.M. & Richards, J.P. 2001. Magmatic and structural controls on the development of porphyry Cu±Mo±Au deposits. Reviews in Economic Geology Vol.14: 157-181.

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