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

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 Eu_N/Eu_N* 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 Eu_N/Eu_N* > 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.