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Cenozoic continental arc magmatism and associated mineralization in Ecuador

Beate, Bernardo
Published in Mineralium Deposita. 2004, vol. 39, no. 2, p. 204-222
Collection Open Access - Licence nationale Springer
Abstract Most of the economic ore deposits of Ecua- dor are porphyry-Cu and epithermal style gold deposits associated with Tertiary continental arc magmatism. This study presents major and trace ele- ment geochemistry, as well as radiogenic isotope (Pb, Sr) signatures, of continental arc magmatic rocks of Ecuador of Eocene to Late Miocene (􏰃50–9 Ma, ELM) and Late Miocene to Recent (􏰃8–0 Ma, LMR) ages. The most primitive ELM and LMR rocks ana- lyzed consistently display similar trace element and isotopic signatures suggesting a common origin, most likely an enriched MORB-type mantle. In contrast, major and trace element geochemistry, as well as radiogenic isotope systematics of the whole sets of ELM and LMR samples, indicate strikingly different evolutions between ELM and LMR rocks. The ELM rocks have consistently low Sr/Y, increasing Rb/Sr, and decreasing Eu/Gd with SiO2, suggesting an evo- lution through plagioclase-dominated fractional crys- tallization at shallow crustal levels (<20 km). TheLMR rocks display features of adakite-type magmas (high Sr/Y, low Yb, low Rb/Sr) and increasing Eu/Gd and Gd/Lu ratios with SiO2. We explain the adakite- type geochemistry of LMR rocks, rather than by slab melting, by a model in which mantle-derived melts partially melt and assimilate residual garnet-bearing mafic lithologies at deeper levels than those of pla- gioclase stability (i.e., >20 km), and most likely at sub-crustal levels (>40–50 km). The change in geo- chemical signatures of Tertiary magmatic rocks of Ecuador from the ELM- to the LMR-type coincides chronologically with the transition from a transpres- sional to a compressional regime that occurred at 􏰃9 Ma and has been attributed by other investigations to the onset of subduction of the aseismic Carnegie ridge. The major districts of porphyry-Cu and epithermal deposits of Ecuador (which have a small size, <<200 Mt, when compared to their Central Andean counterparts) are spatially and temporally associated with ELM magmatic rocks. No significant porphyry-Cu and epithermal deposits (except the epithermal high- sulfidation mineralization of Quimsacocha) appear to be associated with Late Miocene-Recent (LMR, 􏰃8–0 Ma) magmatic rocks. The apparent ‘‘infertility'' of LMR magmas seems to be at odds with the association of major porphyry-Cu/epithermal deposits of the Central Andes with magmatic rocks having adakite-type geo- chemical signatures similar to LMR rocks. The paucity of porphyry-Cu/epithermal deposits associated with LMR rocks might be only apparent and bound to exposure level, or real and bound (among other possi- bilities) to the lack of development of shallow crustal magmatic chambers since 􏰃9 Ma as a result of a pro- longed compressional regime in the Ecuadorian crust. More work is needed to understand the actual metallo- genic potential of LMR rocks in Ecuador.
Full text
Research groups Isotope Geochemistry, Geochronology and Thermochronology
Mineral Resources and Geofluids
(ISO format)
CHIARADIA, Massimo, FONTBOTÉ, Lluis, BEATE, Bernardo. Cenozoic continental arc magmatism and associated mineralization in Ecuador. In: Mineralium Deposita, 2004, vol. 39, n° 2, p. 204-222. doi: 10.1007/s00126-003-0397-5 https://archive-ouverte.unige.ch/unige:19386

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Deposited on : 2012-04-16

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