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Geology, geochronology and structural evolution of La Escondida copper district, Northern Chile


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Urzua, F 2009 , 'Geology, geochronology and structural evolution of La Escondida copper district, Northern Chile', PhD thesis, University of Tasmania.

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La Escondida district is one of the largest copper concentrations within the late Eoceneearly
Oligocene porphyry copper metallogenic belt of northern Chile. The district occurs
in the north-trending Domeyko Cordillera, an orogenic chain associated with the
Domeyko Fault System since the uppermost Late Cretaceous.
The results of a 1:25,000-scale geological mapp111g of La Escondida district have revealed
an intricate juxtaposition of Late Paleozoic to Eocene sedimentary, volcanic and intrusive
rock packages. These occur in fault-bounded structural panels and their locations can be
predicted underneath the post-Eocene unconsolidated alluvial blanket. Previously defined
and new geological units define two major tectonostratigraphic cycles in La Escondida
region: a Late Carboniferous-Permian to Triassic-age Pre-Andean (Gondwanian) cycle;
and Late Triassic-Present Andean cycle. The former includes La Tabla Formation, which
resulted from widespread terrestrial volcanism and cogenetic granitoids. Minor, weakly
mineralised porphyry copper-type intrusions and Michigan-style copper occurrences are
part of this cycle. The Pre-Andean cycle was characterised by caldera complexes,
resurgent intrusions (domes and plutons) and fluvio-lacustrine sedimentation. Major and
trace element compositions of volcanic and intrusive rocks are consistent with a
destructive continental marg111 setting. Calc-alkaline, dominantly siliceous explosive
volcanism and I-type intrusions were emplaced in a mature magmatic arc governed by an
extensional regime.
The Andean cycle in La Escondida district started with deposition of a mixed calcareoussiliciclastic
sequence on the eastern margin of a back-arc bas111, between the Late Triassic
and N eocomian. An almost 2,000 m thick pile of sedimentary rocks was deposited and
subsequently folded and thrusted by the 90-80 Ma Peruvian compressional tectonic phase.
Three successive magmatic events occurred from 81 Ma to 34 Ma. These events were
characterised by volcanic and intrusive episodes that have geochemical signatures typical
of arc and transitional/back-arc settings.
During the Late Cretaceous-lowermost early Paleocene, three north- to northeast elongated
plutonic complexes were emplaced (81-79 Ma Sombrero, 77-72 Ma Cerro Bayo
and 66-64 Ma Torcaza complexes) that range in composition from monzogabbro,
monzodiorite to monzogranite and dacite porphyries. Andesitic volcaniclastic rocks of Las
Torres igneous complex were deposited between ca. 70 and 66 Ma, within a small
tectonovolcanic basin. Effusive volcanism was preceded and probably accompanied by
the intrusion of a north-northeast-oriented dyke swarm of monzogabbro, diorite and
andesite, between 82-70 Ma. Structural data suggest that these north- to northeastelongated
intrusions were emplaced and an extensional to transtensional stress regime.
The dykes were deformed by the K-T deformational tectonic phase at ca. 64-62 Ma,
although those bodies emplaced early were affected by the Peruvian tectonic movements.
The Paleozoic and Mesozoic rocks are unconformably overlain by the late Paleocene-early
Eocene Augusta Victoria Formation. This unit consists of extensive ignimbrite flows, coarse-grained volcano-sedimentary rocks and subordinate basaltic andesite and andesite,
and was deposited between 57 and 53 Ma. Minor intrusive activity at 60-
61 Ma produced andes1te dikes and other subvolcanic bodies as a precursor to widespread
volcanism. Subsequent E-W contractional deformation produced large-scale open folds
with axes that slightly plunge to the north. Folding and associated reverse faulting were
followed by dextral and sinistral strike-slip faulting that only produced mmor offsets.
The middle-late Eocene magmatism started with the intrusion of the 43-41 Ma Cerro
Rincones plutonic complex, the largest in La Escondida district. It contains nine phases
that range from diorite, through monzodiorite and granodionte, to quartz-monzorute and
nncrograrute. Although early discrete phases were preferentially emplaced along fold axes
ill the Augusta Victoria Formation, larger phases d1splay ring-shapes in plane-view.
Emplacement of the -porphyry Cu-style intrusive complexes at La Escondida and
Escondida Nprte-Zaldivar deposits occurred between 38 and 34 Ma, and the Domeyko
Fault System was a major control on the minerahsing illttusions. The kinematic history of
its eastern branch, the Escondida master fault, consisted of an early compressional stage
(<51-50 Ma), followec;l by dextral strike-slip transpressional activity (ca. 40-37 Ma) during
the emplacement of Escondida Notte-Zaldivar porphyries and later sinistral
transpressional movement (> 37 Ma) that controlled the intrusion of La Escondida
porphyry (ca. 38-34 Ma). Deformation continued up to ca. 20 Ma, with a nnrumum
average exhumation rate of ca 0.05 km/m.y. Significant extens10nal and left-lateral
transtensional movements occurred on La Escondida master fault at ca. 18-10 Ma
(Quechua deformation phase) that produced the triangular pull-apart basill of the Salar de
Hamburgo. Dissection of the Hamburgo basin occurred during the early Miocene by
NNE-striking faults.
The 39-38 Ma Capella stocks, 38-36 San Carlos strata volcaniclastic rocks and
microdiorite bodies were respectively emplaced prior to and during minerahsation at La
Escond1da and Escondida Nor_te-Zaldivar. A large family of post-mineralisation
porphyritic d1orite, andesite-diorite porphyry and andesite pyroxene dykes and minor
stocks were also intruded between 38 and 35 Ma. Major and trace elements compositions
of the Eocene intrusions suggest increasing pressure and depth of magma source from the
pre-to post-mmeralisation magmatic products. There was a change from a dry magmatic
source regime (pre-mineralisation Cerro Rmcones and Capella intrusions), through wet
magma (syn-to post-minerahsation porphyrite diorite, andesite-diorite porphyry and San
Carlos strata), and back to dry magma (post-minerahsation pyroxene andesite porphyry
dyke), based on the available geochemical data. Appreciable Late Paleozoic crustal
inheritance has been detected during analyses of zircons from the Andean magmatic
rocks, revealing a crustal role ill the petrogenesiS of these predominantly asthenospheric
mantle wedge-derived rocks.
Two sub-belts of Eocene hydrothermal alteration systems ill La Escondida dlStrict have
been identified during this study: (1) an eastern sub-belt that includes La Escondida and
Escondida Notte-Zaldivar deposits and several penpheral mineralised intrusions; and (2) a
poorly constrained western sub-belt that consists of advanced argilllc altered and silicified
rocks, the most significant of which is the Chlmborazo h1gh sulfidation epithermal AuCu-(
Fe) deposit. Future exploration should be focused along the La Escondida master
faults ill the eastern sub-belt beneath the extended Late Cenozoic alluvial cover as these faults appears to have localised the most productive mineralised systems in the district.

Item Type: Thesis - PhD
Authors/Creators:Urzua, F
Keywords: Copper ores, Geology, Mines and mineral resources
Copyright Holders: The Author
Copyright Information:

Copyright 2009 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references. Vol. 1. Ch. 1. Introduction -- Ch. 2. Tectonostratigraphic setting -- Ch. 3. General geology -- Ch. 4. Structural geology -- Ch. 5. Hydrothermal systems of La Escondida District -- Ch. 6. Geochemistry of igneous rocks -- Ch. 7. Conclusions and recommendations -- Vol. 2. Appendices -- [Vol. 3.] Container holding 21 folded maps

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