The Cenozoic history of volcanism and hydrothermal alteration in the Central Andean Flat-Slab Region: new ⁴⁰Ar-³⁹Ar constraints from the El Indio-Pascua Au (-Ag, Cu) Belt, 29°20′-30°30′ S

Ninety-three new ⁴⁰Ar-³⁹Ar laser step-heating plateau dates for igneous rocks and alteration minerals from the El Indio-Pascua Au-Ag belt permit significant refinement of the Tertiary volcanic stratigraphy and the definition of a succession of alteration events in this major mineralized district. Eight distinct Tertiary stratigraphic units are now recognized (two newly defined in this research): (1) the 30-36 Ma Bocatoma Unit, comprising dioritic and granodioritic shallow-level intrusions; (2) the voluminous 23-26 Ma Tilito Formation, consisting predominantly of dacitic tuffs; (3) the 17.5-21 Ma Escabroso Group made up of extensive successions of andesitic flows and coeval hypabyssal intrusions; (4) the 14-17 Ma, andesitic, Cerro de las Tórtolas Formation and its intrusive lithodeme, the Infiernillo Unit; (5) the dacitic, 11.0-12.7 Ma, Vacas Heladas Formation; (6) the rhyodacitic 7.5-8 Ma Pascua Formation, defined in this study; (7) the 5.5-6.2 Ma rhyolitic Vallecito Formation: and (8) the recently defined rhyolitic 2 Ma Cerro de Vidrio Formation. Magmatic activity decreased markedly following the eruption of the Cerro de las Tórtolas Formation. Hydrothermal activity occurred at least from the late Eocene to the Late Miocene, but economic Au-Ag-Cu mineralization was confined to the 6-9.5 Ma interval, the only observed contemporaneous igneous unit being the restricted Pascua Formation. Epithermal Au-Ag-Cu deposits and major prospects emplaced in this period include, from north to south, Pascua-Lama, Veladero, Sancarrón, Río del Medio, El Indio, Tambo, and Vacas Heladas. The widespread, albeit barren, alteration associated with the Bocatoma, Escabroso, Infiernillo, and Vacas Heladas magmatism indicates that the availability of hydrothermal fluid was not the controlling factor for ore formation, emphasising instead the role of the metal content of the magmas associated with epithermal mineralization, and/ or the requirement for favorable physiographic conditions at the site of ore deposition.