Introduction
Leg 169 was the second leg of a planned two-leg program to investigate the geological, geophysical, geochemical, and biological processes at sediment-covered spreading centers in the northeast Pacific Ocean (Fig.1). Building on the highly successful Leg 139 drilling (Davis, Mottl, Fisher, et al., 1992) our primary goal was to investigate the genesis of massive sulfide deposits by drilling two deposits at different stages of maturity; Middle Valley at the northern end of the Juan de Fuca Ridge and Escanaba Trough at the southern end of the Gorda Ridge. The four primary topics investigated on this leg were 1) the mechanism of formation of massive sulfide deposits at sediment-covered ridges, 2) the tectonics of sedimented rifts and controls on fluid flow, 3) the sedimentation history and diagenesis at sedimented rifts, and 4) the extent and importance of microbial activity in these environments. A series of holes was to be drilled across deposits at each of these sites to determine the sedimentary record of hydrothermal products adjacent to the deposits and to constrain the timing and duration of hydrothermal activity. Alteration and stockwork zones beneath the deposits were to be sampled to constrain the sources of metals and geochemical reactions that control mineralization and the parameters that controlled the fluid flow.
Middle Valley: Geology of the hydrothermal field
Middle Valley, a medium-rate spreading center (58 mm/yr), forms one leg of a Ridge- Transform-Transform unstable triple junction with the Sovanco Fracture Zone and the Nootka fault (Davis and Villinger, 1992). The proximity of Middle Valley to the cold Explorer Plate results in a reduced magma supply and a slow-spreading ridge morphology with a deep and wide axial trough. Current magmatic activity is mostly confined to the adjacent West Valley spreading center indicative of a recent jump in the location of the plate boundary. Proximity of the Middle Valley spreading center to an abundant supply of terrigenous sediment during the Pleistocene sea level lowstand has resulted in burial of the spreading center by 200 to >1000 m of turbiditic and hemipelagic sediment, with sediment thickness increasing to the north. Two areas separated by about 4 km were drilled during Leg 169, the Dead Dog vent field in the area of active venting (Site 858) and the Bent Hill area (Site 856) (Fig. 2).
Bent Hill
Bent Hill is one of a string of small mounds that runs parallel to the eastern rift bounding normal fault scarp. These features lie close to an inferred normal fault that appears to offset basement reflectors, but near-surface sediment layering imaged in seismic reflection profiles appears to be continuous across this fault. Bent Hill is a roughly circular feature 400 m in diameter that has been recently uplifted approximately 50 m (Fig. 3). It is bounded on the west by a steep scarp that parallels the rift-bounding faults and exposes semiconsolidated turbiditic sediment. Bright, reverse polarity seismic reflections that are limited in extent to the area under Bent Hill are interpreted to be generated at the interface between the base of basaltic sills and the underlying sediments (Rohr and Schmidt, 1994).
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 Gorda Ridge spreading center is located offshore of Oregon and northern California and is bounded by the Mendocino Fracture Zone on the south and the Blanco Fracture Zone on the north (Fig. 1). A small offset in the spreading axis at 41°40'N latitude marks the northern boundary of Escanaba Trough, which forms the southernmost part of Gorda Ridge. Escanaba Trough is opening at a total rate of approximately 24 mm/yr and has a morphology consistent with its slow-spreading rate. The axial valley, which is at a depth of 3300 m, increases in width from about 5 km at the north end to more than 15 km near the intersection with the Mendocino Fracture Zone.
South of 41°17'N latitude, the axial valley of Escanaba Trough is filled with several hundred meters (Fig. 5) of turbiditic sediment. The sedimentary cover thickens southward and is a kilometer or more in thickness near the Mendocino Fracture Zone. Turbiditic sediment enters the trough at the southern end and is channeled northward by the axial valley walls (Vallier et al., 1973; Normark et al., 1994). Sedimentation was relatively rapid (up to 5 m/1000 yr) during sea level lowstands in the Pleistocene, and the entire sediment fill of the trough probably was deposited within the last 100,000 years (Normark et al., 1994; Davis and Becker, 1994). A reference hole through this sedimentary package and into basaltic basement was planned to provide background information to evaluate the sedimentary and thermal history in an area away from the hydrothermal upflow zone.
Seismic reflection surveys show that the floor of Escanaba Trough is generally a smooth, flat plain underlain by continuous and relatively undisturbed turbidites. However, local areas along the axis of spreading have irregular seafloor topography characterized by circular hills 0.5 to 1.2 km in diameter that are uplifted 50 to 120 m above the surrounding seafloor. The areas of sediment disruption are sites of recent axial rift igneous activity. The geologic and geophysical evidence suggests that axial rift igneous activity at these sites is manifested by the intrusion of dikes, sills, and laccoliths into the sediment with less abundant volcanic flows (Morton and Fox, 1994; Zierenberg et al., 1994, 1993). Sulfide mineralization has been sampled by dredging, sediment coring, or submersible at four igneous centers within the sediment-covered part of Escanaba Trough. The dominant morphologic feature in the area of operations for Leg 169 was Central Hill (Fig. 6).
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).
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