Fluid-Assisted Aggregation and Assembly of Magnetite Microparticles in the Giant El Laco Iron Oxide Deposit, Central Andes

Abstract

The El Laco iron oxide mineral deposit in the CentralAndes ofChile has attracted significant attention because of its uniquelypreserved massive magnetite orebodies, which bear a remarkable similarityto volcanic products. To date, the outcropping highly vesicular andporous massive magnetite orebodies have received little attentionfrom a microtextural point of view, limiting our understanding aboutthe role of volcanogenic processes on iron mineralization. Here, wereport the chemical composition of vesicular magnetite at El Lacousing EPMA and LA-ICP-MS methods and provide detailed 2D and 3D imagingof the internal structure of these texturally complex magnetite oresby combining SEM observations, synchrotron radiation micro-X-ray fluorescencechemical mapping, and high-resolution X-ray computed microtomography.Our observations reveal the presence of abundant magnetite microsphereswith diameters ranging from & SIM;100 to & SIM;900 & mu;m, aswell as dendritic microstructures forming interconnected networksup to a few millimeters in size. Two-dimensional microtextural andgeochemical imaging of the microspheres show that these features areformed by multiple euhedral magnetite crystals growing in all directionsand occur immersed within a porous matrix conformed by smaller-sized(& SIM;2-20 & mu;m) and irregularly shaped magnetite microparticles.These types of morphologies have been reported in hydrothermal ventsassociated with hydrovolcanic processes and commonly described inhydrothermal synthesis experiments of magnetite microspheres, suggestingprecipitation from iron-rich fluids. A hydrothermal origin for themagnetite microparticles reported here is further supported by theirgeochemical signature, which shows a strong depletion in most minorand trace elements typical from magnetite precipitated from hydrothermalfluids in ore-forming environments. We propose that decompression,cooling, and boiling of fluids triggered massive iron supersaturation,resulting in the nucleation of magnetite microparticles or colloids,followed by self-assembly into larger and more complex microstructures.Our data from El Laco deposit agree with models invoking magmatic-hydrothermalfluids to explain the origin of the deposit and provide new insightson the potential role of iron colloids as agents of mineralizationin volcanic systems.