Chemistry of sulfide deposites of ODP Site 106-649

The Snake Pit active hydrothermal field was discovered at 23°22'N on the Mid-Atlantic Ridge during ODP Leg 106. Among the ten holes drilled in the mound at the foot of an active chimney, only three (649B, 649F, and 649G) had substantial recovery, and produced cores of unconsolidated hydrothermal deposit made up of porous sulfide fragments with minor talc pellets and biological debris, and a few pieces of brassy massive sulfides. Eight representative samples from the 6.5-m-long core from Hole 649B were analyzed for bulk chemistry, both by XRF (major elements) and NAA (trace elements). Major elements average compositions show high Fe (36 wt%), S (37 wt%), and Cu (12 wt%) contents, and minor Zn (6.7 wt%), reflecting a mostly high-temperature deposit. Trace elements are characterized by a high Au content (600 ppb) which could express the maturity of the mound. Mineralogical assemblages show evidence of sequential precipitation, and absence of equilibrium. Major sulfide phases are pyrrhotite, pyrite, Fe, Cu sulfides, marcasite, and sphalerite. Three types of samples are distinguished on the basis of textures and mineral assemblages: type 1, rich in pyrrhotite, with approximately equivalent amounts of Cu, Fe sulfides, and sphalerite and minor pyrite; type 2, rich in Cu, Fe sulfides, which are cubic cubanite with exsolutions and rims of chalcopyrite; and type 3, essentially made up of sphalerite. Type 2 samples likely represent fragments of the inner chimney wall. The presence of talc intergrown with cubic cubanite/chalcopyrite in one big piece from Hole 649G is probably related to mixing of the hydrothermal fluid with seawater.

(Table 7) Fe**3+/Fe**2+ ratios in selected nontronite samples at DSDP Leg 70 Holes

Since its discovery in 1974 (Klitgord and Mudie, 1974), the Galapagos mounds hydrothermal field has received much attention. Sediment samples were taken during Leg 54 of the Deep Sea Drilling Project (DSDP) and by other expeditions to the area (e.g., Corliss et al., 1978). While a hydrothermal origin for the mounds sediments has been generally accepted, several different theories of origin for the mounds themselves have been proposed (e.g., Corliss et al., 1978; Natland et al., 1979; Williams et al., 1979). One of the aims of DSDP Leg 70 was to return to the mounds field and, using the new hydraulic piston cor er described elsewhere in this volume, to obtain more complete recovery of mounds sediments than had previously been possible. It was our hope that this would help in our understanding of the nature and origin of these deposits. In this chapter, we describe the results of chemical analysis of over 250 sediment samples taken during the course of Leg 70.