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Atacamite formation by deep saline waters in copper deposits from the Atacama Desert, Chile: evidence from fluid inclusions, groundwater geochemistry, TEM, and 36Cl data

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Abstract

The presence of large amounts of atacamite in oxide zones from ore deposits in the Atacama Desert of northern Chile requires saline solutions for its formation and hyperarid climate conditions for its preservation. We investigated the nature and origin of atacamite-forming solutions by means of coupling groundwater geochemical analyses with fluid inclusion data, high-resolution mineralogical observations, and chlorine-36 (36Cl) data in atacamite from the Mantos Blancos and Spence Cu deposits. In both deposits, the salinities of fluid inclusions in atacamite are comparable to those measured in saline groundwaters sampled from drill holes. The average salinity of fluid inclusions in atacamite for the Mantos Blancos and Spence deposits (~7–9 and 2–3 wt.% NaCleq, respectively) are strongly correlated to the salinities at which gypsum supersaturates from groundwaters in both deposits (total dissolved solids ~5–9 and 1–3 wt.% NaCleq, respectively). This correlation is confirmed by transmission electron microscopy observations of atacamite-bearing samples, revealing an intimate association between atacamite and gypsum that can be traced down to the nanometer scale. 36Cl data in atacamite provide new lines of evidence concerning the origin and age of the saline waters that formed atacamite in various stratabound and porphyry Cu deposits from the Atacama Desert. All atacamite samples show very low 36Cl-to-Cl ratios (11 × 10−15 to 28 × 10−15 at at−1), comparable to previously reported 36Cl-to-Cl ratios of deep formation waters and old groundwaters. In addition, 36Cl-to-Cl ratios in atacamite correlate with U and Th concentration in the host rocks but are independent from distance to the ocean. This trend supports an interpretation of the low 36Cl-to-Cl ratios in atacamite as representing subsurface production of fissiogenic 36Cl in secular equilibrium with the solutions involved in atacamite formation. Therefore, 36Cl in atacamite strongly suggest that the chlorine in saline waters related to atacamite formation is old (>1.5 Ma) but that atacamite formation occurred more recently (<1.5 Ma) than suggested in previous interpretations. Our data provide new constraints on the origin of atacamite in Cu deposits from the Atacama Desert and support the recent notion that the formation of atacamite in hyperarid climates such as the Atacama Desert is an ongoing process that has occurred intermittently since the onset of hyperaridity.

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Acknowledgements

Support for this study was received from Comisión Nacional de Ciencia y Tecnología de Chile grant # 1070736 to Carlos Palacios, Martin Reich, and Miguel Angel Parada. We are grateful to BHP-Billiton for logistical assistance and for providing access to the Spence deposit and specifically to Martin J. Williams, Mario Sáez, Walter Ruf, and Jorge Peña for their help during open-pit and core sampling. We also thank Anglo American for providing us with samples from Mantos Blancos and for releasing analytical data for publication (research contract D-1012). The transmission electron microscope used in this work was acquired under the Mecesup grant UCH-0205. We thank Christian Nievas and Marcela Robles for TEM sample preparation. Z. Lu at the University of Rochester prepared the AgCl targets; M. Caffee and the AMS group at PrimeLab, Purdue University, carried out the 36Cl determinations; we appreciate their efforts. We acknowledge Bernd Lehmann for his helpful and constructive review of the manuscript.

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Authors and Affiliations

  1. Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile

    Martin Reich, Carlos Palacios & Miguel A. Parada

  2. Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, 14267, USA

    Udo Fehn

  3. Eion Cameron Geochemical Inc., 865 Spruce Ridge Road, Carp, Ontario, K0A 1L0, Canada

    Eion M. Cameron

  4. Ocean Exploration, GNS Science, Lower Hutt, New Zealand

    Matthew I. Leybourne

  5. Departamento de Ingeniería Mecánica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile

    Alejandro Zúñiga

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  1. Martin Reich
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Correspondence to Martin Reich.

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Reich, M., Palacios, C., Parada, M.A. et al. Atacamite formation by deep saline waters in copper deposits from the Atacama Desert, Chile: evidence from fluid inclusions, groundwater geochemistry, TEM, and 36Cl data. Miner Deposita 43, 663–675 (2008). https://doi.org/10.1007/s00126-008-0184-4

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