AN INVESTIGATION OF THE CHEMISTRY OF APATITE IN IRON OXIDE- APATITE (IOA) DEPOSITS USING MICRO-ANALYTICAL METHODS

Iron oxide – apatite (IOA) deposits are an important source of iron ore and potentially as rare earth element (REE) resources owing to the REE enrichment of apatite. Globally, IOA deposits are found in districts that are spatially and temporally associated with subduction zone magmatism. IOA deposits are dominated by magnetite, with lesser hematite that is almost certainly secondary, and varying amounts of apatite and actinolite. Despite significant research aimed at understanding the genesis of IOA deposits, there is no consensus on how the deposits formed. In this study, we focus on apatite, which owing to its ability to incorporate nearly half of the elements in the periodic table, including volatile and fluid-mobile elements such as F, Cl and S, may provide information about the ore fluid(s) responsible for IOA mineralization. The chemical composition of apatite grains from the Los Colorados IOA deposit, located in the Chilean Iron Belt, was quantified by using several micro-analytical techniques. Cathodoluminescence (CL) and energy dispersive spectral mapping were used to assess chemical variations within individual apatite grains, followed by electron probe microanalyses (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to determine bulk, minor and trace element compositions. The data reveal elemental variations between the core and rim of the apatite grains for elements such as Cl, and the variations in Cl concentration correlate with variations in the CL behavior of apatite grains. In general, there is a negative correlation between F and Cl concentrations and a positive correlation between S and Cl concentrations. Further, the F concentration is lowest near the edges of apatite grains, while Cl and S concentrations increase toward the edges of the grains. Areas of apatite grains that exhibit the brightest CL behavior contain elevated concentrations of La and Mn relative to areas of apatite with weak CL intensity. These results are consistent with apatite that (re-)equilibrated with a hydrothermal fluid; however, more work is required to determine whether the findings support or refute the any of the previously proposed models of formation for IOA deposits.