Ore-Bearing Breccias of the Tunantza Epithermal Gold Deposit, Cordillera del Condor, Southeastern Ecuador

The results of a study of the epithermal Tunantza gold deposit located in the Cordillera del Condor, in the sub-Andean zone in southeastern Ecuador, are presented for the first time. A geological scheme, description and genetic classification of breccias and their significance for the gold mineralization of the Tunantza deposit are provided, serving as the basis for a proposed genetic model integrating the evolution of the ore-magmatic system up to the late hydrothermal stages. Geological materials and the results of documentation of surface outlets and underground workings were used; the petrographic study involved 96 thin sections (86 transparent thin sections and ten polished sections). The Tunantza gold deposit is hosted within a complex of phreatomagmatic breccias, which are crosscut by hydrothermal breccias, forming a long-lived ore-magmatic hydrothermal system. Their occurrence is closely related to a rhyolitic quartz–feldspar porphyry stock, which intruded the granodiorites of the Zamora batholith and acted as a sort of “plug” preceding the eruptive explosive phase that gave rise to the breccia body. The mineralization includes breccia bodies enriched and cemented by sulfides, which are surrounded by thin, proximal, subvertical zigzag-shaped veins, reflecting a local explosive character. The results show that a long-lived ore-magmatic hydrothermal system had a multistage formation and should be used as a detailed model for the exploration of epithermal and potentially porphyry deposits.