Abstract

Located in the northern part of the El Indio belt of Chile-Argentina, the 12.6-million ounce (Moz) gold, late Miocene Veladero high-sulfidation epithermal deposit is hosted principally in late Oligocene and middle Miocene volcanic and volcaniclastic rocks and to a lesser extent in the underlying Permian rhyolitic pyroclastic rocks. The principal host rocks are poorly bedded volcaniclastic breccia and minor air-fall tuff of the middle Miocene Cerro de las Tórtolas Formation, which unconformably overlies deformed and structurally interleaved late Oligocene and late Paleozoic rocks. These latter rocks form a steeply dipping fold and thrust belt that was bevelled by the regional Miocene Frontera-Deidad paleogeomorphic surface. The Cerro de las Tórtolas Formation, deposited across the Frontera-Deidad surface, represents at Veladero an ~16 Ma vent complex characterized by domes and their coeval subvolcanic equivalents (Infiernillo intrusive unit), surrounded by coarse, poorly sorted volcaniclastic breccia that grades eastward onto a volcaniclastic apron of bedded and reworked detritus with interbedded air-fall tuff and local lacustrine tuffaceous sandstone and siltstone. Gold at Veladero is hosted in a tabular, north-northwest elongated silicified horizon that cuts across the host stratigraphy and has a subhorizontal top that ranges between 4,200 and 4,400 m above sea level. The silicified horizon lies at the approximate elevation to slightly beneath the 14.5 to 12 Ma Azufreras-Torta surface, which controlled formation of the economic El Indio, Tambo, and Pascua-Lama high-sulfidation gold deposits elsewhere in the El Indio belt. Published late Miocene 40Ar/39Ar ages between 11 and 10 Ma on alunite from the Filo Federico zone at Veladero and from uneconomic prospects nearby indicate that advanced argillic alteration and likely gold deposition is 5 m.y. younger than the ~16 Ma host rocks. The Veladero deposit formed after a change in the local tectonic environment from one of shortening that preceded deposition of the host volcaniclastic sequence to one of extension localized within the vicinity of Veladero that episodically disrupted the host volcanic sequences, reactivated the steeply dipping thrust faults, and displaced the Frontera-Deidad and Azufreras-Torta paleogeomorphic surfaces. Regional tectonic relationships suggest that this transition did not reflect a fundamental change in the overall shortening that dominates the Cenozoic of the Chilean-Argentinean Andes. Instead, local extension around Veladero resulted from the inherent gravitational instability created by topography in excess of 4,000 m above sea level.

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