Iron and Oxygen Isotope Signatures of Magnetite in Iron Ore of Kiruna Type

Abstract: Stable isotope analysis of iron and oxygen isotopes in magnetite of Kiruna-type from Sweden and Chile have been conducted to further the understanding of the formation, and advance the analysis methods of these deposits. For this, data from SIMS (secondary ion mass spectroscopy), EBSD (electron backscatter diffraction) and triple oxygen analysis (by laser fluorination) are used. Some researchers have found that crystal orientation can affect the oxygen and iron isotope ratios when analysing magnetite by SIMS, complicating the process. EBSD analysis therefore aided by finding the orientations of the crystals prior to SIMS analysis. Magnetite from Kiruna have been tested for use as SIMS reference material, but it did not prove to be suitable at present time. Further homoegeneity tests can be useful before declaring the sample as unsuitable. Preliminary iron isotope results from SIMS on magnetite from Kiirunavaara indicate an internal variation of δ 56 Fe between -0.72-0.70 per mil, with an uncertainty of 0.38 per mil (1σ). The variations could not be determined to match crystal orientations. Triple oxygen analyses show very low ∆'17 O on Kiruna samples, -357 to -171 ‰, indicating a strong MIF-O (mass-independently fractionated oxygen isotope composition) component. The MIF-O signal is also present in ∆'17 O records in evaporites from similar ages (1.7 - 1.9 Ga) (Crockford et al. 2019), which are known to partly contain oxygen derived from atmospheric O 2 . The MIF-O signal was not present in the Chilean magnetite samples, however, they are within the ∆'17 range of evaporites (Crockford et al.2019) from a similiar age (0 Ma vs 2 Ma). It is proposed that magmatic fluids interacted with evaporites, exchanging oxygen isotopes, before forming magnetite of Kiruna-type, thereby allowing the magnetite to contain part of the atmospheric isotopic record.