Halogens, trace element concentrations, and Sr-Nd isotopes in apatite from iron oxide-apatite (IOA) deposits in the Chilean iron belt: Evidence for magmatic and hydrothermal stages of mineralization

The ratio of the halogens F and Cl in apatite is highly sensitive to changes in the composition of an evolving silicate melt or aqueous fluid. For this reason apatite chemistry is widely used as a monitor of halogen behavior in magmatic-hydrothermal systems. Apatite is an ubiquitous mineral in iron oxide – apatite (IOA) mineral deposits, where P and volatiles such as F, Cl, H2O and S play a major role in ore genesis. In this study, we present a combination of textural, micro-analytical and isotopic data for apatite from three Andean IOA deposits (Carmen, Fresia and Mariela) of Early Cretaceous age from the Coastal Cordillera of northern Chile. Apatite textures and compositions show evidence of post-crystallization alteration. Apatite is predominantly zoned with respect to Cl and F, showing a decoupled geochemical behavior between these two elements. Overall, four types of apatite or domains were identified in the analyzed grains based on the XCl-apatite/XF-apatite and XCl-apatite/XOH-apatite ratios determined using the atomic proportions of F, Cl and OH. F-apatite is characterized by a XCl-apatite/XF-apatite < 0.1 and a XCl-apatite/XOH-apatite = 0.03–2; Cl-apatite displays a XCl-apatite/XF-apatite > 25 and a XCl-apatite/XOH-apatite > 4.5 and up to 3000; Cl-OH-F-apatite has a XCl-apatite/XF-apatite = 0.15–8 and a XCl-apatite/XOH-apatite = 0.25–5.45; and Cl-OH-apatite shows a XCl-apatite/XF-apatite = 8–75 and a XCl-apatite/XOH-apatite = 0.5–3. Carmen apatite is mostly F-apatite, but shows Cl-OH compositions restricted to rims and fractures, whereas apatite from Mariela is dominantly Cl-apatite and Cl-OH-apatite. Apatite from Fresia have variable compositions between Cl-OH-F, Cl-OH- and Cl-apatite, where Cl- and Cl-OH-apatites are characterized by an enrichment of S, Na, Sr and Fe relative to F-apatite. Most notably, S and Na correlate with Cl. In Carmen and Fresia, Cl-OH-apatite is slightly depleted in LREEs, Th and U, a finding consistent with micro-textural evidence of metasomatic processes including coupled dissolution-reprecipitation and formation of secondary monazite and xenotime inclusions. In contrast, apatite from Mariela exhibits no depletion in LREEs and displays a homogeneous distribution of Th and U between the different apatite types, with no monazite and xenotime inclusions. The textural types of apatite identified in this study, coupled with the halogen and trace element composition of apatite, are consistent with modification of primary, F-apatite by interaction with hydrothermal fluids, which led to the formation of Cl-OH-apatite and Cl-apatite. In addition, the initial 87Sr/86Sr ratios and εNd values of apatite (0.7038 to 0.7050 and −0.3 to +6.5, respectively), calculated considering a 130 Ma age, are consistent with a magmatic origin for the primary F-apatite with minimal or no crustal contribution. Thus, textural, geochemical, and isotopic results of apatite support a magmatic-hydrothermal origin for these Andean IOA deposits with variable degrees of metasomatic overprint as evidenced by the formation of Cl-OH and Cl-apatite.