The genesis of iron oxide-apatite (IOA) deposits, which have been historically a major source of iron for our society, has remained elusive for more than half a century, with proposed genetic models that span the spectrum from purely igneous to purely hydrothermal processes. Much of this long-standing debate has been centered in the Plio-Pleistocene El Laco deposit in the Chilean Altiplano, which is the youngest and one of the best preserved example of this mineralization type on Earth. Its massive magnetite orebodies emplaced on the flanks of an andesitic stratovolcano host more than a gigaton of iron and have attracted significant attention because of their uniquely preserved volcanic-like features which bear a striking resemblance to basaltic lava flows. Therefore, the El Laco deposit is considered as a key location to understand the processes that form IOA systems in volcanic settings.
In this talk I will present the results of a comprehensive study that combined fieldwork with a multi-instrumental microanalytical approach for geochemical and microtextural characterization of ore and volcanic rock samples, which allowed dissecting the El Laco system from its roots to the surface. In particular, I will show how microtextures and chemical composition of magnetite and associated silicate minerals can help us to better understand the processes involved in the formation and evolution of this enigmatic deposit. The results that will be shown support a genetic model that explains the formation of the El Laco deposit by a combination of magmatic and hydrothermal processes operating during the evolution of a volcanic system.