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

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 (delta Fe-56 = 0.28-0.39%, with average value of 0.35%), the magnetite in the veined magnetite-apatite ores (i.e., veined magnetite; delta Fe-56 = 0.16-0.41%, with average value of 0.26%), the magnetite in scoriaceous ores (delta Fe-56 = 0.26-0.34%, with average value of 0.28%) and the magnetite found in degassing cavities (delta Fe-56 = 0.27-0.28%, with average value of 0.28%) yielded Fe isotope compositions consistent with the igneous magnetite in the host fresh andesites (delta Fe-56 = 0.13-0.46%, with average value of 0.23%). On the other hand, the veined magnetite also contains slightly higher delta O-18 (0.65-5.98%) than the general igneous magnetite (delta O-18 = +1.5-+4.5%), consistent with the O isotopes of massive magnetite as previously reported (beta O-18 = 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 orthomagmatic origin. Additionally, the El Laco fresh andesites display delta O-18 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.