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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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