New Insights for the Formation of Kiruna-Type Iron Deposits by Immiscible Hydrous Fe-P Melt and High-Temperature Hydrothermal Processes: Evidence from El Laco Deposit

Abstract

The world-renowned Pliocene El Laco iron deposit in northern Chile is the youngest and the best-preserved Kiruna-type deposit in the world. The genesis of the El Laco Kiruna-type iron deposit—i.e., whether it is magmatic or hydrothermal in origin—is a long-standing controversy. The interstitial Fe-P phase lined by early formed magnetite crystals observed in massive ores at El Laco is morphologically and geochemically consistent with that produced by recent immiscible experiments, confirming that the massive magnetite ores were products of complete solidification of an iron-rich mush comprising early crystallized magnetite and an interstitial immiscible Fe-P melt. The hydrothermal features in geochemistry of massive magnetite are similar to the features of magnetite in altered andesites, implying a superimposed hydrothermal process. The occurrence of melt inclusions with high homogenization temperatures (>700°C) hosted by the apatite in Cristales Grandes ores indicates that the veined ores formed shortly after the massive ores genetically related to the magmatic system. Some vesicles in the massive ores and the magnetite scoriae, previously interpreted as compelling evidence of volcanic structures, are demonstrated to be the residual pore spaces formed after interstitial phases between magnetite grains were removed by postmagmatic hydrothermal fluids and the accumulated debris eroded from massive ores with minor altered andesites, respectively. Thus, we propose a two-step genetic model in which massive ores resulted from magmatic processes and then were modified by subsequent magmatic-hydrothermal alteration. This model could be applied to the metallogenesis of most Kiruna-type iron deposits.