V52B-02
A Holistic Model That Physicochemically Links Iron Oxide - Apatite and Iron Oxide – Copper – Gold Deposits to Magmas
Friday, 18 December 2015: 10:35
310 (Moscone South)
Adam Charles Simon1, Martin Reich2, Jaayke Knipping1, Laura Bilenker3, Fernando Barra4, Artur Deditius5, Craig Lundstrom6 and Ilya N Bindeman7, (1)Univ of Michigan, Ann Arbor, MI, United States, (2)University of Chile, Department of Geology and Andean Geothermal Center of Excellence (CEGA), Santiago, Chile, (3)University of Michigan, Ann Arbor, MI, United States, (4)Department of Geology, Universidad de Chile, Santiago, Chile, (5)Murdoch University, Murdoch, Western Australia, Australia, (6)Univ Illinois Urbana Champaign, Urbana, IL, United States, (7)University of Oregon, Department of Geological Sciences, Eugene, OR, United States
Abstract:
Iron oxide-apatite (IOA) and iron oxide-copper-gold deposits (IOCG) are important sources of their namesake metals and increasingly for rare earth metals in apatite. Studies of natural systems document that IOA and IOCG deposits are often spatially and temporally related with one another and 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 Fe and O isotope data and high-resolution trace element (e.g., Ti, V, Mn, Al) data of individual magnetite grains from the world-class Los Colorados (LC) IOA deposit in the Chilean Iron Belt to elucidate the origin of IOA and IOCG deposits. Values of d56Fe range from 0.08‰ to 0.26‰, which are within the global range of ~0.06‰ to 0.5‰ for magnetite formed at magmatic conditions. Values of δ18O for magnetite and actinolite are 2.04‰ and 6.08‰, respectively, consistent with magmatic values. Ti, V, Al, and Mn are enriched in magnetite cores and decrease systematically from core to rim. Plotting [Al + Mn] vs. [Ti + V] indicates that magnetite cores are consistent with magmatic and/or magmatic-hydrothermal (i.e., porphyry) magnetites. Decreasing Al, Mn, Ti, V is consistent with a cooling trend from porphyry to Kiruna to IOCG systems. The data from LC are consistent with the following new genetic model for IOA and IOCG systems: 1) magnetite cores crystallize from silicate melt; 2) these magnetite crystals are nucleation sites for aqueous fluid that exsolves and scavenges inter alia 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 metal-oxides and metal-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.
