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Fertility Assessment of Tectono-Magmatic Cycles in the Tres Cerrillos Prospect (Western Cordillera of Ecuador)

Abstract:

This study explores the mineralizing potential and the magmatic controls of Miocene-Pliocene magmas in the Western Cordillera of Ecuador, evaluating the temporal evolution and petrogenesis of magmas associated with multiple porphyry Cu systems at the Tres Cerrillos prospect. Within the prospect, four distinct magmatic cycles at ~11, ~10.6, ~8.7, and ~2.7 Ma were identified, spanning the mid-Miocene to Pliocene. Petrographic, geochemical, and geochronological data reveal a transition from barren to fertile behavior within each cycle, marked by ramp-ups in key geochemical fertility proxies (Sr/Y, V/Sc, La/Yb, Dy/Yb, Zr/Y, and (EuN/Eu*)/YbN). These fertile magmatic signatures are associated with magmas evolving at high-pressure and high-H2O conditions in the lower crust, leading to amphibole and garnet fractionation. Assimilation-fractional crystallization modeling of rare-earth elements indicates a high degree of assimilation of the oceanic Pallatanga basement during the evolution of the Tres Cerrillos magmas. All magmatic cycles have a duration of ~0.5 Ma; however, potentially longer ramp-up periods in fertility proxies, evidenced by inherited zircons and ⁴⁰Ar/³⁹ Ar geochronology, place the Cielito and El Pantanal targets as the most prospective at Tres Cerrillos. A long-lived ramp-up of fertility indicators is considered a hallmark of supergiant porphyry copper deposits. Exhumation phases (~15–13, ~10–9, and ~6–5 Ma) related to periods of increased compression in Ecuador precede the onset of the magmatic cycles. These compressional phases could have favored magma storage and differentiation at deep crustal levels before magmas were transferred to upper crustal reservoirs under more neutral stress conditions. The transition from a compressional to a more neutral tectonic stress regime could have been modulated by the oblique subduction of the topographically irregular Carnegie Ridge beneath the Ecuadorian continental margin, which highlights the importance of subduction dynamics in the formation of porphyry copper deposits.