DISCUSSION

Two questions must be answered to address how structures affect the flow of hydrothermal fluids through the sediment pile to create massive sulfide mounds. The first question concerns the geochemistry of the sulfides and whether hydrothermal fluids interacted extensively with the sediment or whether a distinct basalt signature remains in the system. The second question answers more specifically how the sulfide feeder units are created and how they result in massive sulfide mounds.

Geochemical signatures of the BHMS indicate a basalt source modified by reaction with sediments (Goodfellow and Franklin, 1993; Zierenberg et al., 1993). Permeable flow of hydrothermal fluid through 500 m (depth to basalt basement identified in Hole 856H) of sediment would likely result in a geochemical signature more like Escanaba Trough sulfides, which indicate a greater sediment component (Goodfellow and Franklin, 1993; Zierenberg et al., 1993). Because the sulfides in the BHMS do have an intermediate basalt signature, focused fluid flow must be invoked to transport hydrothermal fluid from depth. This idea, that focused fluid flow is required to produce massive sulfide deposits, is supported by physical evidence such as hydrothermal vent fields forming at the intersections of faults (Rona and Clague, 1989). In addition, thermal models based on massive sulfide deposition also suggest that focused fluid flow is required to rapidly advect heat and transport metals to the surface (Fehn and Cathles, 1979; Strens and Cann, 1982; Lowell and Rona, 1985).

Even though thermal models of massive sulfide deposition require focused fluid flow, and the geochemical signatures of the BHMS deposit suggest focused fluid flow, no fault zone or permeable vertical horizon was recovered during drilling of Leg 169. However, this does not require that fluid flow be governed by porous flow from depth. Indirect evidence exists for the presence of a hydrothermal conduit, possibly a fault, that transports fluid from basement to the sediment layers near the surface. First, the Bent Hill series of mounds follow the trend of a normal fault that offsets basement reflectors but does not intersect the uppermost sediment layers (Davis and Villinger, 1992; Davis, Mottl, Fisher, et al., 1992). Even though the fault may now be inactive, as suggested by the lack of offset in the uppermost sedimentary layers, the fault may represent an open conduit for fluid flow. Second, downhole logging of Hole 856H indicates that a fault may have been intersected at a depth of 221-239 meters below seafloor (mbsf) and/or 250-270 mbsf (Fouquet, Zierenberg, Miller, et al., 1998). Within these intervals, low resistivity and high porosity were recorded, which suggest increased permeability. These zones also coincide with particularly low core recovery (~7%) and a change in pore-fluid chemistry (Fouquet, Zierenberg, Miller, et al., 1998). Third, the distinct lack of vertical or subvertical veining and the predominance of horizontal replacement in units such as the DCZ would suggest that the primary permeable horizon in these units is horizontal and that fluids were not flowing up through and across these horizons but, rather, were flowing along the horizon. These textures would indicate that hydrothermal fluids flowed into permeable layers from a conduit rather than flowing into the horizon from below through distributed porous flow. And finally, the very nature of ocean drilling precludes the intersection of near-vertical fault conduits. A near-vertical drill string is unlikely to intersect a near-vertical fault. Furthermore, the friable nature of brittle fault material reduces the likelihood of recovering the fault material even if a fault were intersected.

Whereas no direct evidence exists for a fault that transmits sulfide depositing hydrothermal fluids from depth, direct evidence does exist for horizontal flow of hydrothermal fluids through the sediments and limited vertical transport of fluid through veins. Evidence includes the dominant sulfide textures--sulfide precipitation within the primary sedimentary pore structure and subvertical vein networks that generally crosscut the horizontal sedimentary (and sulfide) fabric. Regardless of what subunit in the sulfide feeder system is studied, the infiltration and vein textures remain remarkably similar even if the sulfide mineralogy differs. This indicates that the same structural mechanisms are at work throughout the feeder system. It should also be noted that sulfide infilling of pore spaces, especially those of Subunit VIC, are predominantly found within coarser grained sediments, whereas the sulfide-veined units are predominantly in the mud to siltstones (Fig. F12).

A simple model, similar to one proposed by Zierenberg et al. (1993) for fluid flow in clastic sulfides, can be proposed for the structural evolution of and hydrothermal circulation within the BHMS. Hydrothermal fluids are transported away from a hydrothermal conduit, most likely a fault, along permeable sandy turbidite layers. As the hydrothermal fluids conductively cool, they precipitate sulfides within the pore spaces of the sediments, similar to models of sulfide deposition proposed for stratiform copper deposits (Fox, 1984; Brown, 1992). Hydrothermal fluids are confined to permeable horizons by the mud- and clay-rich layers that bound them. If sulfide precipitation effectively inhibits further permeable fluid flow, fluid pressures may become elevated within the sandy horizons. As fluid pressure increases, the effective pressure in the unit decreases, thereby initiating fracturing (Phillips, 1972; Cox et al., 1995). Once fractures open, hydrothermal fluids will be drawn into the crack through the induced pressure gradient, where they will then probably precipitate sulfides (Etheridge et al., 1984). If the crack remains open, fluids may flow through the fracture network from one permeable horizon to the next. This mechanism most likely operates only at a small scale between narrow horizons and does not act as the dominant fluid transport mechanism (Fig. F13). This statement is supported by the fact that most subvertical veins exhibit only one sequence of hydrothermal precipitate, rather than multistage mineralization. Additional evidence that precipitation of hydrothermal minerals in permeable horizons creates pressure regimes capable of inducing fracture is that most subvertical sulfide veins crosscut the horizontal sulfide impregnation textures. This indicates that horizontal fluid flow came first. Only rarely do subvertical veins directly feed a horizontal vein.

In addition to flow through fractures created by induced pressure regimes, vertical transport of hydrothermal fluids may occur through fracture networks created by tectonic stresses at the ridge. If these fractures remain open, they would provide an obvious pathway for hydrothermal fluids. This mechanism is preferred for the few centimeter-wide veins recovered in the drilled cores that showed evidence for multiple episodes of vein filling. The limited recovery of large aperture subvertical sulfide veins that transmit fluids between permeable horizons may be explained by the sampling bias of vertically drilled cores. It is likely that many more of these veins exist within the sulfide mound and do successfully transport fluids from one permeable horizon to the next, but the bias of the drilled cores precludes their recovery.

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