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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 Tortolas 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 define...