Halogens in Silicic Magmas and Their Hydrothermal Systems

Abstract

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 sol-ubility, 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 F 2 O −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