F-, CL- AND K-RICH AMPHIBOLES IN THE IRON DEPOSITS OF THE HUDSON HIGHLANDS, NEW YORK

F-, Cl- and K-rich amphiboles of the calcic amphibole group, such as hastingsite, pargasite +/- ferro-edenite series with K2O up to 3.2 wt.%, Cl up to 3.66 wt.% and/or F up to 2.26 wt.%, have been identified in the iron deposits of the Hudson Highlands. These amphiboles are associated with a sulfide mineral sequence that paragenetically overlaps the magnetite ore. They formed as metasomatic replacements of the original rock-forming amphiboles (hastingsite sensu lato) and/or pyroxenes. Samples were analyzed by optical microscopy, electron microprobe, Mössbauer and single crystal X-ray diffraction. The chemical data show a positive linear correlation between the concentrations of K and Cl, and a positive logarithmic correlation between both IVAl and VIFe (total) and Cl. The negative correlation between the Mg# and Cl concentration is consistent with the Mg-Cl avoidance rule. The calculated and Mössbauer Fe3+/Fe2+ ratios support the crystallization at relatively low oxygen fugacity in a reducing environment.

Mössbauer spectroscopy suggests that 20-36% of the total Fe is Fe3+in the 10 analyzed samples and that Fe2+ is distributed among the M(1), M(2), and M(3) sites, with a preference for M(2) occupancy. Quadratic modeling based on chemistry and diffraction data indicate complete partitioning into M(2). Crystal structure refinements were carried out for fluoro-potassichastingsite and potassichastingsite. Mg partitioned among M(1), M(2), and M(3) sites. The A-site occupancy, based on chemical data, was calculated for all the analyzed amphiboles between 0.65 to 0.97. The K-dominant and F/Cl-rich species shows the highest A-site occupancy. In both refined structures, the A site was found to be modeled best using the A(m) and A(2) splitting of the site, with K dominantly in the A(M).

At least two chemically distinct fluids seem to be responsible for the formation of halogen- and potassium-rich amphiboles. The first was a medium-temperature, F-rich fluid, followed and mixed with a Cl-rich one, both accompanying the deposition of sulfides. Halogen depletion of these fluids toward the end of the mineralizing process formed the halogen-free amphiboles.