PORPHYRY AND EPITHERMAL DEPOSITS WITHIN THE SPATIAL AND TEMPORAL CONTEXT OF VOLCANO-PLUTONIC SYSTEMS: ANTOFALLA VOLCANIC COMPLEX, NW ARGENTINA

The Antofalla Volcanic Complex (AVC) lies on the NW-trending Archibarca lineament, which controls the location of the Eocene-Oligocene Escondida porphyry Cu deposit in Chile, and several Neogene volcanic centers in Argentina. These Neogene complexes are minimally eroded, but gullying and sector collapse locally expose deeper levels where hydrothermal alteration and epithermal styles of mineralization can be observed. Porphyry mineralization may exist at deeper levels by analogy with nearby deposits such as Escondida and Bajo de la Alumbrera, but are not exposed in this area. However, high-temperature, high-salinity fluid inclusions in hydrothermal quartz clasts in an ignimbrite from the Escorial volcano attest to the existence of magmatic hydrothermal systems at depth.

The AVC contains two significant zones of epithermal alteration and mineralization ca. 20 km apart, and each covering ca. 3 km². They are exposed through overlying unaltered lava flows, and are thus an integral part of the evolution of the volcanic centre. Spatially and temporally, however, they represent very small parts of the overall system: the AVC has a radius of ca. 25 km, covers an area of ca. 2,100 km², and ranges in age from 10.9 to 1.6 Ma. The complex also displays a wide range of volcanic products from primitive basaltic andesites to rhyolites, with an early shoshonite-trachydacite suite and a later basaltic andesite-dacite suite. Rhyolites formed throughout this magmatic history, and appear to be at least partly crustally-derived. The transition from early shoshonitic to later calc-alkaline compositions took place over a period of <2 m.y., and is interpreted to reflect the time required to develop a mature MASH zone at the base of the crust. Development of mineralization in the AVC is thus considered to be a minor facet in the evolution of a much larger tectonomagmatic system, and exploration for such deposits needs to be viewed in this broader context.