DIRECT EVIDENCE OF A MAGMATIC-HYDROTHERMAL HYPERSALINE ORE FLUID AT THE ENIGMATIC EL LACO IRON OXIDE APATITE DEPOSIT

Iron oxide-apatite (IOA) deposits (also known as Kiruna-type or magnetite-apatite deposits) contain tens to hundreds of millions of tons of Fe, and significant amounts of rare earth elements, P, Co, and V. These deposits are commonly hosted in genetically-unrelated andesitic rocks formed in arc environments. Styles of mineralization include massive tabular magnetite, breccias, lenses, veins, stockworks and pegmatite-like ore bodies. Proposed genetic models include: 1) metasomatic replacement of andesite via basinal brines; 2) silicate liquid immiscibility; 3) ascent of a magnetite-bearing magmatic-hydrothermal brine. Here we focus on the Pleistocene andesite-hosted El Laco IOA deposit, Chile, that is comprised of six coeval ore bodies consisting of ~98% magnetite and minor clinopyroxene. Outcropping massive magnetite ore bodies display magnetite with flow-textures and degassing structures that are similar to basaltic lava flows. Field and drill core observations reveal that the ore bodies are located in ring fractures around the central Pico Laco and extend for several hundred meters. The aim of this study is to constrain the nature and source of the ore fluid(s) that formed this deposit. Here we present data for clinopyroxene-hosted fluid inclusions in drill core samples. Primary fluid inclusions contain halite, sylvite, anhydrite, and opaque phases that have been reported to be magnetite and/or hematite, and a vapor phase. First homogenization occurs at ~400°C with some inclusions homogenizing fully at 740°C, and others not homogenizing at 1100°C after holding for 20 minutes. Freezing stage experiments failed to produce a change in phase composition. LA-ICP-MS data reveal that the fluids are 40-50 wt% NaCl eq. and have S concentrations of ~3-10 wt%. These data are consistent with previous studies that document the presence of a magmatic-hydrothermal hydrosaline fluid. Considering the high solubility of magnetite in hydrosaline fluids at magmatic conditions, and the significant decrease in magnetite solubility with decreasing pressure and temperature, these fluid inclusion data are consistent with formation of the massive magnetite ore bodies by precipitation of magnetite from a magmatic-hydrothermal brine.