Core drilled during Leg 169 provides a remarkable opportunity to characterize and evaluate the structural evolution of a massive sulfide mound. To determine the lateral extent of mineralization and to characterize the sulfide system with depth, we drilled along a north-south and an east-west transect and drilled through the apex of the Bent Hill Massive Sulfide (BHMS) to a depth of 500 m. At this depth, basalt flows interpreted as volcanic basement were recovered (Fouquet, Zierenberg, Miller, et al., 1998). Both the massive sulfide and the underlying sulfide feeder systems were successfully recovered in three holes, providing an outstanding sequence of core with which to analyze the structural mechanisms for massive sulfide formation at sedimented ridges.
Previously, studies of sedimented-covered ridge sulfide mounds have concentrated on the composition of the sulfides to constrain the genesis of the sulfide system (Ames et al., 1993; Goodfellow and Franklin, 1993; Butterfield et al., 1994; Campbell et al., 1994; Davis and Fisher, 1994; Duckworth et al., 1994; Zierenberg et al., 1994). Equally as important, however, are studies that attempt to characterize the structural evolution of the sulfide mound. Economic geologists have long recognized the importance of understanding the structure of the hydrothermal system to explore economic grade ores. Unfortunately, the poor recovery of densely fractured materials from the ocean crust has significantly reduced the feasibility of structural studies in active seafloor deposits. Therefore, the study of the structural evolution of sulfide mounds on the seafloor has largely been limited to information obtained through seismic analyses (Macdonald and Luyendyk, 1977; Karson and Rona, 1990; Davis et al., 1992; Davis and Villinger, 1992; Davis and Becker, 1994). One viable means by which the structural framework of massive sulfide deposits can be constrained has largely been ignored in drilled core. Vein textures provide a valuable record of the pathways by which hydrothermal fluids flow. This paper derives a model of hydrothermal circulation at the BHMS deposit based upon vein fabrics and infilling textures in the sulfide feeder system of the BHMS deposit.