Magmatic evolution of pre-ore volcanics and porphyry intrusives associated with the Altar Cu-porphyry prospect,Argentina |
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| Affiliation: | 1. Sciences de la Terre, Université du Québec à Chicoutimi, Chicoutimi G7H 2B1, Canada;2. Departamento de Geología, Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile;1. ARC Centre of Excellence in Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia;2. Department of Geology, Lakehead University, 955 Oliver Rd, Thunder Bay, ON P7B 5E1, Canada;3. First Quantum Minerals Limited, 8th Floor, 543 Granville St, Vancouver, British Colombia V6C 2VC, Canada;1. Departamento de Geología y Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla #803, Santiago de Chile, Chile;2. ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;4. Instituto de Geología, Universidad Nacional Autónoma de México Ciudad Universitaria, 04510 México, D.F., Mexico;1. Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Campus do Vale, Porto Alegre, RS 91501-970, Brazil;2. Instituto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro, Brasília, DF 70904-970, Brazil;3. Centro de Pesquisas Geocronológicas, Universidade de São Paulo, Rua Lago 562, Cidade Universitária, São Paulo, SP 05508-080, Brazil;4. Companhia de Pesquisas e Recursos Minerais, SGAN 603, Brasília, DF 70830-902, Brazil |
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| Abstract: | Altar is a Cu-porphyry deposit related to several small plagioclase porphyry intrusions of the late Miocene formed on the margin of the Flat-Slab segment along the Andean Cordillera in north-west Argentina. New stratigraphic and structural mapping supported by geochemistry and geochronology of pre-ore volcanics at Altar has revealed that a period of ～6–7 Ma of volcanism during the late Oligocene-early Miocene formed ～4000 m of volcano-stratigraphic succession making up the Pachón Formation. It represents a period dominated by explosive to effusive eruption in a dynamic arc basin with local ash fall and flow deposition in lacustrine and fluvial sites. Volcanism is typified by medium- to high-K calc-alkaline arc magmatism with a shift from mafic compositions at the base to felsic rocks at the top of the formation containing zircons aged 21.9 ± 0.2 Ma (2 Std.Dev, U–Pb). A clear geochemical separation exists between early Miocene pre-ore volcanics that show signatures akin to non-adakitic, normal arc, extensional tectonic settings conducive of chemical differentiation at shallow crustal levels and correlate with intra-regional Abanico and Farellones Formations; and the middle to late-Miocene Cu-mineralised porphyry intrusions. After a break of ～9 Ma in the geological record at Altar, these Cu-fertile bodies are emplaced entirely within the Pachón Rhyolite and represent adakite-like magmas with fractionation trends evolving from a lower crustal MASH zone. This distinction is controlled by a change from an extensional to compressive tectonic regime in the region during the middle Miocene in which magmas were stalled in the lower crust for an extended period, subsequently became enriched in metals and then formed several Cu-porphyry bodies which were emplaced during a relatively short period towards the late Miocene. |
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| Keywords: | Flat-Slab Geochemistry Miocene Porphyry Geochronology U–Pb zircon age |
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