THE ROLE OF S AND CL IN THE EXSOLUTION AND EVOLUTION OF MAGMATIC-HYDROTHERMAL FLUIDS AND SUBSEQUENT ORE-METAL TRANSPORT
Results of recent experiments on: (1) S, Cl, CO2, and H2O solubility in silicate melts, (2) anhydrite solubility in Cl-enriched fluids, and (3) the partitioning of Cl and S between apatite-melt-fluid(s) demonstrate that Cl and S enhance processes of fluid exsolution and that initial saturation in H2O is not relevant to some mineralizing magmas. Moreover, geologically relevant abundances of magmatic S reduce Cl solubility in felsic melts and as a result increase the likelihood that brine or vapor plus brine (enriched in H, Na, K, Ca, Cl, and S species) will exsolve “early”. Interestingly, with modest concentrations of alkali chlorides in magmatic fluids, the solubility of anhydrite can be quite significant; the S contents of some Cl-bearing run-product fluids approach 30 wt.%. Newly determined partition coefficients for Cl and S distribution between apatite, felsic melt, and fluid(s) are applied to the interpretation of the volatile abundances of natural apatites from mineralized plutons and their barren volcanic equivalents; one observation is that the Cl contents of nascent hydrothermal fluids in a variety of subduction-related magmas exceed the Cl concentrations generally accepted for such magmatic systems. Thus, the roles played by S and Cl in ore-metal sequestration by magmatic fluids begin much earlier, during magma evolution, than previously thought.