IRON OXIDE – APATITE, IRON OXIDE – COPPER – GOLD DEPOSITS AND MAGMAS: A BUBBLY CONNECTION

Iron oxide-apatite (IOA) and iron oxide-copper-gold deposits (IOCG) are often spatially and temporally related with one another and with coeval magmatism. However, a genetic model that accounts for observations of natural systems remains elusive, with few observational data able to distinguish among working hypotheses that invoke meteoric fluid, magmatic-hydrothermal fluid, and immiscible melts. Here, we use high-resolution trace element concentrations in magnetite, hematite and pyrite, high-precision Fe and O stable isotope data of magnetite and hematite grains, δD of magnetite and actinolite, and Re and Os in magnetite and pyrite from the Los Colorados IOA and Candelaria and Mantoverde IOCG deposits in the Chilean Iron Belt to elucidate the origin of IOA and IOCG systems. At Los Colorados, Ti, V, Al, and Mn are enriched in magnetite cores and decrease systematically from core to rim, a trend consistent with magmatic and/or magmatic-hydrothermal magnetites. High Co/Ni ratios of pyrite from Los Colorados are also consistent with a magmatic-hydrothermal origin. δD values for magnetite and actinolite indicate a mantle source for H. Values of d⁵⁶Fe and d¹⁸O for magnetite and hematite from all deposits indicate a magmatic source for Fe and O. The Re-Os systematics overlap data from Andean porphyry Cu-Mo deposits and are consistent with a magmatic-hydrothermal origin. Together, the data are consistent with a genetic model wherein 1) magnetite cores crystallize from silicate melt; 2) these magnetite crystals are nucleation sites for aqueous fluid that exsolves and scavenges Fe, P, S, Cu, Au from silicate melt; 3) the magnetite-fluid suspension is less dense that the surrounding magma, allowing ascent; 4) as the suspension ascends, magnetite grows in equilibrium with the fluid and takes on a magmatic-hydrothermal character (i.e., lower Al, Mn, Ti, V); 5) during ascent, magnetite, apatite and actinolite are deposited to form IOA deposits; 6) the further ascending fluid transports Fe, Cu, Au and S toward the surface where hematite, magnetite and sulfides precipitate to form IOCG deposits. This model is globally applicable and explains the observed temporal and spatial relationship between magmatism and formation of IOA and IOCG deposits.