Leg 169--Sedimented Ridges II

Co-Chiefs: Yves Fouquet and Robert Zierenberg	Cruise Dates: 22 August-17 October, 1996
Staff Scientist: Jay Miller	Operations Superintendent: Gene Pollard

The goal of Leg 169 was to drill into two active hydrothermal systems to investigate the genesis and evolution of massive sulfide deposits formed at sediment-covered ridges. These two systems, Bent Hill/Dead Dog in Middle Valley on the Juan de Fuca Ridge, and Central Hill in Escanaba Trough on the Gorda Ridge, were recognized to host deposits of variable chemistry and degree of maturity. The preliminary results of this leg, enhanced by those generated in post-cruise research, will shed light on the longevity of these systems and the chemical and temporal variability of fluids and sulfides attending them. Additionally, our successful reinstrumentation of CORKed boreholes will provide in situ monitors of geologic processes in Middle Valley.

The Bent Hill massive sulfide (BHMS) deposit has 3 distinct parts. A 100-m-thick conical mound of massive sulfide formed at the seafloor underlain by a 100-m-thick feeder zone comprised of a sediment host with crosscutting veins filled with Cu-Fe sulfides and pyrrhotite. One of the most spectacular discoveries of this leg, however, is the presence of an intensely silicified horizon at the base of the feeder zone which is underlain by a horizon of intense alteration and replacement of the host sediment by Cu-rich sulfides. This high grade (~16% Cu) stratiform mineralization zone may reflect a high temperature aquifer capped by an impermeable silicification front, and also represents a potential economic target for mineralized deposits on land.

A second mound 350 m south of BHMS was also sampled, and is characterized by multiple, stacked mineralized horizons with significantly higher ore grades (40% Zn and 15% Cu) than was sampled at BHMS. A high grade Cu ore horizon was also intercepted at the same stratigraphic interval as was sampled at BHMS. Another highlight of this expedition was the creation two new vigorous hydrothermal vents in what had been considered an inactive part of the system. The creation of these vents has attracted media attention (Science, Geotimes) and resulted in NSF funding an immediate follow-up expedition with an ROV to observe and monitor these new vents.

Active hydrologic experiments were conducted by removing existing CORKs, sampling high temperature fluids, and reinstrumenting two boreholes. By sealing one hole at an active hydrothermal site first, we expect that the installation of the second in a cold reference hole 2 km away will induce a transient pressure pulse we can monitor in the active system. An OBS array deployed around the our area of operations will detect any induced seismicity resulting from our operation.

Previous investigations recognized distinctive chemical signatures for massive sulfides from Middle Valley and Escanaba Trough. Middle Valley sulfides appear to have basalt as a major source component, whereas Escanaba Trough sulfides indicate only sediment as a metal source. Drilling several holes in deposits exhibiting a range of hydrothermal activity and sulfide deposition demonstrated that the thickness of the sulfides at Central Hill is little more than what is exposed at the seafloor. The lack of any feeder zone indicates these sulfides formed by pervasive diffuse venting of hydrothermal fluid over a short time span, as opposed to the long-lived, focused hydrothermal discharge which is responsible for sulfide deposition at Middle Valley.

Objectives

In the following preliminary summary of results, individual interpretations are related to the science objectives listed below.

Mechanism of formation of massive sulfide deposits at sediment covered ridges

Size and geometry of sulfide deposits and hydrothermal alteration zones
Compositional variations within and between deposits
Source of metals in massive sulfides
Constraints on fluid temperatures and compositions that deposited massive sulfide
Composition of fluids in producing boreholes
Timing and duration of hydrothermal activity
Formation of hydrothermal mounds in active vent fields

Tectonics of sedimented rifts and controls on fluid flow

Controls on igneous activity at sedimented rifts and the importance of sill emplacement
Permeability and structural controls on hydrothermal circulation
Interrelationship of faulting and fluid flow
Constraints for hydrologic modeling
Factors controlling fluid flow on the scale of individual vent complexes and the importance of sub-seafloor fluid mixing

Sedimentation history and diagenesis at sedimented rifts

Source and deposition rate of sediments
Diagenetic reactions in high heat flow regimes
Organic matter alteration and generation of hydrothermal petroleum

Extent and importance of bacterial activity

Role of bacteria in oxidation of organic matter, reduction of sulfate, and precipitation of carbonate
Activity of thermophilic bacteria in sealed boreholes at known temperatures and fluid compositions
Extent of thermophilic and nonthermophilic bacteria in the subsurface and relationship to hydrothermal fluid compositions
Comparison of bacterial populations in active and inactive hydrothermal deposits

Preliminary results

The Bent Hill Massive Sulfide is a conical mound 100 m thick. (objective 1)
The underlying feeder zone mineralization is 100 m thick. (objective 1, objective 2)
There exists an intersected stratiform high-grade copper zone below stockwork mineralization. (objective 1)
Zn/Cu ratio increases upward and outward from the core of BHMS. (objective 1, objective 2)
ODP/TAMU drilled a stacked sequence of thick, high-grade massive sulfide and feeder mineralization. (objective 1)
ODP/TAMU operations created two new vigorous hydrothermal vents. (objective 2)
Achieved CORK objectives at Holes 858G and 857D. (objective 2)
Demonstrated high temperature phase separation for Escanaba Trough hydrothermal fluid. (objective 2)
Determined that immature Escanaba Trough deposits are formed by recent, diffuse venting; mature Middle Valley deposits are formed by long-term, recurrent focused flow. (objective 1, objective 2)
Established extremely high sedimentation rates for sediment fill of the Escanaba Trough. (objective 3)
Achieved high resolution sampling for thermophilic and hyperthermophilic microorganisms. (objective 4)
The U.S. National Science Foundation has responded rapidly to scientists' request to revisit this area of the Northeast Pacific. In October 1996, the research vessel Thompson, outfitted with a robotic vehicle operated from the ship, began to study the geology, chemistry, and biology of these new hydrothermal vents.

Operations

On Leg 169 two old and broken CORKs had to first be fished out of the re-entry cone. This was not a small task in itself. The hostile environment caused the two Corks to become corroded and thus they pulled apart in pieces while attempting their recovery. Remedial fishing trips were required which resulted in the successful recovery of the remaining portions and allowed the installation of two new CORKs as planned.