Abstract

Although iodine is rare in crustal settings, previous studies have documented its occurrence in the supergene zones of base and precious metal ore deposits in arid environments. In this report, we present a novel application of the iodine-129 (129I) isotope tracer to iodine-rich samples from the world’s largest supergene Cu profile at the Chuquicamata deposit in northern Chile, where anomalous concentrations of this element have been reported. All supergene marshite (CuI) samples from Chuquicamata and iodine-rich soil above the Mansa Mina deposit have 129I/I isotope ratios (~190–560 × 10−15) that are significantly lower than those of surface waters (i.e., 1,500 × 10−15), indicating that iodine was most likely derived from a reservoir of marine origin rather than a meteoric and/or atmospheric source. Geochemical modeling shows that the long-term dilution of iodine-rich deep formation waters by meteoric water during the main stage of supergene alteration (~40–10 Ma) resulted in fluid 129I/I ratios that are within the range of mineral/soil 129I/I ratios observed at Chuquicamata, strongly suggesting that iodine was remobilized from the Lower Jurassic to mid Cretaceous marine basement. Results from this study show that iodine isotopes can be successfully applied to trace the origin and nature of supergene fluids in iodine-rich oxide blankets, leach caps, and soils above buried Cu deposits, and also to constrain the timescales of fluid circulation during supergene enrichment.

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