Time relationships between volcanism–plutonism–alteration–mineralization in Cu-stratabound ore deposits from the Michilla mining district, northern Chile: a 40Ar/39Ar geochronological approach (original) (raw)

Abstract

The Michilla mining district comprises one of the most important stratabound and breccia-style copper deposits of the Coastal Cordillera of northern Chile, hosted by the Middle Jurassic volcanic rocks of the La Negra Formation. 40Ar/39Ar analyses carried out on igneous and alteration minerals from volcanic and plutonic rocks in the district allow a chronological sequence of several magmatic and alteration events of the district to be established. The first event was the extrusion of a thick lava series of the La Negra Formation, dated at 159.9 ± 1.0 Ma (2_σ_) from the upper part of the series. A contemporaneous intrusion is dated at 159.6 ± 1.1 Ma, and later intrusive events are dated at 145.5 ± 2.8 and 137.4 ± 1.1 Ma, respectively. Analyzed alteration minerals such as adularia, sericite, and actinolite apparently give valid 40Ar/39Ar plateau and miniplateau ages. They indicate the occurrence of several alteration events at ca. 160–163, 154–157, 143–148, and 135–137 Ma. The first alteration event, being partly contemporaneous with volcanic and plutonic rocks, was probably produced in a high thermal gradient environment. The later events may be related either to a regional low-grade hydrothermal alteration/metamorphism process or to plutonic intrusions. The Cu mineralization of the Michilla district is robustly bracketed between 163.6 ± 1.9 and 137.4 ± 1.1 Ma, corresponding to dating of actinolite coexisting with early-stage chalcocite and a postmineralization barren dyke, respectively. More precisely, the association of small intrusives (a dated stock from the Michilla district) with Cu mineralization in the region strongly suggests that the main Michilla ore deposit is related to a magmatic/hydrothermal event that occurred between 157.4 ± 3.6 and 163.5 ± 1.9 Ma, contemporaneous or shortly after the extrusion of the volcanic sequence. This age is in agreement with the Re–Os age of 159 ± 16 Ma obtained from the mineralization itself (Tristá-Aguilera et al., Miner Depos, 41:99–105,2006).

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Acknowledgments

This work was financially supported by the Institut de Recherche pour le Développement (IDR), a MECESUP-Chile scholarship (D Tristá–Aguilera), and Minera Michilla S.A. Geologists and assistants of the Department of Geology of Minera Michilla S.A are also gratefully thanked for allowing us to undertake this study and its geological contribution. M. Manetti is thanked for analytical assistance. Two anonymous reviewers and J Le Roux are thanked for their helpful comments on the previous version of this manuscript.

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1. Verónica Oliveros
   Present address: Departamento de Ciencias de la Tierra, Universidad de Concepción, P.O. Box 160-C, Concepción, Chile

Authors and Affiliations

1. Departamento de Geología, Universidad de Chile, P.O. Box 13518-21, Santiago, Chile
   Verónica Oliveros, Diego Morata & Luis Aguirre
2. Departamento de Ciencias Geológicas, Universidad Católica del Norte, Av Angamos 0610, Casilla 1280, Antofagasta, Chile
   Dania Tristá-Aguilera & Shoji Kojima
3. CNRS-IRD-UNSA UMR 6526 Géosciences Azur, Parc Valrose, Université de Nice-Sophia Antipolis, 06108, Nice, France
   Gilbert Féraud
4. Superintendencia de Geología, Minera Michilla S.A., km 110 camino a Tocopilla, Mejillones, Segunda Región de Antofagasta, Chile
   Fernando Ferraris

Authors

1. Verónica Oliveros
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2. Dania Tristá-Aguilera
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3. Gilbert Féraud
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4. Diego Morata
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5. Luis Aguirre
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6. Shoji Kojima
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7. Fernando Ferraris
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Correspondence toVerónica Oliveros.

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Oliveros, V., Tristá-Aguilera, D., Féraud, G. et al. Time relationships between volcanism–plutonism–alteration–mineralization in Cu-stratabound ore deposits from the Michilla mining district, northern Chile: a 40Ar/39Ar geochronological approach.Miner Deposita 43, 61–78 (2008). https://doi.org/10.1007/s00126-007-0147-1

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• Received: 29 March 2006
• Accepted: 31 May 2007
• Published: 28 July 2007
• Issue Date: January 2008
• DOI: https://doi.org/10.1007/s00126-007-0147-1

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