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Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters?

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Abstract

Atacamite, a copper hydroxychloride, is an important constituent of supergene oxide zones of copper deposits in northern Chile, whereas in similar deposits elsewhere, it is rare. In Chile, it has generally been assumed to be a primary constituent of the supergene zones. There are two difficulties with this supposition. The first is that atacamite requires saline water for its formation, whereas supergene oxidation was caused by percolating, oxygenated meteoric water, mainly rainwater. The second is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water. Supergene enrichment of copper deposits in northern Chile extended over a long period, 44 to 9 Ma, being terminated by the onset of hyperaridity. During this period, there was at least intermittent rainfall, exposing previously formed atacamite to dissolution or phase change. Furthermore, atacamite-bearing oxide zones in several deposits are directly overlain by thick Miocene alluvial gravels; the stream waters that transported these gravels would have permeated the oxide zones. In some deposits, atacamite-bearing assemblages occur both in the oxide zones and in contiguous gravels. We suggest that atacamite-bearing oxide assemblages are more likely to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 Ma. A hyperarid climate made possible evaporation and concentration of chloride in meteoric waters. In this paper, we discuss another source of saline waters to modify oxide zones. Dewatering of the Domeyko Basin expelled brines along faults, some of which had earlier guided the location of porphyry deposits. At the Spence porphyry copper deposit, saline waters, which δD vs δ 18O isotope analyses identify as basinal brines, are presently rising through the deposit, then flowing away along the base of the covering gravels. Compositions of these waters lie within the stability fields of atacamite and brochantite, the two minerals that comprise the oxide zone. Evidence is presented for other porphyry deposits, Radomiro Tomic and Gaby Sur, that basinal brines may have been involved in the late formation of atacamite.

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Acknowledgments

Our work at Spence would not have been possible without the generous help of Ollie Bonham and Jack Currie of RioChilex. Alvaro Puig, Roorigo Diaz, and Victor Araya, all of Codelco, facilitated a visit by EMC to Radomiro Tomic, and Bob Clark of Actlabs provided kindly the Enzyme Leach data for the Radomiro Tomic soils. The work at Spence and Gaby Sur was carried out as part of a Canadian Mining Industry Research Organization study. We thank the director of that organization, Richard Alcock, and 26 company sponsors for their support. George Steele, exploration manager for Rio Tinto, provided kindly a vehicle and a geological technician. Daniel Salinas of Rio Tinto and Alexi Ramirez of RioChilex worked with us in the field. We thank them for their energy and skill and for their sense of humor. Gwendy Hall, Judy Vaive, and Peter Belanger of the Geological Survey of Canada provided the water analyses, and Eric Hoffman of Actlabs, the Enzyme Leach analyses of the Spence and Gaby Sur soils. We thank Terry Arcuri and Mark Hannington for helpful reviews.

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Authors and Affiliations

  1. Eion Cameron Geochemical Inc., Carp, Ottawa, ON, K0A 1L0, Canada

    Eion M. Cameron

  2. Ocean Exploration, GNS Science, Box 30-368, Lower Hutt, New Zealand

    Matthew I. Leybourne

  3. Department of Geology, Universidad de Chile, P.O. Box 13518-21, Santiago, Chile

    Carlos Palacios

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  1. Eion M. Cameron
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  2. Matthew I. Leybourne
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  3. Carlos Palacios
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Correspondence to Eion M. Cameron.

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Cameron, E.M., Leybourne, M.I. & Palacios, C. Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters?. Miner Deposita 42, 205–218 (2007). https://doi.org/10.1007/s00126-006-0108-0

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