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Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile

By
José Perelló
José Perelló
1
Antofagasta Minerals S.A., Apoquindo 4001, piso 18, Las Condes, Santiago, 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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Constantino Mpodozis
Constantino Mpodozis
1
Antofagasta Minerals S.A., Apoquindo 4001, piso 18, Las Condes, Santiago, Chile
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Humberto Brockway
Humberto Brockway
1
Antofagasta Minerals S.A., Apoquindo 4001, piso 18, Las Condes, Santiago, Chile
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Héctor Posso
Héctor Posso
3
Anaconda Perú, Avenida Paseo de la República 3245, piso 3, San Isidro, Lima, Peru
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Published:
January 01, 2012

Abstract

The porphyry copper mineralization at Los Pelambres is contained in two contiguous deposits, Los Pelambres (Cu-Mo) and Frontera (Cu-Au), which together constitute the third largest copper concentration (∼36 million metric tons (Mt) Cu) in the Miocene to early Pliocene belt of central Chile. Los Pelambres is centered on a composite, N-oriented, ∼ 4.5- × 2.5-km precursor quartz diorite stock emplaced within the regional, NNW-striking, E-vergent Los Pelambres reverse fault. The fault places intensely deformed Late Cretaceous volcanic and late Oligocene to early Miocene volcanic and volcanosedimentary rocks of the Los Pelambres Formation over gently folded early Miocene volcanic rocks of the Pachón Formation. Copper-gold mineralization at Frontera is hosted mainly by andesite of the Pachón Formation.

Hydrothermal alteration at Los Pelambres-Frontera conforms to the classic zonal pattern in which a potassic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypo-gene metal resource is hosted by multiple veinlet generations within potassic alteration, of which type 4 (quartz ± K-feldspar ± biotite ± sericite ± phengite ± andalusite ± corundum), A, and B types are volumetrically and economically the most important. The type 4 veinlets are regularly distributed throughout Los Pelambres and Frontera, whereas highest intensities of A and B veinlets display a spatial correlation with at least 20 small (∼ 200-m diam), SE-plunging magmatic-hydrothermal centers. These centers comprise one or more intermineral porphyry intrusions of dacitic (porphyry B) and andesitic (porphyry A) compositions along with igneous and hydrothermal breccias, the apical parts of which contain aplite and pegmatite pods. These centers acted as a series of miniature porphyry copper deposits whose coalescence generated the Los Pelambres-Frontera ore-body. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chal-copyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing copper mineralization in biotitized andesite at Frontera. The sericitic alteration is largely pyritic, but a NE-striking, SE-dipping corridor of D-type veinlets that overprints the potassic alteration in the northwestern quadrant of Los Pelambres contains copper sulfosalts. The internal portions of this corridor are characterized by advanced argillic assemblages, defining the roots of a once more extensive lithocap.

On the basis of detailed U-Pb zircon dating, the intrusive magmatism at Los Pelambres-Frontera lasted ∼ 3.8 m.y., from emplacement of the precursor Los Pelambres stock between ∼ 14 and 12.5 Ma, through generation of numerous porphyry B and A phases and associated magmatic-hydrothermal centers between ∼ 12.3 and 10.5 Ma, to intrusion of late mineral porphyry at Frontera at ∼ 10.2 Ma. Similarly, the copper, molybdenum, and gold mineralization was introduced during a protracted interval of ∼ 1.7 m.y., between 11.8 and 10.1 Ma, as constrained by Re-Os molybdenite geochronology. The entire system cooled to nearly ambient temperatures by ∼ 8 Ma, as supported by temporally overlapping K-Ar, Ar/Ar, and (U-Th)/He ages, and was exposed to the effects of supergene oxidation and immature enrichment by ∼ 5 Ma. Plio-Pleistocene glaciation partially eroded a former, more widespread supergene chalcocite blanket, the remnants of which accounted for the bulk of the ore mined during the first 10 years of the Los Pelambres open-pit operation.

The southeast-inclined geometry of the entire Los Pelambres-Frontera system, including the porphyry centers and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to synmineral tilting. The tilting accompanied regional tectonic uplift during crustal shortening and thickening, which were controlled by thick-skinned reverse faults active ∼ 60 km farther east in Argentina.

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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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