Geological, structural and geochronological framework of the Veladero North area, Cordillera Frontal, Argentina
Creators: name: Charchaflie, Diego
Publisher: name: University of British Columbia
Year: 2009

description: The Veladero North area, located in the northern part of the El Indio-Pascua Belt within the Cordillera Frontal of Argentina and Chile, contains gold reserves and resources that combined exceed 370 tons (12 Moz). Throughout the Belt, mineralization is related to Miocene magmatic activity and both high- and low-sulfidation systems have been mined. The Miocene volcanic rocks form several formations separated by unconformities from underlying Carboniferous to Triassic volcanic and plutonic rocks. Whereas age and structural relationship of these rocks are well known in Chile, the geology of the Argentinan flank is poorly documented. This study defines the stratigraphy, geology and tectonic evolution of the Veladero North area and clarifies the regional framework of the Cordillera Frontal in Argentina. Thrust faults that dip steeply (70° to 90°) to the west and back-thrusts that dip steeply (70° to 90°) to the east define the overall geometry of the Miocene fold and thrust belt around the Veladero North area. The thrust panels comprise Permian (259 ± 0.7 Ma to 254 ± 4.2 Ma, U-Pb) rhyolitic volcanic and volcaniclastic rocks assigned to the Guanaco Sonso Formation and late Oligocene to early Miocene (24.5 ± 0.2 Ma to 22.8 ± 1.7 Ma, U-Pb) andesitic to dacitic pyroclastic and sedimentary rocks equivalent to the Tilito Formation. Middle Miocene (15.8 ± 1 Ma, U-Pb and 12.7 ± 0.9 Ma to 11.0 ± 0.2 Ma, ⁴⁰Ar-³⁹Ar) pyroclastic rocks and shallow intrusives of the Cerro de las Tortolas Formation and Infiernillo Unit, and Vacas Heladas Formation in the study area unconformably overlie the fold and thrust belt. These strata are sub-horizontal or dip shallowly (&lt;20°) to the east, and are separated from the thrusted rocks by two regional, low-relief unconformities. A Pliocene (2.1 ± 0.5 Ma, ⁴⁰Ar-³⁹Ar) rhyolitic dome in the eastern part of the district forms the Cerro de Vidrio Formation, the youngest recognized unit of the region. Previously published and new geochronological data suggest that late Paleozoic to Triassic magmatism in the Cordillera Frontal occurred as a series of plutonic and volcanic episodes isolated by periods of magmatic quiescence. The Choiyoi Group apparently comprises two independent volcanic units emplaced before and after a 25 m.y. volcanic lull, of which, the older and widespread Permian Guanaco Sonso Formation is exposed in the Veladero North area. Structural relations and geochronology indicate that the largest part of Tertiary deformation in the study area occurred throughout the Miocene, and is best explained by multiple phases of shortening. The youngest host rock of the Veladero North epithermal deposit is a volcanic succession dominated by heterolitic bedded breccias, volcaniclastic sandstones and dome-related rocks equivalent to the (16 Ma to 14.9 Ma) Cerro de las Tortolas Formation and its intrusive counterpart, the Infiernillo Unit. If the mineralization in Veladero North is the same age as that in the rest of El Indio-Pascua Belt (9.5 Ma to 6 Ma), the volcaniclastic package is then several million years older than the mineralization. Thus, the genesis of the package bears little relationship to gold mineralization except for forming a porous host rock for it.