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The Giant El Teniente Breccia Deposit: Hypogene Copper Distribution and Emplacement

By
M. Alexandra Skewes
M. Alexandra Skewes
Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399
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Alejandra Arévalo
Alejandra Arévalo
Superintendencia Geología, El Teniente, CODELCO-Chile, Rancaguua, Chile
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Ricardo Floody
Ricardo Floody
Superintendencia Geología, El Teniente, CODELCO-Chile, Rancaguua, Chile
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Patricio H. Zuñiga
Patricio H. Zuñiga
Superintendencia Geología, El Teniente, CODELCO-Chile, Rancaguua, Chile
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Charles R. Stern
Charles R. Stern
Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399
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Published:
January 01, 2002

Abstract

The giant El Teniente copper-molybdenum deposit, located in the Andes of central Chile, is one of the world's largest known copper deposits, containing estimated resources of >75 × 106 t of fine copper in ore with grades greater than 0.67 percent. El Teniente has been described in the past as a porphyry deposit developed around a Pliocene dacite porphyry stock, with 80 percent of its copper mineralization hosted in Miocene andesites. However, new mapping—both regional and in underground mine workings—along with petrological studies, indicates that El Teniente, like the other giant Miocene and Pliocene copper deposits in central Chile, is actually best classified as a breccia deposit. Most of the high-grade hypogene copper at El Teniente occurs in and surrounding multiple magmatic-hydrothermal breccia pipes. Mineralized breccia complexes, with copper content >1 percent, have vertical extents of >1.5 km, and their roots are as yet unknown. These breccias are hosted in a pervasively biotite-altered and mineralized mafic intrusive complex composed of gabbros, diabases, and porhyritic basalts and basaltic andesites, and not in andesite extrusive rocks. The multiple breccias in El Teniente include copper- and sulfide-rich biotite, igneous, tourmaline, and anhydrite breccias, generated by the exsolution of magmatic fluids from cooling plutons, and also magnetite and rock-flour breccias. Surrounding biotite breccias, a dense stockwork of biotite-dominated veins has produced pervasive biotite alteration, and copper mineralization characterized by chalcopyrite >> bornite + pyrite. Later veins, with various proportions of quartz, anhydrite, sericite, chlorite, tourmaline, feldspars, and copper sulfide minerals, formed in association with the emplacement of younger breccias and felsic porphyry intrusions. These generated sericitic alteration in the upper levels of the deposit, and in some cases contributed more copper to the deposit, but in other cases eliminated or redistributed preexisting mineralization. Both the Teniente Dacite porphyry and the central rock-flour breccia of the Braden pipe, the dominant lithostructural unit in the deposit, are copper poor. Their emplacement at a late stage in the development of the deposit created a relatively barren core, surrounded by a thin (~150-m) zone of bornite > chalcopyrite, in the larger main area of chalcopyrite-rich, biotitealtered mafic rocks and mineralized breccias.

The multistage development of breccia emplacement, alteration and copper mineralization at El Teniente occurred over a time span that was greater than 2 m.y., between >6.4 and 4.4 Ma, at the end of a more than 10-m.y. episode of Miocene and Pliocene magmatic activity, and just prior to the eastward migration of the Andean magmatic arc as a consequence of decreasing subduction angle. Decreasing subduction angle also caused crustal thickening, uplift and erosion, resulting in telescoping of the various breccias and felsic intrusions in the deposit. El Teniente is located at the intersection of major north-south, northwest-southeast, and northeast-southwest Andean structures, but what actually focused magmatic activity and mineralization at this one locality for so long remains an unsolved problem, the solution of which would provide an important tool for exploration of similar giant deposits.

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Contents

Special Publications of the Society of Economic Geologists

Integrated Methods for Discovery: Global Exploration in the Twenty-First Century

Richard J. Goldfarb
Richard J. Goldfarb
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Richard L. Nielsen
Richard L. Nielsen
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Society of Economic Geologists
Volume
9
ISBN electronic:
9781629490335
Publication date:
January 01, 2002

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