The comparisons between Middle Valley and Escanaba Trough suggest the following generalities. Creation of new oceanic crust by seafloor spreading and burial of oceanic crust by sedimentation occurred at approximately the same rate in Middle Valley, such that the uppermost oceanic crust is a transitional zone of sheeted sills. Slower spreading at Escanaba, along with exceptionally rapid sedimentation rates, suggests that much of Escanaba basement formed by seafloor eruption of basalt that was later buried by sediment. Periodic large-scale eruptions led to the emplacement of large intrusions near the sediment/basalt interface, such as the one postulated to have uplifted Central Hill, and occasionally to the emplacement of basalt flows over the sediment fill, as observed to the east of Central Hill.
The Central Hill area has experienced a recent high-temperature event that flushed the sediment column with high temperature hydrothermal fluids. Evidence of the passage of these fluids through the sediment is the extensive alteration of the organic matter in the sediment and local alteration of sediment to chlorite. Massive sulfide deposits formed from these fluids are extensive, but the mineralization is predominately located at the sediment/water interface and feeder zone-style mineralization is rare. The sulfides are dominated by high-temperature hexagonal pyrrhotite, but the sulfides are enriched in elements such as Pb, As, Sb, and Sn that are derived from the sediments.
In contrast, the hydrothermal system responsible for the development of massive sulfide deposits in Middle Valley seems to have been long lived and highly focused allowing the formation of massive sulfide >100 m thick. The Bent Hill deposit is underlain by an extensive hydrothermal feeder zone. A silicified horizon below the base of the feeder zone provided a hydrologic seal on the system that alternately allowed focused fluid upflow where breached by fault zones and resulted in extensive subseafloor hydrothermal replacement mineralization when fluids were prevented from venting to the seafloor. Repeated development of hydrothermal systems at ODP Mound are indicated by stratigraphically stacked massive sulfide lenses, each underlain by associated feeder zone mineralization. This suggests that deeper structures, such as ridge-parallel normal faults or the transition from sediment-rift type to normal oceanic crust controlled the location of hydrothermal discharge.