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Volcanic setting of the Bajo de la Alumbrera porphyry Cu-Au deposit, Farallon Negro Volcanics, Northwest Argentina


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Harris, AC, Bryan, SE and Holcombe, RJ 2006 , 'Volcanic setting of the Bajo de la Alumbrera porphyry Cu-Au deposit, Farallon Negro Volcanics, Northwest Argentina' , Economic Geology, vol. 101, no. 1 , pp. 71-94 , doi: 10.2113/101.1.71.

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The late Miocene Farallon Negro volcanics, comprising basaltic to rhyodacitic volcano-sedimentary rocks,
host the Bajo de la Alumbrera porphyry copper-gold deposit in northwest Argentina. Early studies of the geology
of the district have underpinned the general model for porphyry ore deposits where hydrothermal alteration
and mineralization develop in and around porphyritic intrusions emplaced at shallow depths (2.5-3.5 km)
into stratovolcanic assemblages. The Farallon Negro succession is dominated by thick sequences of volcanosedimentary
breccias, with lavas forming a minor component volumetrically. These volcaniclastic rocks conformably
overlie crystalline basement-derived sedimentary rocks deposited in a developing foreland basin
southeast of the Puna-Altiplano plateau. Within the Farallón Negro volcanics, volcanogenic accumulations
evolved from early mafic to intermediate and silicic compositions. The younger and more silicic rocks are
demonstrably coeval and comagmatic with the earliest group of mineralized porphyritic intrusions at Bajo de
la Alumbrera.
Our analysis of the volcanic stratigraphy and facies architecture of the Farallon Negro volcanics indicates
that volcanic eruptions evolved from effusive to mixed effusive and explosive styles, as magma compositions
changed to more intermediate and silicic compositions. An early phase of mafic to intermediate volcanism was
characterized by small synsedimentary intrusions with peperitic contacts, and lesser lava units scattered widely
throughout the district, and interbedded with thick and extensive successions of coarse-grained sedimentary
breccias. These sedimentary breccias formed from numerous debris- and hyperconcentrated flow events. A
later phase of silicic volcanism included both effusive eruptions, forming several areally restricted lavas, and explosive
eruptions, producing more widely dispersed (up to 5 km) tuff units, some up to 30-m thickness in proximal
sections. Four key features of the volcanic stratigraphy suggest that the Farallon Negro volcanics need not
simply record the construction of a large steep-sided polygenetic stratovolcano: (1) sheetlike, laterally continuous
debris-flow and other coarse-grained sedimentary deposits are dominant, particularly in the lower sections;
(2) mafic-intermediate composition lavas are volumetrically minor; (3) peperites are present throughout
the sequence; and (4) fine-grained lacustrine sandstone-siltstone sequences occur in areas previously thought
to be proximal to the summit region of the stratovolcano. Instead, the nature, distribution, and geometry of volcanic
and volcaniclastic facies suggest that volcanism occurred as a relatively low relief, multiple-vent volcanic
complex at the eastern edge of a broad, >200-km-wide late Miocene volcanic belt and on an active foreland
sedimentary basin to the Puna-Altiplano. Volcanism that occurred synchronously with the earliest stages of porphyry-
related mineralization at Bajo de la Alumbrera apparently developed in an alluvial to ring plain setting
that was distal to larger volcanic edifices.

Item Type: Article
Authors/Creators:Harris, AC and Bryan, SE and Holcombe, RJ
Keywords: Porphyry, Copper, Gold, Stratovolcano, Lava Domes, Bajo de la Alumbrera
Journal or Publication Title: Economic Geology
ISSN: 0361-0128
DOI / ID Number: 10.2113/101.1.71
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