Relative oxidation states of magmas inferred from Ce“IV”/Ce“III” in zircon: Application to porphyry copper deposits of northern Chile

Julian R. Ballard, J. Michael Palin, Ian H. Campbell

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    Abstract

    Major- and trace-element compositions of zircons and whole rocks from 14 barren and seven ore-bearing calc-alkaline intrusions from the Chuquicamata- El Abra porphyry copper belt of northern Chile have been measured in situ by excimer laser ablation “ELA” ICP-MS. These data permit the Ce“IV”/Ce“III” ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce“IV” and Ce“III”. Zircon Ce“IV”/Ce“III” and EuN/EuN* ratios, and by inference magmatic oxidation states, generally increase from older, mafic to younger, felsic units. Within this sequence, porphyry copper mineralization is directly associated only with intrusions with zircon Ce“IV”/Ce“III” > 300 and EuN/EuN* > 0.4. Such trends can be understood in terms of interdependent relations between oxygen fugacity, sulfur specification and solubility, and chalcophile element partitioning in silicate magmas. Because zircon occurs in most calc-alkaline intrusions and is resistant to subsolids alteration, zircon Ce“IV”/Ce“III” ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic-hydrothermal Cu±Au mineralization. The approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks.

    Original languageEnglish
    Pages (from-to)347-364
    Number of pages18
    JournalContributions to Mineralogy and Petrology
    Volume144
    Issue number3
    DOIs
    Publication statusPublished - Dec 2002

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