Triple oxygen isotope variations in magnetite from iron-oxide deposits, central Iran, record magmatic fluid interaction with evaporite and carbonate host rocks

Stefan T.M Peters,Farhad Torab,Bernd Lehmann,Andreas Pack,Trevor Ireland,Seann J Mckibbin,Narges Alibabaie,Niloofar Nayebi,Davood Raeisi

doi:10.1130/g46981.1

Abstract

Abstract Oxygen isotope ratios in magnetite can be used to study the origin of iron-oxide ore deposits. In previous studies, only 18O/16O ratios of magnetite were determined. Here, we report triple O isotope data (17O/16O and 18O/16O ratios) of magnetite from the iron-oxide–apatite (IOA) deposits of the Yazd and Sirjan areas in central Iran. In contrast to previous interpretations of magnetite from similar deposits, the triple O isotope data show that only a few of the magnetite samples potentially record isotopic equilibrium with magma or with pristine magmatic water (H2O). Instead, the data can be explained if magnetite had exchanged O isotopes with fluids that had a mass-independently fractionated O isotope composition (i.e., MIF-O), and with fluids that had exchanged O isotopes with marine sedimentary carbonate rocks. The MIF-O signature of the fluids was likely obtained by isotope exchange with evaporite rocks of early Cambrian age that are associated with the IOA deposits in central Iran. In order to explain the triple O isotope composition of the magnetite samples in conjunction with available iron isotope data for magnetite from the deposits, we propose that magnetite formed from magmatic fluids that had interacted with evaporite and carbonate rocks at high temperatures and at variable water/rock ratios; e.g., magmatic fluids that had been released into the country rocks of a magma reservoir. Additionally, the magnetite could have formed from magmatic fluids that had exchanged O isotopes with SO2 and CO2 that, in turn, had been derived by the magmatic assimilation and/or metamorphic breakdown of evaporite and carbonate rocks.

Highlights

The formation of iron-oxide–apatite (IOA)deposits is much debated
The δ18O values of these samples are typical for magnetite that is in equilibrium with magma and with magmatic water; i.e., typical for “orthomagmatic magnetite” (δ18O = 1‰–4‰; Taylor, 1968)
The majority of the samples, have Δ′17O0.528 values ≪−40 ppm. Those samples cannot have been in equilibrium with magma or with pristine magmatic water when they formed

Summary

Introduction

The formation of iron-oxide–apatite (IOA)deposits is much debated. Studies on the origin of IOA deposits have often focused on the δ18O values of magnetite, which were suggested to reflect isotopic equilibrium with magmas or with magmatic water; i.e., H2O in equilibrium with magma (e.g., Jami et al, 2007; Nyström et al, 2008; Knipping et al, 2015; Bilenker et al, 2016; Johnson et al, 2016). We determined the triple O isotope compositions of 23 magnetite samples from IOA deposits in the Yazd and Sirjan areas in central Iran (Fig. 1).

Results

Conclusion