Abstract
Magnetite is the main constituent of iron oxide–apatite (IOA) deposits, which are a globally important source of Fe and other elements such as P and REE, critical for modern technologies. Geochemical studies of magnetite from IOA deposits have provided key insights into the ore-forming processes and source of mineralizing fluids. However, to date, only qualitative estimations have been obtained for one of the key controlling physico-chemical parameters, i.e., the temperature of magnetite formation. Here we reconstruct the thermal evolution of Andean IOA deposits by using magnetite thermometry. Our study comprised a > 3000 point geochemical dataset of magnetite from several IOA deposits within the Early Cretaceous Chilean Iron Belt, as well as from the Pliocene El Laco IOA deposit in the Chilean Altiplano. Thermometry data reveal that the deposits formed under a wide range of temperatures, from purely magmatic (~ 1000 to 800 °C), to late magmatic or magmatic-hydrothermal (~ 800 to 600 °C), to purely hydrothermal (< 600 °C) conditions. Magnetite cooling trends are consistent with genetic models invoking a combined igneous and magmatic-hydrothermal origin that involve Fe-rich fluids sourced from intermediate silicate magmas. The data demonstrate the potential of magnetite thermometry to better constrain the thermal evolution of IOA systems worldwide, and help refine the geological models used to find new resources.
Highlights
Magnetite is the main constituent of iron oxide–apatite (IOA) deposits, which are a globally important source of Fe and other elements such as P and REE, critical for modern technologies
The thermal evolution of IOA deposits remains poorly constrained and, to date, obtaining quantitative temperature data directly from magnetite mineralization has been challenging. This raises uncertainties on genetic models proposed to explain the genesis of Andean IOA deposits, which over the years have invoked: (1) immiscible Fe, P, and carbonate-sulfate-rich m elts[29,30,31,32,33]; (2) metasomatic r eplacement[17,34,35]; and (3) magmatic-hydrothermal processes[10,11,21,24,25,36,37]
Temperature estimations for Andean IOA deposits have been determined using oxygen isotope thermometry between magnetite-actinolite and magnetite-pyroxene pairs[19,23,24,27,29], thermometry of magnetite-ilmenite pairs[38], actinolite thermometry based on Fe c ontents19,22,27, 1Department of Geology and Millennium Nucleus for Metal Tracing Along Subduction, FCFM, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. 2Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. 3Department of Earth and Environmental Sciences, University of Michigan, 1100
Summary
Magnetite is the main constituent of iron oxide–apatite (IOA) deposits, which are a globally important source of Fe and other elements such as P and REE, critical for modern technologies. The microtextures and trace element and isotopic compositions of magnetite samples from Andean IOA deposits have been widely studied[10,11,15,17,18,19,20,21,22,23,24,25,26,27,28] These studies scaffold a framework to better understand the ore-forming processes, revealing that the Fe ore forms by mechanisms that involve growth of magnetite under a wide range of conditions, spanning from high-temperature, purely igneous settings, to lower-temperature, fluid-dominated hydrothermal environments. Despite these advances, the thermal evolution of IOA deposits remains poorly constrained and, to date, obtaining quantitative temperature data directly from magnetite mineralization has been challenging. This allowed us to confirm the robustness of this thermometer, as well as elucidating micro-analytical uncertainties that lead to temperature overestimations and potential misinterpretations
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