The Cascabel Porphyry Cu-Au-Ag District, Northern Ecuador

Abstract The Cascabel Cu-Au-Ag porphyry cluster is part of the Eocene metallogenic belt of the northern Western Cordillera of Ecuador, and formed during east-directed, low-angle subduction and eastward migration of the Macuchi arc. Cascabel is part of the Imbaoeste mining district, a NE-trending mineralized belt located between the regional-scale Chimbo-Toachi shear zone to the west and the Calacalí-Pujilí-Pallatanga fault to the east. The basement rocks of the Cascabel district are gabbros and mafic volcanic breccias. They are overlain by late Eocene volcanic and volcanosedimentary rocks interpreted to be part of the submarine to transitional Paleocene to late Eocene Macuchi arc. Quaternary conglomerates and alluvial deposits cover the Eocene volcanic rocks locally. Three fault systems—oriented northeast, northwest, and north-northwest—have been identified at Cascabel, with the NW-trending faults being the principal system associated with the emplacement of mineralized dikes, porphyry-style veins, hydrothermal alteration zones, and late breccias. The Cascabel district contains three principal Cu-Au(-Ag) porphyry deposits: Alpala, Tandayama-America, and Aguinaga, together with several other porphyry prospects. Each porphyry deposit has a central potassic alteration domain surrounded by synchronous propylitic alteration assemblages (chlorite ± epidote ± actinolite) and mineralized chlorite-white mica alteration assemblages that overprinted potassic and propylitic alteration. Late alteration assemblages, mainly phyllic, minor argillic, and lesser advanced argillic alteration, developed principally around faults and overprinted the early-formed alteration assemblages. Multiple intrusive phases compose steeply NE-plunging mineralized intrusive complexes at Cascabel’s three major porphyry centers. The intrusions—which vary from equigranular to porphyritic and include diorites, microdiorites, quartz diorites, hornblende quartz diorites, tonalites, and rare granodiorite and granite—have been grouped as early, syn-, intra-, and late mineralization and host porphyry-style veins and Cu-Au(-Ag) mineralization. Several minor postmineralization intrusions have been identified that lack significant mineralization. The intrusive rocks at Cascabel mostly range from subalkaline basalt to dacite compositions and have characteristic primitive mantle-normalized rare earth element (REE) profiles with negative Nb anomalies and subduction-related geochemical signatures, consistent with a magmatic arc origin. They were sourced from a primitive, hydrous parental magma and became progressively more fractionated with each successive intrusive event, from diorites to tonalites and granites, suggesting that magmatic fractionation was fundamental to the intrusive history at Cascabel. New zircon U-Pb dating results from Cascabel indicate multiphase intrusive activity occurred over a period of ~4 m.y., from 39.40 ± 0.60 to 35.07 ± 0.76 Ma. Re-Os geochronological data highlights that mineralization occurred in a more restricted time interval from 39.06 ± 0.20 to 38.36 ± 0.229 Ma. The synmineralization QD10 quartz diorites at Alpala (39.40 ± 0.60 to 38.00 ± 0.80 Ma) have distinctive positive Eu anomalies, listric-shaped REE patterns, and fertile zircon compositions, implying the involvement of oxidized hydrous magmas in mineralization at Cascabel. Mineralization occurred in a compressional continental arc setting, producing a giant Cu-Au deposit at Alpala and providing encouragement for porphyry exploration elsewhere in Ecuador’s Eocene magmatic arc.