EXSOLUTION OF MAGMATIC VOLATILE PHASES FROM CL-ENRICHED MINERALIZING GRANITIC MAGMAS AND IMPLICATIONS FOR ORE METAL TRANSPORT

To understand Cl dissolution in aluminosilicate liquids and the exsolu tion of Cl-rich magmatic volatile phases, experiments were conducted t o determine the solubility of NaCl, KCI, and H2O in felsic liquids at 0.5 and 2 kbar. The Cl content of H2O-poor, NaCl-saturated, and KCl-sa turated silicate liquids is low (i.e., less than or equal to 1.3 wt%) to very low (i.e., approximate to 0.2 wt% wt%) and varies with changes in pressure and composition; the Cl concentration increases with the F concentration and the molar (Al + Na + Ca + Mg/Si) ratio of the liqu id and decreases with increasing activity of H2O in the system. Exsolu tion of a volatile phase depends on the partial pressures of all disso lved volatiles, and low Cl solubilities in NaCl-saturated and KCl-satu rated silicate liquids imply that exsolution of a Cl-bearing volatile phase will occur ''early'' in Cl-bearing granitic magmas, i.e., prior to extensive melt crystallization and/or at comparatively low water fu gacities. The solubility behavior of H2O and Cl is very similar to tha t of CO2 and H2O in felsic liquids. Small quantities of CO: are known to facilitate volatile phase exsolution (Holloway, 1976), and in a sim ilar manner volatile phases may exsolve ''early'' in Cl-enriched grani te magmas. Whereas 5 to 6 wt% dissolved H2O is necessary for volatile phase exsolution from a CO2-free and Cl-free haplogranite liquid at 2 kbar and 800 degrees C, a Cl-rich brine will exsolve if the liquid con tains only 1 wt% H2O and 0.26 wt% Cl at the same conditions. These new solubility data are interpreted in light of H2O, F, and Cl concentrat ions in felsic liquids, determined from silicate melt inclusions, to c onstrain the exsolution of Cl-bearing, magmatic volatile phases from m ineralizing granitic magmas. Felsic magmas genetically associated with Cu-porphyry and Mo-porphyry mineralization contain sufficient H2O and Cl to become saturated with respect to a hypersaline brine without st rong pressure reduction, boiling of the volatile phase (i.e., exsoluti on of immiscible vapor and brine), or strong volatile enrichment resul ting from extensive crystal fractionation. Experiments were also condu cted to investigate the solubility of Mo in highly saline volatile pha ses coexisting with granitic liquids at 2 and 0.5 kbar. The apparent p artition coefficient for Mo in the volatile phase(s) relative to silic ate liquid, (DM,), is defined as [the computed concentration of Mo in a volatile phase or phases/the measured concentration of Mo in granite glass]. D(Mo) ranges from 8 to 80 as the NaCl and KCl content of the volatile phase(s) increases from 15 to 90 wt%. Because Mo does not co mplex with Cl in aqueous fluids, it appears that Mo solubility may be a strong function of the activity of Na and K in alkali chloride-rich volatile phases. Copyright (C) 1997 Elsevier Science Ltd.