Abstract

Apatite grains from the Los Colorados iron oxide-apatite (IOA) deposit, the largest IOA deposit in the Chilean Iron Belt (CIB), exhibit significant intracrystalline spatial variability with respect to the concentrations of F, Cl, and OH and trace elements. Statistical interrogation of the compositional data indicates that individual apatite grains contain spatially discrete F-rich and Cl-rich domains. The chemical composition of the F-rich domains is consistent with apatite growth from silicate melts, whereas the chemical composition of the Cl-rich domains is consistent with apatite growth from a magmatic-hydrothermal fluid that cooled as it percolated outward from the Los Colorados fault-the structural control for emplacement of the ore body-into the surrounding brecciated diorite and andesite host rocks. Apatite in the deposit is intimately intergrown with magnetite and actinolite for which trace element, Fe, H, and O stable isotope data indicate a combined magmatic/magmatic-hydrothermal genesis for the deposit. The compositional data for apatite are consistent with a genetic model wherein F-rich apatite cores crystallized with magnetite from silicate melt, followed by exsolution of a magmatic-hydrothermal fluid during decompression of the parent magma. Experimental studies demonstrate that magmatic-hydrothermal volatile phase bubbles preferentially nucleate and grow on the surfaces of apatite and magnetite microlites during decompression of a magma body. Continued degassing of the melt results in the volatile phase sweeping up apatite and magnetite microlites, and forming a magnetite-apatite-fluid suspension that is buoyant in the magma chamber, and ascends from the source magma along faults during regional extension. Halite-saturated fluid inclusions in magnetite, which is paragenetically equivalent to apatite at Los Colorados, indicate that the magmatic-hydrothermal fluid was a brine, which allows this fluid to efficiently scavenge Cl, P, rare earth elements, and other fluid-compatible elements from the silicate melt. During ascent, the X-Cl/X-F ratio of apatite increases as it grows from the evolving Cl-rich magmatic-hydrothermal fluid during decompression and cooling.