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.

上新世El Laco铁矿床是世界上最年轻，保存最完好的基律纳型矿床，其成因已经争议了数十年。大量磁铁矿（δ ⁵⁶ FE = 0.28-0.39‰，0.35‰平均值），在脉磁铁矿磷灰石矿石的磁铁矿（即，脉磁铁矿;δ ⁵⁶ FE = 0.16-0.41‰，0.26平均值‰），在scoriaceous矿石磁铁矿（δ ⁵⁶ FE = 0.26-0.34‰，0.28平均值‰）和磁铁矿在脱气腔实测值（δ ⁵⁶ FE = 0.27-0.28‰，0.28平均值‰），得到铁同位素组成与宿主新鲜安山岩火成岩磁铁矿一致（δ ⁵⁶Fe = 0.13–0.46‰，平均值为0.23‰）。在另一方面，所述脉磁铁矿还略高包含δ ¹⁸ O（0.65-5.98‰）比一般火成岩磁铁矿（δ ¹⁸ O = + 1.5 + 4.5‰），进行大规模的磁铁矿的氧同位素一致如先前报道（δ ¹⁸ O = 4.3-5.0‰）。磁铁矿的Fe和O同位素均表明El Laco磁铁矿矿石可能源自高温正磁成因。另外，所述的El LACO新鲜安山岩显示δ ¹⁸O值为10.34–12.06‰，可能无法与矿石中的磁铁矿平衡。通过将Fe-O同位素的数据与地质特征相结合，我们提出El Laco磁铁矿可能不是源自含铁安山岩熔体，而是源自铁基富铁质母体熔体。