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New Chronology for El Teniente, Chilean Andes, from U-Pb, 40Ar/39Ar, Re-Os, and Fission-Track Dating: Implications for the Evolution of a Supergiant Porphyry Cu-Mo Deposit

By
Victor Maksaev
Victor Maksaev
Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
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Francisco Munizaga
Francisco Munizaga
Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
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Michael McWilliams
Michael McWilliams
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115
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Mark Fanning
Mark Fanning
Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
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Ryan Mathur
Ryan Mathur
Geology Department, Juniata College, 1700 Moore Street, Huntington, Pennsylvania 16652
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Joaquin Ruiz
Joaquin Ruiz
Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040E Fourth Street, Tucson, Arizona 85721-0077
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Marcos Zentilli
Marcos Zentilli
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5
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Published:
January 01, 2005

Abstract

Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40Ar/39Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40Ar/39Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40Ar/39Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district.

Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40Ar/39Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40Ar/39Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y.

The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartzsericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40Ar/39Ar).

The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105° ± 20°C, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40Ar/39Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody.

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Contents

Geological Society, London, Engineering Geology Special Publications

Andean Metallogeny: New Discoveries, Concepts, and Updates

Richard H. Sillitoe
Richard H. Sillitoe
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José Perelló
José Perelló
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César E. Vidal
César E. Vidal
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Society of Economic Geologists
Volume
11
ISBN electronic:
9781629490359
Publication date:
January 01, 2005

GeoRef

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