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Magnetic properties related to hydrothermal alteration processes at the Escondida porphyry copper deposit, northern Chile

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Abstract

Fluid–rock interaction related to the circulation of hydrothermal fluids can strongly modify the physicochemical properties of wall rocks in porphyry Cu deposits. These processes can also produce compositional and textural changes in ferromagnetic minerals, which can be quantified using magnetic methods. In the Escondida porphyry Cu deposit of northern Chile, each hydrothermally altered lithology is characterized by a discrete assemblage of Fe–Ti oxide minerals. These minerals have distinctive bulk magnetic susceptibility (K bulk), temperature-dependent magnetic susceptibility, and magnetic hysteresis parameters. Selectively altered rocks (i.e., potassic and chloritic alteration types) exhibit the highest K bulk values (>3.93 × 10−3 SI units), and their hysteresis parameters indicate multidomain magnetic mineral behavior. This suggests that these rocks are composed of the coarsest magnetic grain sizes within the deposit. Optical analyses and susceptibility–temperature curves confirm that the magnetic signals in selectively altered rocks are mainly carried by secondary magnetite. In contrast, pervasively altered rocks (i.e., quartz-sericite and argillic alteration types) exhibit low K bulk values (<1.93 × 10−4 SI units) and contain smaller pseudo-single domain magnetic grain assemblages. This is consistent with the destruction and/or reduction in size of magnetite under acidic conditions. The results therefore demonstrate a genetic relationship between the hydrothermal alteration processes, Fe–Ti oxide minerals, and magnetic properties of the wall rock in the Escondida deposit. These magnetic methods can be considered a sensitive and efficient petrophysical tool for the identification and semi-quantification of alteration assemblages, and facilitating the recognition and mapping of discrete hydrothermal zones during exploration and operation of porphyry Cu deposits.

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Acknowledgments

This research was funded by projects UCN-MEL 25.02.02/31.10.10.99, MECESUP 0711, and CONICYT 78092009. We thank Minera Escondida Ltd. for providing technical and logistical support during field and sampling activities. Analyses and interpretation of X-ray diffraction data were provided by Mr. F. Álvarez and Dr. N. Guerra, respectively. We also thank Dr. E. Medina for his valuable collaboration on mineralogical studies of Fe–Ti oxide minerals using scanning electron microscopy and Mr. G. Miranda for his assistance in preparation of some figures.

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  1. Departamento de Geología, Facultad de Ingeniería, Universidad de Atacama, Copayapu 485, Copiapó, Chile

    K. Riveros

  2. Departamento de Ciencias Geológicas, Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile

    K. Riveros, E. Veloso, E. Campos & A. Menzies

  3. Minera Escondida Ltd, Avenida de la Minería 501, Antofagasta, Chile

    W. Véliz

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Riveros, K., Veloso, E., Campos, E. et al. Magnetic properties related to hydrothermal alteration processes at the Escondida porphyry copper deposit, northern Chile. Miner Deposita 49, 693–707 (2014). https://doi.org/10.1007/s00126-014-0514-7

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