STRATIGRAPHY AND GEOLOGICAL SETTING OF EL DOMO VMS DEPOSIT WITHIN THE EOCENE MACUCHI SUBMARINE ARC, CENTRAL ECUADOR

El Domo is a Zn-Cu-Ag-Au volcanogenic massive sulfide deposit (VMS) recently discovered within the Paleocene – Eocene submarine arc rocks of the Macuchi Formation in central Ecuador.The VMS is hosted at the interface between a rhyodacite at the footwall and a volcano-sedimentary sequence at the hangingwall of the deposit. The VMS contain variable proportions of pyrite, sphalerite, chalcopyrite and minor galena in an undeformed tabular body. Mineralization was produced by open space filling and replacement in volcanic and volcaniclastic rocks. Replacement below the sea floor was an efficient mechanism for preserving the VMS.

Whole rock geochemistry shows that the volcanic rocks that host the massive sulfide have both, tholeiitic and calc-alkaline affinities. The change from calc alkaline rhyodacitic rocks to tholeiitic mafic volcanics is coeval with the onset of massive sulfide deposition. Geochemical analyses, depositional structures and alteration of the related igneous and volcaniclastic rocks from both, the footwall and hangingwall to the massive sulfide, show that these rocks were deposited in a submarine volcanic arc environment, in which bimodal mafic extrusive volcanic and volcaniclastic successions predominated. Lu–Hf isotope data indicate εHf values between +11 to +13, pointing to a juvenile magmatic source for the Eocene rhyodacite. This juvenile source suggests the influence of the oceanic plateau basement of the Macuchi Fm.

The host facies were deposited by submarine volcanic eruptions during the Eocene as inferred by a ⁴⁰Ar/³⁹Ar age of 41.49 ± 0.37 Ma (2σ) obtained from the volcanic succession overlying the massive sulfide, and a U/Pb zircon age of 42.13 ± 0.54 Ma (2σ) from the footwall rhyodacite..

VMS mineralization occurred in an extensional setting with a regional control of NNE oriented crustal-scale faults created during collisional events preceding the development of the submarine volcanic arc. These regional strike slip faults created extensional basins within a regional transpressive setting. Extension provided structural conduits for magmatic and hydrothermal discharge and formation of the massive sulfide. The onset of volcanism and local extension is also consistent with an increase in the oblique convergence of the Farallon Plate and the South American Plate.