TRACKING THE BEHAVIOR OF SULFUR IN FELSIC INTRUSIONS USING S-ISOTOPES

            Saturation of a felsic magma in sulfide can remove Cu and Au, preventing formation of a metal-rich vapor phase (Jugo et al., 1999). Most felsic intrusive rocks contain small amounts of magmatic sulfides as inclusions in the silicate phases and sulfate in anhydrite or aptite. Insights into the magmatic history of sulfur in these magmas can be gained from the isotopic compositions of these two types of sulfur (at 800^oC, coexisting sulfide and sulfate differ by about 5 per mil). Previous studies have looked at the whole rock sulfur-isotopic composition of I-type granitoids. The technique of Rice et al. (1993) extracts sulfide and sulfate separately, allowing evaluation of isotopic equilibrium and providing information on degassing of the intrusion and modification of magmatic sulfides.

            We have measured sulfur isotope compositions of sulfide and sulfate fractions in felsic intrusions from Hanover-Fierro (NM) and the Wasatch Range (UT). The Hanover-Fierro stock is exceptional in having clearly magmatic anhydrite, whereas the other stocks do not. Sulfide, sulfate, and whole rock d³⁴S value of the Hanover-Fierro stock are –2.3, +6.7, and +5.8, respectively. This corresponds to an equilibrium temperature of 650^oC (Ohmoto and Rye, 1974) indicating near-solidus equilibration. A second sample of the Hanover-Fierro stock that lacks extractable sulfate (no anhydrite is present and apatite lacks detectable sulfate) has a sulfide d³⁴S value of  –0.8 per mil, reflecting degassing from  shallow parts of the intrusion that prevented sulfate saturation and drove d³⁴S value to lighter values.

            The Alta and Clayton Peak stocks in the Wasatch Range also lack extractable sulfate and have sulfide d³⁴S value values of 3.8 to 6.9 per mil. Lower values would be expected if the sulfide were in equilibrium with sulfate. The Little Cottonwood stock contains sulfide and extractable sulfate with d³⁴S values of 4.2 and 6.5 per mil, respectively, which indicate an impossible equilibrium temperature of 1450^oC. This could be explained if the sulfide and sulfate minerals did not crystallize from the melt at the same time or if the sulfide isotopic composition was modified during degassing of the intrusion. These preliminary results suggest that sulfide and sulfate isotopes provide useful information on the behavior of sulfur in felsic magmas.