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Geologic Overview of the Escondida Porphyry Copper District, Northern Chile

By
Miguel Hervé
Miguel Hervé
1
Minera Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile
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Richard H. Sillitoe
Richard H. Sillitoe
2
27 West Hill Park, Highgate Village, London N6 6ND, England
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Chilong Wong
Chilong Wong
1
Minera Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile
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Patricio Fernández
Patricio Fernández
1
Minera Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile
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Francisco Crignola
Francisco Crignola
1
Minera Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile
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Marco Ipinza
Marco Ipinza
1
Minera Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile
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Felipe Urzúa
Felipe Urzúa
3
BHP Billiton, 10 Marina Boulevard 50-01, Marina Bay Financial Centre Tower 2, Singapore 018983
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Published:
January 01, 2012

Abstract

The giant Escondida district in northern Chile, discovered in 1981, includes the major porphyry copper deposits at Escondida-Escondida Este, Escondida Norte-Zaldívar, Pampa Escondida, and two small deposits (the Escondida cluster), besides the Chimborazo deposit. The district contains at least 144 million metric tons (Mt) of copper. The Escondida district is part of the middle Eocene to early Oligocene porphyry copper belt, which follows the trench-parallel Domeyko fault system, a product of the Incaic transpressional tectonic phase. At the district scale, the major N-striking Portezuelo-Panadero oblique-reverse fault juxtaposes latest Carboniferous to Early Permian igneous basement with an andesitic volcanic sequence of late Paleocene to early Eocene age, both of which host the porphyry copper mineralization. Immediately before and during porphyry copper formation, a thick siliciclastic sequence with andesitic volcanic products intercalated toward the top (San Carlos strata) filled a deep basin, generated by clockwise rigid-block rotation, within the confines of the Escondida cluster. The presence of these volcanic rocks suggests that an eruptive center was still active within the confines of the Escondida cluster when deposit formation began.

The deposits are all centered on multiphase biotite granodiorite porphyry stocks, which were predated by dioritic to monzodioritic precursors and closely associated with volumetrically minor, but commonly high-grade, magmatic-hydrothermal breccias. The earliest porphyry phases consistently host the highest grade mineralization. Alteration-mineralization zoning is well developed: potassic and overprinted gray sericite assemblages containing chalcopyrite and bornite at depth; more pyritic chlorite-sericite and sericitic zones at intermediate levels; and shallow advanced argillic developments, the remnants of former lithocaps that could have attained 200 km2 in total extent. The latter are associated with high-sulfidation, copper-bearing sulfide mineralization, much of it in enargite-rich, massive sulfide veins. The Escondida and Escondida Norte-Zaldí-var deposits, formed at ∼ 38 to 36 Ma, are profoundly telescoped, whereas the earlier (∼ 41 Ma) Chimborazo and later (∼ 36–34 Ma) Escondida Este and Pampa Escondida deposits display only minor telescoping, suggesting that maximal Incaic uplift and erosion took place from 38 to 36 Ma.

The Portezuelo-Panadero and subsidiary longitudinal faults in the district—inverted normal structures that formerly delimited the eastern side of a Mesozoic backarc basin—were subjected to sinistral transpression prior to deposit formation (pre-41 Ma), which gave rise to the clockwise block rotation responsible for generation and initial synorogenic filling of the San Carlos depocenter. The Escondida district was then subjected to transient dextral transpression during emplacement of the NNE- to NE-oriented porphyry copper intrusions and associated alteration and mineralization (∼ 38–34.5 Ma). This dextral regime had waned by the time that a N-trending, late mineral rhyolite porphyry was emplaced at Escondida Este and was replaced by transient sinistral transpression during end-stage formation of NW-striking, high and intermediate sulfidation, massive sulfide veins and phreatic breccia dikes. Since 41 Ma, the faults in the district have undergone no appreciable displacement because none of the porphyry copper deposits shows significant lateral or vertical offset.

Renewed uplift and denudation characterized the late Oligocene to early Miocene, during which the extensive former lithocap was largely stripped and incorporated as detritus in a thick piedmont gravel sequence. Development of hematitic leached capping and attendant chalcocite enrichment zones, along with subsidiary oxide copper ore, was active beneath the topographic prominences at Escondida, Escondida Norte-Zaldívar, and, to a lesser degree, Chimborazo from ∼ 18 to 14 Ma, but supergene activity was much less important at the topographically lower, gravel-covered Pampa Escondida deposit. After ∼ 14 Ma, supergene processes were soon curtailed by the onset of hyperaridity throughout much of northern Chile.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
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Michael Harris
Michael Harris
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Francisco Camus
Francisco Camus
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Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

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