CHLORINE IN HIGH-GRADE IRON FORMATION “HYDROUS” MINERALS – INTERPLAY AMONG TEMPERATURE, PRESSURE, CRYSTAL CHEMISTRY AND METAMORPHIC FLUID

The amount of chlorine (Cl) present in provisionally “hydrous” minerals influences mineral stability and may serve as a monitor of the evolving fluid phase during progressive metamorphism. Chlorine contents of amphibole and biotite vary as a function of temperature, pressure, crystallochemical factors, and fluid composition. The capability for these minerals to serve as monitors of Cl in coexisting aqueous fluids is particularly effective in Fe-rich amphibole and biotite such as those found in high-grade iron formations.

Archean (2.8-2.9 Ga) iron formations from the eastern Beartooth Mountains, Montana are typified by dominantly anhydrous mineral assemblages of quartz + magnetite + orthopyroxene + garnet ± clinopyroxene ± plagioclase that have equilibrated during granulite facies conditions of ~775–800°C and 6–6.5 kbar. These relatively anhydrous iron formations contain minor amounts of prograde metamorphic Cl-rich amphibole and biotite that occur as inclusions in orthopyroxene and garnet as well as matrix minerals. The high-grade Fe-rich amphiboles (mostly Cl-rich potassic-hastingsite and magnesio-ferri-hornblende) and biotites contain concentrations of Cl reaching up to 2.9 wt% and 3.4 wt%, respectively. The biotites also contain up to 10.5 wt% BaO and 6.9 wt% TiO₂. The substitution of Cl into amphibole and biotite is more probable if the anion sites are enlarged – generally due to a decrease in tetrahedral rotation and/or an increase in the overall unit cell dimensions. These features are reflected by the strong positive correlation between Cl and X_Fe, ^IVAl and K (in amphibole). The influence of progressively more saline aqueous fluid compositions with progressive metamorphism is reflected by matrix amphiboles and biotites being more chlorine-rich than the inclusions in orthopyroxene and garnet. Based on calculations using the anion contents in biotite, the coexisting aqueous fluids were highly saline with X_H2O < 0.3, f_HCl/f_H2O = 0.4 to 1.1 and log(f_HF/f_HCl) = -2.7 to -3.9. In addition, the fluid was relatively oxidizing (~NNO) and produced Fe³⁺-rich amphiboles. These data provide evidence for fluid compositions of high-grade brines and high chlorine levels that increase the stability field of “hydrous” minerals into granulite facies conditions.