Metallogenesis of the Miocene El Indio-Pascua gold-silver-copper belt, Chile/Argentina: Geodynamic, geomorphological and petrochemical controls on epithermal mineralisation

The El Indio-Pascua Au (-Ag, Cu) Belt, straddling the Chile-Argentina border between Latitudes 29°20' and 30°30'S, is one of the worlds premier epithermal districts, containing the world class El Indio, Pascua and Veladero deposits and prospects. The geology of the district is dominated by an entirely subaerial upper Eocene to Upper Pliocene calc-alkaline volcanic succession overlying a Paleozoic to Lower Jurassic basement consisting of predominantly felsic intrusive and volcanic rocks. This transect of the Central Andean orogen has been considered to have been amagmatic since the Late Miocene as a result of flattening of the angle of subuction of the oceanic plate.; The Tertiary succession is herein subdivided into eight distinct stratigraphic units, two of which are introduced in this study. The 30–36 Ma Bocatoma Intrusive Unit, consisting of diorites and granodiorites, intrudes the basement and is overlain by a succession of predominantly dacitic ignimbrites of the 23–26 Ma Tilito Formation, locally exceeding 1200 m thickness. A regionally-persistent regolith horizon separates this unit from the similarly voluminous, but andesitic, 17.5–21 Ma, Escabroso Group. The petrographically similar andesites and diorites of the succeeding 14–17 Ma Cerro de las Tórtolas Formation and the Infiernillo intrusive lithodeme record a diminution in magmatic activity. Magma output was drastically reduced and discontinuous thereafter. The Middle to Upper Miocene units include the dacitic 11–12.7 Ma Vacas Heladas Formation, the very restricted dacitic to rhyodacitic Pascua Formation, dated at 7.5–8 Ma, and the rhyolitic tuffs of the 5.5–6.2 Ma Vallecito Formation. The latter was previously assumed to be the youngest in the district, but the newly discovered Cerro de Vidrio rhyolite dome 8 km E of Veladero is herein shown to be 2 Ma in age.; Hydrothermal alteration was widespread in the district and is associated with various magmatic units, including the Bocatoma Intrusive Unit, the Escabroso Group, the Infiernillo Intrusive Unit, the Vacas Heladas Formation, the Pascua Formation and the Vallecito Formation. Economic mineralisation, however, formed only between ca. 6 and 9.5 Ma, the only coeval igneous unit being the Pascua Formation.; Accurate geochronological data for the Middle to Upper Miocene volcanic and intrusive units were a prerequisite for the delimitation of the ages of a succession of regional pediplains, considered to be rapidly eroded planar landforms. (Abstract shortened by UMI.)...