Abstract

The Bent Hill Massive Sulfide (BHMS) in the Middle Valley comprises three major mineralized parts: (1) a 100-m-thick conical mound of massive sulfide formed at the seafloor and then partly buried by sediment; (2) an underlying 100-m-thick feeder zone which consists of subvertical crosscutting veins filled with Cu-Fe sulfide and pyrrhotite, and where the intensity of veining decreases with depth and the style of mineralization changes to predominantly subhorizontal impregnation and replacement of sediment; and (3) the surprising discovery at the base of the feeder zone of a 4-m-thick, strongly silicified horizon underlain by a 13-m-thick zone of intense alteration and replacement of the host sandstones by Cu-Fe sulfides and chlorite (Deep Copper Zone). This zone of high grade (up to 16% Cu) stratiform copper mineralization may represent an aquifer of hydrothermal fluid flowing laterally into a permeable sedimentary horizon. This aquifer is capped and isolated by an impermeable silicification front. Pore fluid derived from below this horizon is distinct from that sampled above and, has the low chlorinity signal typical of the only vent known in the area prior to Leg 169. Hole 1038F penetrated this horizon and was vigorously venting hydrothermal fluid after drilling ceased.

The metal zonation observed in ancient massive sulfide deposits is also present in the BHMS. Continued hydrothermal circulation through the massive sulfide after its initial deposition converted much of the primary pyrrhotite to pyrite magnetite and has remobilized metals like zinc which were reprecipitated at the top and on the sides of the mound at lower temperatures. Much of the copper transported in the hydrothermal fluid was deposited below the seafloor in the stockwork zone and in the Deep Copper Zone.

A second mound, the Ore Drilling Program massive sulfide (ODPMS) mound, occurs 350 m south of the BHMS. A single hole was drilled near the top of the mound, 50 m south of the only known natural active vent. The results were nothing less than spectacular. Hole 1035H penetrated three stacked zones of massive and semi-massive sulfide mineralization along with their feeder zones. Metal grades are much higher than those encountered at BHMS with some samples exceeding 40% Zn and 15% Cu. Most explorationists in the mining industry will never have the experience of drilling a hole that intersects as much high-grade ore as at Hole 1035H. However, the true value of this deposit is in the complex record of deposition, recrystallization, and remobilization of metal recorded through the multiple hydrothermal stages that remained focused beneath this mound throughout its history. We were not able to test the continuity of mineralization between the ODPMS and BHMS mounds, but a zone of high-grade stratiform copper mineralization was intersected at approximately the same stratigraphic horizon as the zone under BHMS. Hole 1035H is also now the third known hydrothermal vent in the Bent Hill area.

Although creating new vents in the Bent Hill area was not part of the pre-cruise plans, conducting active hydrological experiments in the Dead Dog vent field, which occurs 4 km to the northwest, was a high-priority objective of Leg 169. We successfully removed the existing CORK from Hole 858G, recovering the first CORK-hosted hydrothermal chimney deposits in the process, and sampled 272°C hydrothermal fluids from the borehole. The borehole was then reinstrumented with a new temperature string and pressure transducer and a new CORK was installed. The damaged CORK in Hole 857D was recovered and replaced with an 898-m-long thermistor string and a new CORK in a technologically difficult operation that was efficiently executed by the Ocean Drilling Program (ODP) engineering group and the Sedco staff. Rapid downflow of cold bottom water into this hole was confirmed. This may lead to a pressure pulse that is potentially detectable in Hole 858G, 1.6 km to the north, and may lead to induced seismicity, which was being monitored by an array of Ocean Bottom Seismometers deployed prior to drilling. A transect of short holes across the Dead Dog active hydrothermal mound demonstrated that the mound is young and was formed by build-up and collapse of anhydrite chimneys, rather than by subsurface deposition and internal inflation.

A high priority objective was to establish the differences between the mature hydrothermal system developed at Bent Hill and the young hydrothermal system in Escanaba Trough. These systems differ in more than their state of evolution. Metals in the Middle Valley sulfide deposits seem to be dominantly derived from basaltic rocks, whereas in Escanaba Trough, the composition of the deposits shows extensive contribution of metals from the sediment. Massive sulfide recovered from Central Hill suggests that the thickness of massive sulfide differs little from the amount exposed above the seafloor (5-15 m). The absence of a well-developed veined feeder zone indicates pervasive diffuse venting of hot fluid over a short period of time rather, than long-lived, focused high temperature discharge, as it was the case at Bent Hill.

A hydrothermal component in the pore fluid from Escanaba Trough indicates that hydrothermal fluid flow was relatively recent. Both low and high-salinity fluids are present indicating phase separation, followed by segregation of most of the low-salinity fluids into an unconsolidated sand unit in the interval from 70 to120 mbsf. Concentration of alkalies and other elements indicates that the hydrothermal fluids have interacted extensively with sediment, even though most of the recovered sediment is not extensively altered. Organic matter maturation confirms that the sediments have seen at least a brief pulse of high temperature.

The sedimentary fill of the Escanaba Trough records some of the highest sedimentation rates observed in the deep sea. The sparsity of biostratigraphic control, however, makes it difficult to quantify the rates of deposition. A change of provenance from older sediment derived predominantly from the Klamath Mountains to more recent sediment derived predominantly from the Columbia River drainage was indicated by spot coring on Deep Sea Drilling Project (DSDP) Leg 5. Leg 169 obtained a nearly continuous record of the 500 m of sediment fill at the reference hole at Site 1037 which was drilled into basaltic basement. Most of the sediment is derived from turbidity currents that enter the southern end of Escanaba Trough and flow northward along the ridge axis. The unique "box canyon" setting of Escanaba Trough traps even the thickest turbidite flows leading to both high sedimentation rates and unusually thick deposits of muddy turbidites. An acoustically transparent layer that is traceable in seismic records throughout Escanaba Trough was determined to be a 45-m-thick depositional package containing abundant well sorted fine- to medium-grained sand with little admixed clay and silt. We hope that radiocarbon dating of pristine looking wood fragments in this unit will supplement sparse paleontological data.

To 169 Scientific Results

To 169 Table of Contents

Publications Homepage

ODP Homepage