Constraints of Fe-O isotopes on the origin of magnetite in the El Laco Kiruna-type iron deposit, Chile

Abstract

The Pliocene El Laco iron deposit is the youngest and best-preserved Kiruna-type deposit in the world, the genesis of which has been on dispute for several decades. The massive magnetite (δ56Fe = 0.28–0.39‰, with average value of 0.35‰), the magnetite in the veined magnetite-apatite ores (i.e., veined magnetite; δ56Fe = 0.16–0.41‰, with average value of 0.26‰), the magnetite in scoriaceous ores (δ56Fe = 0.26–0.34‰, with average value of 0.28‰) and the magnetite found in degassing cavities (δ56Fe = 0.27–0.28‰, with average value of 0.28‰) yielded Fe isotope compositions consistent with the igneous magnetite in the host fresh andesites (δ56Fe = 0.13–0.46‰, with average value of 0.23‰). On the other hand, the veined magnetite also contains slightly higher δ18O (0.65–5.98‰) than the general igneous magnetite (δ18O = +1.5–+4.5‰), consistent with the O isotopes of massive magnetite as previously reported (δ18O = 4.3–5.0‰). Both the Fe and O isotopes of the magnetite demonstrate that the El Laco magnetite ores might be derived from a high-temperature ortho-magmatic origin. Additionally, the El Laco fresh andesites display δ18O values of 10.34–12.06‰, which might be not equilibrated with the magnetite in the ores. By combining the data of Fe-O isotopes with the geological features, we propose that the El Laco magnetite ores might not originate from the ore-hosting andesitic melt, but from a more mafic Fe-rich parental melt.