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Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS

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Abstract

The El Laco magnetite deposits consist of more than 98 % magnetite but show field textures remarkably similar to mafic lava flows. Therefore, it has long been suggested that they represent a rare example of an effusive Fe oxide liquid. Field and petrographic evidence, however, suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs. We determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from various settings at El Laco and compared them with magnetite from both igneous and hydrothermal environments. This new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite “lava flows” formed. The trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite, including primary magnetite phenocrysts from the unaltered andesite host rocks at El Laco. Instead, the El Laco magnetite is most similar in composition to hydrothermal magnetite from high-temperature environments (>500 °C), such as iron oxide-copper-gold (IOCG) and porphyry-Cu deposits. The magnetite trace elements from massive magnetite are characterised by (1) depletion in elements considered relatively immobile in hydrothermal fluids (e.g. Ti, Al, Cr, Zr, Hf and Sc); (2) enrichment in elements that are highly incompatible with magmatic magnetite (rare earth elements (REE), Si, Ca, Na and P) and normally present in very low abundance in magmatic magnetite; (3) high Ni/Cr ratios which are typical of magnetite from hydrothermal environments; and (4) oscillatory zoning of Si, Ca, Mg, REE and most high field strength elements, and zoning truncations indicating dissolution, similar to that formed in hydrothermal Fe skarn deposits. In addition, secondary magnetite in altered, brecciated host rock, forming disseminations and veins, has the same composition as magnetite from the massive lenses. Euhedral magnetite lining both open-spaced veins in the brecciated host rock and along the walls of large, hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and/or physio-chemical conditions of the fluid. Thus, the chemical fingerprint of magnetite from the supposed El Laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows, by dissolution and precipitation of magnetite from high-temperature fluids, rather than a magmatic origin from an effusive Fe oxide liquid.

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Acknowledgments

This project was funded by NSERC, DIVEX, Vale, the Canadian Research Chair in Magmatic Metallogeny, and in part by the Geological Survey of Canada’s TGI4 program. We thank S. Medhi, D. Savard and M. Choquette for their assistance with laser ablation-ICP-MS and microprobe analyses, respectively. We thank SGA Chilean guides and D. Genna for their aid with sample collection and photos. Thanks are given to F. Tornos and L. Fontboté for additional samples from El Laco and to H. Mumin, L. Corriveau, J. Richards and D. Naslund for interesting and stimulating discussions. We are very grateful to comments from 3 reviewers (D. Burrows, D. Naslund and 1 anonymous) and Editor, B. Lehmann.

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  1. Sarah A. S. Dare

    Present address: Department of Earth Sciences, University of Ottawa, Ottawa, ON, K1H 6N5, Canada

Authors and Affiliations

  1. Département Sciences Appliquées, Université du Québec à Chicoutimi, Saguenay, QC, G7H 2B1, Canada

    Sarah A. S. Dare & Sarah-Jane Barnes

  2. Département Géologie, Université Laval, Québec, QC, G1V 0A6, Canada

    Georges Beaudoin

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  1. Sarah A. S. Dare
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  2. Sarah-Jane Barnes
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Correspondence to Sarah A. S. Dare.

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Editorial handling: B. Lehmann

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Dare, S.A.S., Barnes, SJ. & Beaudoin, G. Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS. Miner Deposita 50, 607–617 (2015). https://doi.org/10.1007/s00126-014-0560-1

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