Cenozoic continental arc magmatism and associated mineralization in Ecuador

Abstract

Most of the economic ore deposits of Ecuador are porphyry-Cu and epithermal style gold deposits associated with Tertiary continental arc magmatism. This study presents major and trace element 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 analyzed 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 SiO 2 , suggesting an evolution through plagioclase-dominated fractional crystallization at shallow crustal levels (<20 km). The LMR rocks display features of adakite-type magmas (high Sr/Y, low Yb, low Rb/Sr) and increasing Eu/Gd and Gd/Lu ratios with SiO 2 . We explain the adakitetype 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 plagioclase stability (i.e., >20 km), and most likely at sub-crustal levels (>40-50 km). The change in geochemical signatures of Tertiary magmatic rocks of Ecuador from the ELM-to the LMR-type coincides chronologically with the transition from a transpressional 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 highsulfidation 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 geochemical 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 possibilities) to the lack of development of shallow crustal magmatic chambers since $9 Ma as a result of a prolonged compressional regime in the Ecuadorian crust. More work is needed to understand the actual metallogenic potential of LMR rocks in Ecuador.

Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

“…One hypothesis is that the signature is retained from slab melting (e.g., Bourdon et al 2002Bourdon et al , 2003Samaniego et al 2002;Hidalgo et al 2007). Another hypothesis considers them as resulting from lower crustal assimilation-fractional crystallization (AFC) and partial melting processes (e.g., Kilian et al 1995;Monzier et al 1999;Garrison and Davidson 2003;Chiaradia et al 2004a;Garrison et al 2006). Major and trace element variations as well as radiogenic isotope compositions coupled to petrographic observations indicate that the adakite-like products of each volcanic center investigated are the result of multiple magmatic processes, including fractional crystallization, assimilation and mixing within the crust.…”

“…Transition from non-adakitic to adakite-like rocks in Ecuador Chiaradia et al (2004a, b) pointed out a clear difference between the geochemical evolution of Middle to Late Miocene (pre-6 Ma) non-adakitic rocks of southern Ecuador and Latest Miocene to Pliocene (post-6 Ma) adakitelike volcanic rocks. The present study (as Bourdon et al 2002;Samaniego et al 2002) illustrates that such an evolution is also recorded by Quaternary volcanic centers of northern Ecuador, like Chacana, with the difference that the transition from non-adakitic to adakite-like rocks in this case occurs \1 My ago.…”

“…In a series of recent papers (e.g., Bourdon et al 2002Bourdon et al , 2003Samaniego et al 2002;Hidalgo et al 2007;Hoffer et al 2008), the adakite-like rocks of Ecuador have been considered the result of slab melting, but other works on the same rocks suggest the possible role played by magmatic processes at lower crustal levels (Kilian et al 1995;Arculus et al 1999;Monzier et al 1999;Garrison and Davidson 2003;Chiaradia et al 2004a;Bryant et al 2006;Garrison et al 2006), a petrogenetic process invoked also to explain the geochemical signatures of southern Colombian volcanoes (e.g., Droux and Delaloye 1996;Calvache and Williams 1997).…”

The genetic association of adakites and Cu–Au ore deposits

“…For example, some adakitic rocks do not have any deposits (Chiaradia et al 2004;Huang et al 2008), whereas some rocks without clear adakitic geochemical features are ore-bearing, for example some porphyry Cu and epithermal Au deposits in Ecuador (Chiaradia et al 2004) and small porphyry Cu deposits in the western Luzon (Imai 2002); in both cases calc-alkaline andesites are the host rocks. Therefore, it has been argued that high oxygen fugacity, rather than adakitic magma composition, is essential to the formation of porphyry Cu deposits (Imai 2002;Bissig et al 2003).…”

Evolution of the East Philippine Arc: experimental constraints on magmatic phase relations and adakitic melt formation

“…This has resulted in alternative models of adakite genesis that advocate the fractionation of garnet and/or amphibole-bearing mineral assemblages from typical arc basaltic magmas or rocks (e.g. Atherton and Petford 1993;Castillo et al 1999;Garrison and Davidson 2003;Tulloch and Kimbrough 2003;Chiaradia et al 2004;Macpherson et al 2006;Macpherson 2008). Eiler et al (2007) proposed a further possibility; this was that adakitic magma might be produced by very low degrees of partial melting of mantle wedge peridotite.…”