Halogens in Silicic Magmas and Their Hydrothermal Systems

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Halogens, mainly F and Cl, play key roles in the evolution and rheology of silicic magmas, magmatic-hydrothermal transition, partitioning of metals into aqueous fluids, and formation of ore deposits. Similarity of ionic radii of O, hydroxyl, and F, and a much greater size of Cl are responsible for (i) higher solubility, hence compatibility of F in silicate melts, (ii) greater lattice energies of fluorides, therefore their more refractory character and lower solubilities in fluids, and (iii) higher hardness of F as ligand for complexing, leading to a distinct spectrum of metal-fluoride versus metal-chloride complexes. In the F-rich systems, the interaction of F with rock-forming aluminosilicates corresponds to progressive fluorination by the thermodynamic component F2O−1. Formation of F-bearing minerals first occurs in peralkaline and silica-undersaturated systems that buffer F concentrations at very low levels (villiaumite, fluorite). The highest concentrations of F are reached in peraluminous silica-saturated systems, where fluorite or topaz are stable. Coordination differences and short-range order effects between [NaAl]–F, Na–F versus Si–O lead to the fluoride-silicate liquid immiscibility, which extends from the silica–cryolite binary to the peralkaline albite–silica–cryolite ternary and to peraluminous topaz-bearing systems, where it may propagate to solidus temperatures in the presence of other components such as Li. Differentiation paths of silicic magmas diverge, depending on the Ca-F proportions. In the Ca-rich systems, the F enrichment is severely limited by fluorite crystallization, whereas the Ca-poor magmas evolve to the high F concentrations and saturate with topaz, cryolite, or immiscible multicomponent fluoride melts (brines). These liquids preferentially partition and decouple high-field strength elements and rare-earth elements (REE), and are responsible for the appearance of non-chondritic element ratios and/or lanthanide tetrad effects. Continuous transition from volatile-rich silicate melts to hydrothermal fluids is unlikely, although two fluids—hydrous halide melts and solute-poor aqueous fluids—may often exsolve simultaneously. The fluoride ligand is responsible for the effective sequestration of hard cations, mainly REE, Th, U, and Zr, into the hydrothermal fluids. In the Cl-dominated systems, the maximum concentrations in silicate melts are significantly lower than those of F due to the absence of bonding between Cl and network-forming cations in the melt structure. The typical Cl-rich phase in felsic magmas is an aqueous ± carbonic fluid phase; the saturation of which limits the attainable concentration of Cl in the silicate melt. The more depolymerized the structure of the silicate melt is, the more easily metal-chloride species are accommodated. Therefore, metaluminous rhyolites are characterized by the highest fluid/melt partition coefficients for Cl as well as the lowest maximum dissolved Cl concentration. Chlorine is dominantly present as NaCl, KCl, CaCl2, FeCl2, and HCl species in aqueous magmatic fluids; their relative proportions are strongly influenced by silicate melt composition, pressure and total dissolved chloride concentration. The activity coefficients of metal-chloride species in the aqueous fluid are strongly dependent on pressure and total chloride concentration, and so is the volatile/melt partition coefficient of Cl. The increase of pressure strongly promotes Cl partitioning into the fluid phase, whereas increased chloride concentrations in the fluid work against it, especially if vapor-brine immiscibility occurs anchoring the activity of major chloride species in the system. Chloride ions are dominant, or at least take the form of significant complex forming ligands for a broad range of economically important elements found in magmatic-hydrothermal ore deposits such as Cu, Au, Mo, Pb, Zn, Sn, and W. Therefore, Cl has significant effect on the volatile/melt and vapor/brine partition coefficients of these elements, and at least partially controls the likelihood of the formation of economic ore mineralization.