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

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.

Evidence for iron-rich sulfate melt during magnetite(-apatite) mineralization at El Laco, Chile

“…Thus, our results point to similar processes, whereby liquids dominated by sulfate or carbonate are produced by assimilation of evaporites and/or carbonates by mafic to intermediate silicate melts. This is also in accord with recent studies of triple O isotopes in Mt(-Ap) deposits, which indicate interaction between magmatic fluids, evaporites, and carbonate rocks (Peters et al, 2020). These results do not preclude the influence of other fluid(s) or processes in forming the range of mineralization styles at El Laco or other Mt(-Ap) deposits, but provide evidence for a key role of sulfaterich melts as part of the ore-forming process.…”

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

“…Nevertheless, magnetite  18 O values from the deposits that we studied (1.9‰ ≤ δ 18 O ≤ 6.9‰) agree well with the range of magnetite  18 O values found in IOA deposits (e.g., Pea Ridge: 1.0‰ ≤ δ 18 O ≤ 7.0‰; Childress et al, 2016). Aftabi and Mohseni also state that the "erratic" distribution of the magnetite triple oxygen isotope data in Peters et al (2020) disagrees with an IOA origin of the magnetite. This statement is incorrect as well.…”

“…argue that the magnetite deposits studied by Peters et al (2020) are banded iron formations (BIFs), rather than iron-oxide apatite (IOA) deposits. A putative BIF origin for the iron deposits in central Iran was previously advocated by the same authors in Comments on other papers, and refuted in the corresponding Replies by Jami et al (2009), Bonyadi et al (2011), andGhazi andMoazzen (2020).…”

Experimental Evolution of Magnetite Nanoparticle Resistance in Escherichia coli

“…Magnetite NPs exist in nature, and engineered FeNPs are widely used in a range of applications due to their advanced optical properties [ 18 ]. FeNPs can be used to determine oxygen concentrations [ 22 , 23 , 24 ]. FeNPs are involved in orientation, navigation, and iron metabolism in prokaryotic and eukaryotic cells [ 22 , 25 ].…”