Skip to Main Content
Book Chapter

Ore Breccias in the Rio Blanco-Los Bronces Porphyry Copper Deposit, Chile

By
Vargas R. Ricardo
Vargas R. Ricardo
CODELCO-Chile, Division Andina, Saladillo
,
Chile
Search for other works by this author on:
Lewis B. Gustafson
Lewis B. Gustafson
5320 Cross Creek Lane, Reno, Nevada 89511
Search for other works by this author on:
Monica Vukasovic
Monica Vukasovic
Search for other works by this author on:
Tidy F. Enrique
Tidy F. Enrique
CODELCO-Chile, Gerencia Exploracioones, Santiago
,
Chile
Search for other works by this author on:
M. Alexandra Skewes
M. Alexandra Skewes
Department of Geological Sciences
,
University of Colorado, Boulder, Colorado 80303
Search for other works by this author on:
Published:
January 01, 1999

Abstract

Detailed logging of core from drill holes in the Sur-Sur and La Americana breccias in the Andina portion of the Rio Blanco-Los Bronces porphyry copper deposit, supported by petrography, geochemistry, and study of fluid inclusions, has documented zonal and temporal patterns in ore breccias over 1,600 vertical meters. Breccia textures progress from incipient crackle breccia with tourmaline veining to increased rounding of fragments and filling of open spaces by mineralization products and rock flour, reflecting many repeated pulses of brecciation. Both angular breccias and rounded clast breccias with rock-flour matrix have been mineralized primarily by the quiescent flow of hydrothermal solutions between pulses of brecciation. Sharp contacts mark boundaries between pulses separated by a period of thorough cementation, while diffuse, gradational contacts mark boundaries between more closely spaced pulses. Refracturing of the rock mass continued after consolidation of rock-flour matrix breccias, as documented by local angular rebrecciation and ubiquitous postbreccia veining.

The angular, tourmaline breccia with chalcopyrite-pyrite at Sur-Sur grades downward, with decreasing tourmaline and increasing biotite in the matrix, into breccia with biotite-alkali feldspar alteration and chalcopyrite-bornite. Variably tourmalinized rock-flour breccias at La Americana extend about 400 m higher than Sur-Sur, with upward increasing ratios of specularite/tourmaline and pyrite/chalcopyrite, and decreasing grades of Cu and Mo. Minor dikes of porphyry have intruded the breccias but are themselves fragmented and appear to be contemporaneous with brecciation. Dikes that have intruded deep, high-grade Sur-Sur breccia display intense biotite alteration and disseminated chalcopyrite-bornite. A high-grade interval of La Americana breccia has been intruded by a dike with intense sericite-chalcopyrite alteration. Dikes intruding poorly mineralized La Americana breccias are barren and are not biotized. The Sur-Sur breccias were formed contemporaneously with early-stage porphyry copper mineralization at depth, and are cut by a sequence of quartz-molybdenite and sulfide veins with sericitic halos that are typical of the evolution of veining in porphyry copper systems.

In both the Sur-Sur and La Americana breccia matrices, highly saline fluid inclusions, saturated with NaCl (over filling temperatures from 225° to 500°C), coexist with vapor-rich inclusions and with fluid-rich inclusions from 150° to 450°C and 2 to 30 wt percent NaCl equiv. High-salinity fluids are most abundant at depth and with higher copper grades, while liquid-rich and vapor-rich fluids are dominant near surface, particularly at La Americana. This, and prior stable isotope evidence, is compatible with the interpretation that magmatic fluids, derived from magma giving rise to porphyry dikes and the PΔV energy for brecciation, were primarily responsible for mineralization of the ore breccias. Intermixed meteoric water, however, may have been responsible for the huge volume and complex reworking of the breccias, and for apparently wide fluctuations in temperature, pressure, fo2, and salinity, which are suggested by fluctuations in magnetite-hematite and anhydrite saturation in the breccias. This district represents nearly an end member in the wide range of variations that are characteristic of porphyry copper mineralization. The breccias are copper ores because they were formed and mineralized by intrusions derived from a differentiated magma chamber which became saturated with typical porphyry copper-ore fluids. Location, complexity, and geochemistry offer the explorationist clues to the relatively rare tourmaline breccia, which may be ore bearing.

You do not currently have access to this article.

Figures & Tables

Contents

Special Publications of the Society of Economic Geologists

Geology and Ore Deposits of the Central Andes

Brian J. Skinner
Brian J. Skinner
Search for other works by this author on:
Society of Economic Geologists
Volume
7
ISBN electronic:
9781629490311
Publication date:
January 01, 1999

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal