2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 2
Presentation Time: 8:20 AM


WEBSTER, James D., American Museum of Natural History, Department of Earth and Planetary Sciences, Central Park West at 79th St., New York, NY 10024, jdw@amnh.org

Chloride and variably oxidized species of sulfur are highly effective ligands involved in the complexation and transport of Cu, Au, and other ore metals in hydrothermal fluids. These volatile components also play a fundamental role in the initial exsolution of fluids in some magmas and in their subsequent chemical evolution into mineralizing hydrothermal fluids. Given the importance of S and Cl in these processes, knowledge of their abundances and behavior during the evolution of mineralizing magmas is essential. Investigations on Cu-, Au-, ± Mo-mineralized porphyries and their subduction-zone volcanic equivalents have shown that some magmas contain wide ranges in S and Cl (e.g. for Cl, from several to < 70 wt.% NaCl-equivalent). The evidence includes Cl-rich fluid inclusions, S-rich volcanic gases, Cl- ± S-enriched melt inclusions, the occurrence of magmatic anhydrite, and abundances of Cl and S in apatite phenocrysts.

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.