SUMMARY

Despite the technical challenges presented during the drilling program and the overall poor core recovery, multidisciplinary research on board ship and subsequently at many laboratories around the world has successfully achieved most major objectives of Leg 193. We now know that Pual Ridge was constructed by many lava flows ranging from andesite to rhyodacite, displaying coherent and brecciated facies and with some volcaniclastic horizons. Several paleoseafloor horizons have been identified, but there are no accumulations of hemipelagic sediment at these to indicate major hiatuses in volcanic activity, nor is there unequivocal evidence of fossil chimneys or other hydrothermal deposits at paleoseafloor positions. No plutonic rocks were encountered by drilling, so we have no further insights into the inferred intrusive or magma chamber that represents the heat engine to drive the hydrothermal system and a possible source of fluids and metals.

Detailed pictures of subsurface alteration phenomena and processes have emerged. The evidence favors one major alteration event pervasively imposed on all but the youngest lavas at the crest of Pual Ridge. It is unclear how the alteration systems link at depth between Snowcap (1635–1645 mbsl), where diffuse low-temperature venting occurs at seabed outcrops of altered volcanic rock, and Roman Ruins (1680–1690 mbsl, 1.0 km away), where focused high-temperature venting occurs at sulfide chimneys. Both sites display a change downhole from cristobalite-bearing assemblages to quartz-bearing alteration assemblages, but this occurs at a shallower level below Site 1189. Clay mineral assemblages dominate both sites but show no pronounced vertical change in character, reflecting advective heating by hydrothermal fluid cells rather than a conductive thermal gradient. Isotopic estimates of alteration temperatures in the quartz domains are in the range 220–300C; those in the cristobalite domain are not yet delineated. The presence of hydrothermal K feldspar and enrichments in K, Ba, and U at Roman Ruins constitute the principal lateral differences between alteration assemblages at Sites 1188 and 1189, respectively.

Two intervals of semimassive sulfide were cored. Semimassive sulfide near the base of Hole 1189A involved dilation and dispersal of clasts within a volcaniclastic horizon, with precipitation of quartz and sulfides in newly created matrix space. The other, in a Stockwork Zone directly below the cased top of Hole 1189B, was formed by extreme and repetitive dilation and mineral deposition in altered volcanic rock. Its paragenetic sequence from early sulfides to late anhydrite reflects progressive dilution by seawater of a high-temperature hydrothermal fluid sourced from deeper in the system. By preferentially removing Fe from hydrothermal fluids, subseafloor deposition of pyrite in the Stockwork Zone underlying Roman Ruins may contribute to chimney enrichments in base and precious metals.

Strontium isotopic evidence indicates mixing between seawater and a high-temperature hydrothermal fluid that itself has a component of seawater intermixed with fluids derived from igneous or magmatic sources at deeper levels than those explored by drilling. Fluid inclusions in anhydrites establish that phase separation or boiling has occurred in fractures that became veins and breccia matrixes within the pervasively altered volcanic sequence, and possibly at deeper levels than those drilled during alteration itself.

The presence of pyrophyllite-bearing acid sulfate alteration indicating former presence of SO2 represents the main new evidence, not unequivocal, for magmatic fluid components. Curiously, this acid sulfate style of alteration has limited occurrence at Site 1188 and is absent from Site 1189 where subsurface mineralization and seafloor chimneys are more prominent. The sulfur isotope characteristics of some pyrites and anhydrites also indicate a magmatic component mixed with variable proportions of seawater-derived sulfur. REE distribution patterns in anhydrites are considered to imply transport by fluorine complexes and thus a magmatic ligand source.

Geochemical assessments reveal extensive mass transfer during alteration but establish that Cu and trace chalcophile metals were not significantly leached from the drilled volcanic sequence and did not thereby contribute to the metal contents of seabed chimneys. Likewise, the gangue barite of chimneys does not derive from leached wallrock barium. While leaching at unexplored deeper levels is not ruled out, the concept of a magmatic origin for enriched base and precious metals in PACMANUS chimneys remains attractive.

Elevated porosity and permeability of the altered volcanic rocks facilitated diffusion during alteration processes, but fracturing is the more important control on fluid flow. Restriction of physical property and isotopic data to essentially two vertical drill profiles with uncertain hydrologic connectivity does not allow construction of definitive fluid flow models for the overall hydrothermal system. High porosity and consequent volume expansions appear imposed by excess fluid pressures during alteration, possibly related to phase separation at deeper levels in the system. Volume expansion provides an alternative mechanism to tectonic fracturing for breaches of the impervious capping of unaltered volcanic rock that, perhaps temporarily, become sites of chimney formation.

The presence has been established of a microbial biosphere within the active hydrothermal system, extending to ~130 mbsf, below which the system is sterile. The maximum temperature for microbial life here has not been quantified as a consequence of insufficient meaningful borehole temperature measurements. Indications of mineralized microbes were found, but biomineralization does not appear to be a significant process below the seabed as compared to some chimneys.

From the discussions in this Synthesis chapter there are clearly many issues of detail in the PACMANUS hydrothermal system that remain unresolved or uncertain. Definite scope remains for further research on cores and logging data from Leg 193, particularly for integrated studies directed at samples and correlated resisitivity images that have been specifically selected to address those issues.

For the longer term, PACMANUS is now established as a most appropriate site for continued drilling investigations. We have so far explored only the upper levels of this modern analog of ancient ore-forming systems. The next steps required to further advance our understanding are (1) to drill even deeper toward the inferred intrusive body and its surrounding high-temperature reaction zone, (2) to attempt deeper penetration below the Satanic Mills site for comparison of hydrothermal products and processes with Roman Ruins, and (3) to drill sufficiently deep holes between these sites to establish continuity or otherwise of alteration and mineralization patterns between them and to investigate their relationships with thickness variations of the unaltered volcanic capping. Leg 193 has demonstrated that these requirements are technically feasible under the Integrated Ocean Drilling Program (IODP), although coring methods that increase recovery rates will be highly desirable.

NEXT