Geology of El Domo deposit in central Ecuador: a VMS formed on top of an accreted margin

Cristian Vallejo, Francisco Soria, Fernando Tornos, German Naranjo, Bayardo Rosero, Fredy Salazar, Ryan Cochrane. 2016. Geology of El Domo deposit in central Ecuador: a VMS formed on top of an accreted margin. Mineralium Deposita 51, 3, 389-409, DOI: 10.1007/s00126-015-0616-x

El Domo is a Cu-Au-Zn-Ag volcanogenic massive sulfide deposit (VMS) within the Paleocene-Eocene submarine arc rocks of the Macuchi Formation in central Ecuador. The mineralization includes massive and semi-massive sulfides located at the interface between a submarine rhyodacite dome complex and overlying mafic volcanic and volcaniclastic rocks, including chaotic and unsorted debris flow deposits. Whole rock geochemical analyses show that the felsic rocks belong to a calc-alkaline sequence, whereas the mafic assemblages have a tholeiitic affinity, reflecting a magmatic association typically found in many VMS districts. This geochemical change temporally coincides with the formation of the massive sulfides. New zircon Hf isotopic data collected from a hanging wall rhyodacite suggest that the presence of an over-thickened crust of oceanic plateau affinity may have influenced the composition of the magmatic rocks. The timing of massive sulfide formation is constrained by an Ar-40/Ar-39 age of 41.49 +/- 0.37 Ma (2 sigma) from the volcaniclastic sequence which hosts the massive sulfides and a U/Pb zircon age date of 42.13 +/- 0.54 Ma (2 sigma) from the footwall rhyodacite; both ages are interpreted to provide minimum and maximum ages for mineralization. The massive sulfides are zoned and contain variable proportions of pyrite, sphalerite, and chalcopyrite. Minor accumulations of galena, bornite, tennantite, stromeyerite, and proustite are also present; the latter two minerals are the major carriers of silver whereas gold typically occurs as minute-free grains or in solid solution with silver. Ore minerals are intergrown with variable proportions of chlorite, barite, quartz, anhydrite, and smectite. The footwall rhyodacite hosts an irregular stringer zone which is rich in pyrite and gypsum and is characterized by intense quartz-sericite-pyrite alteration. The hanging wall volcaniclastic lithologies exhibit pervasive but vertically limited (8-10 m) silicification, chloritization, and argillization. Both felsic and mafic glass-rich rocks located near the contact between rhyodacites and mafic volcaniclastic rocks are replaced by massive to disseminated mineralization, likely controlled by their originally reactive and porous nature. Replacement appears to have taken place at very shallow depths, 10-20 m below the seafloor, as shown by: (1) the presence of massive sulfide clasts within the uppermost volcaniclastic horizons and (2) the absence of mineralization and alteration in the overlying debris flows. No evidence of exhalation on the seafloor has been found, and replacement below the sea floor was probably an efficient mechanism for preserving the VMS within such a highly dynamic volcanic system. Regionally, mineralization occurred within an extensional submarine arc setting whereby massive sulfides were preferentially controlled by the reactivation of NNE-SSW crustal-scale strike slip faults created during an earlier plate collision. Local stress regimes resulted in the formation of pull-apart basins within a regional transpressive setting and provided conduits for magmatic and hydrothermal discharge. The onset of volcanism and local extension coincided with an increase in the oblique convergence of the Farallon and the South American Plate.

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