The 35-m high Bent Hill massive sulfide (BHMS) mound is located 100 m south of Bent Hill. A twin peaked massive sulfide mound with a single active vent at its north end is located approximately 330 m farther south. These mounds are aligned parallel to the N-S scarp that constitutes the western side of Bent Hill. The BHMS mound is extensively weathered to iron oxyhydroxides and partially buried by sediment. During Leg 139, Hole 856H penetrated 94 m of massive sulfide before the hole had to be abandoned due to inflow of heavy sulfide sand from the upper weathered section of the borehole wall. A strong magnetic anomaly across this mound is related to the occurrence of magnetite and has been modeled to suggest that mineralization continues at least another 30 m below the level drilled and possibly much deeper (Tivey, 1994).
The morphology, degree of oxidation, and the lack of sediment cover on the mounds south of the BHMS deposit indicate that these deposits are younger than the Bent Hill deposit. A single 264°C hydrothermal vent is present on the northern mound. Contoured heat flow values for the Bent Hill area show high values centered around this active vent (Davis and Villinger, 1992). The composition of the vent fluid is similar to those from the Dead Dog vent field, but this vent has lower salinity and only half as much dissolved Ca (Butterfield et al., 1994).
Dead Dog Vent Field
The principal center of hydrothermal activity in Middle Valley is the Dead Dog vent field. Contoured heat flow values show a concentric high which is coincident with a side scan acoustic anomaly (Fig. 4) that outlines the 800-m-long and 400-m-wide vent field. Sediment thickness over the fault block in the area surrounding the vent field is approximately 450 m and overlies a sill-sediment complex that forms the transition to oceanic crust (Davis, Mottl, Fisher, et al., 1992). However, hard acoustic reflectors that occur only immediately beneath the vent field were confirmed by drilling on Leg 139 to be the top of a volcanic edifice at only 250 m depth. The presence of more permeable volcanic basement penetrating up into the sediment cover acts as a conduit that focuses flow of hydrothermal fluid to the seafloor (Davis and Fisher, 1994). The vent field contains at least 20 active vents with exhalative fluid temperatures ranging up to 276°C (Ames et al., 1993). Active vents occur predominantly on top of 5 to 15-m-high sediment-covered mounds a few tens of meters in diameter. Available data from piston cores and ODP Hole 858B suggested that subsurface deposition of anhydrite, Mg-rich smectite, and sulfide minerals contribute to the growth of the mounds. Because the high temperature hydrothermal fluid is strongly depleted in both Mg and SO4, the abundance of these minerals in the subsurface requires that cold seawater (with abundant Mg and SO4) is drawn into the subsurface by the vigorous upflow at the active vent sites.
A major step towards the establishment of seafloor observatories was taken on Leg 139 by instrumentation of two sealed boreholes in the Middle Valley hydrothermal field using the CORK system. One of the objectives of Leg 169 was opening these instrumented boreholes in order to allow sampling of hydrothermal fluids. Reinstrumentation of these holes was planned to allow active experimentation on induced seismicity in a seafloor hydrothermal system and hole-to-hole hydrologic experimentation designed to constrain the physical and hydrologic properties that control hydrothermal flow on the scale of an entire vent field.
The western, sediment-covered part of Central Hill contains the most extensive sulfide deposits observed in Escanaba Trough. The massive sulfide deposits on the west and southeast flanks of Central Hill are actively venting hydrothermal fluid, and the area on the northern flank shows indications of very recent hydrothermal activity, suggesting that these deposits are all part of the same hydrothermal system. The best explored and most hydrothermally active area of sulfide mineralization on Central Hill extends west and north from the northern end of the sediment-covered hill top. On the northern flank of the hill, massive sulfide extends more than 270 m from north to south and more than 100 m from east to west, but the western edge of the deposit has not been defined with certainty. Within this area there is nearly continuous outcrop of massive sulfide. All of the active hydrothermal vents occur in an area with abundant sulfide mounds along the northwestern flank of the hill. The major element composition of the end-member hydrothermal fluid of two actively discharging vents 275 m apart is identical (Campbell et al., 1994), a result that is consistent with the hypothesis that this large mineralized area is a single hydrothermal system hydrologically interconnected at depth.
Sulfide samples collected at the surface of the deposit are dominantly pyrrhotite with variable amounts of isocubanite and chalcopyrite and minor sphalerite, galena, lollingite, arsenopyrite, and boulangerite. Sulfate occurs as barite crusts and chimneys on massive sulfide and intergrown barite-anhydrite in active vents. When compared to Middle Valley, the abundance of barite and enrichment of metals such as lead, arsenic, antimony, and bismuth indicate extensive contribution from sediment source rocks (Koski et al., 1994). Precious metals are significantly enriched relative to Middle Valley massive sulfide. Sediment alteration associated with formation of massive sulfides is dominated by talc, Mg-rich chlorite, or Mg-rich smectite (Zierenberg et al. 1994).
To 169 Middle Valley Results-Bent Hill Area
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