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Hydrothermal pyrite samples from Holes 477, 477A, and 478 have sulfur isotope values ranging from 10 to 11%. Samples with negative sulfur isotope values generally have low sulfide sulfur contents and reflect mixed bacterial and hydrothermal sulfur sources. At higher sulfur contents, the hydrothermal component predominates, producing positive isotope values. Hydrothermal sulfide derives from reduction of seawater sulfate and may contain a significant basaltic component. Hydrothermal anhydrite is restricted to a narrow zone beneath a dolerite sill at Site 477 and, because of partial sulfate reduction in the circulating waters, has isotopic values (23.5-25%), heavier than seawater. INTRODUCTION Sulfur isotope studies are a powerful tool in interpreting the origin of hydrothermal ore deposits (Ohmoto and Rye, 1979). Under ideal conditions, sulfur isotope studies can provide information on the average sulfur isotope value of the sulfur source, the temperature of ore formation, and the types of changes in physical or chemical conditions that cause ore precipitation. Numerous studies of ancient massive sulfide deposits have indicated that sulfur isotope values of sulfides average about 15% lighter than contemporaneous seawater sulfate (Sangster, 1968). Thus, some process involving the reduction of seawater sulfate during ore formation is indicated. Mottl et al. (1979), however, have shown that sulfides in oceanic tholeiites are directly mobilized during experimental basalt-seawater interaction and that sulfate reduction can occur simultaneously. Detailed sulfur isotope studies of presently active submarine hydrothermal sulfide depositing systems offer the best opportunity for resolving the relative roles of basaltic and seawater sulfur sources in these and analogous ancient systems. Indeed, Shanks and Bischoff (1980) indicate a range of sulfur isotope values from 1 to 14%0 for sulfides in the metalliferous sediments of the Atlantis II Deep in the Red Sea. These values suggest that direct mobilization of basaltic sulfide is of minor importance in the Red Sea system. Similarly, the sulfur isotope values of hydrothermal sulfides at 21 °N on the East Pacific Rise are about 1 to 4% (Hekinian et al., 1980), again suggesting an important contribution of sulfide from the reduction of seawater sulfate. Many economically important massive sulfide deposits occur in extensional basins related to continental margin sedimentation (Faro, Sullivan, McArthur River, Mt. Isa). These areas are perhaps analogous to the Guaymas Basin. Lonsdale et al. (1980) have recently described a masCurray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64: Washington (U.S. Govt. Printing Office). sive talc deposit in the North Trough of the Guaymas Basin (Fig. 1), which contains minor amounts of pyrrhotite and smectite. In addition, submersible observations of large, empty, white clam shells indicate extinct hydrothermal vents in the North Trough. High heat flow and hydrothermal minerals in Holes 477 and 477A in the South Trough of the Guaymas Basin (Fig. 1) suggest similar, currently active hydrothermal processes. Indeed, recent submersible observations and sampling operations (Lonsdale, personal communication) in the South Trough have revealed active hydrothermal mound deposits with associated thermal plumes and hot-spring animal communities similar to those at the Galapagos Rift and East Pacific Rise. Samples containing talc, barite, and base metal sulfides were collected. A sulfur isotope value of 3.94% has been measured on pyrrhotite from the North Trough talc deposit (Lonsdale et al., 1980). This value is somewhat atypical of submarine hydrothermal deposits but is nearly identical to isotopic values determined on sulfide stalactites from active vent areas of the Galapagos rift. Lonsdale et al. (1980) have suggested that such sulfide results from the reduction of seawater sulfate in underlying sediments by bacterial and hydrothermal processes. We undertook the present study of sulfides in hydrothermal alteration zones in the Guaymas sediments to further elucidate the mechanisms of sulfide formation in this setting. METHOD Samples containing hydrothermal minerals were carefully chosen from altered zones beneath a coarse-grained dolerite sill about 40 meters thick in Holes 477 and 477A (Figs. 1, 2). An additional sample was selected from altered sediments just above a dolerite intrusion, at least 100 meters thick, in Hole 478. Brief lithologic descriptions and sample locations are listed in Table 1 and Figure 2. The upper six samples from Hole 477 contain anhydrite and pyrite. Anhydrite samples were hand picked from anhydrite-rich areas within the cores. Pyrite occurs in at least small amounts in all samples. Large (1-3 mm) euhedral pyrite crystals occur in Sample 477-22-2, 27-29 cm and were separated by hand picking. Shipboard petrographiç studies had indicated the presence of an hexagonal sulfide, probably pyrrhotite, in the lower portions of Holes 477 and 477A. Because most of the samples were fine grained, sulfur-bearing phases were separated for isotopic analysis by the following successive

Sulfur Isotope Studies of Hydrothermal Anhydrite and Pyrite, Deep Sea Drilling Project Leg 64, Guaymas Basin, Gulf of California