Ocean Drilling Program, United States

In the Ocean Drilling Program's only foray to an active seafloor hydrothermal system hosted by felsic volcanic rocks at a convergent plate margin, deep penetrations were achieved at two contrasted sites within the PACMANUS field (Manus backarc basin, Papua New Guinea). Just 1.0 km apart, these sites are characterized, respectively, by diffuse lowtemperature venting at the seabed (Site 1188, Snowcap site; 1650 meters below sea level [mbsl]) and focused high-temperature venting (Site 1189, Roman Ruins; 1700 mbsl). Shallow holes at a background location remote from known hydrothermal activity (Site 1190) and at a second high-temperature chimney field (Site 1191, Satanic Mills) failed to drill beyond unaltered felsic lavas which at Sites 1188 and 1189 form an impervious cap (as thick as 35 m) to an underlying, pervasively altered lava sequence with occasional volcaniclastic horizons. To the maximum depth drilled (387 meters below seafloor [mbsf]), alteration assemblages are characterized by clay minerals and ubiquitous disseminated pyrite. Hydrothermal K-feldspar at Site 1189 differentiates it from Site 1188 where, by contrast, several intervals of pyrophyllite-bearing acid sulfate alteration suggest input from magmatic volatiles. At both deeply penetrated sites the dominant silica phase in alteration assemblages changes downhole from opal-A at the transition from overlying unaltered lava to cristobalite and then to quartz. The boundary between the cristobalite and quartz domains is gradational between 60 and 110 mbsf in Hole 1188A under Snowcap but is sharper and shallower ( approximately 25 mbsf) in Hole 1189A on the fringes of the Roman Ruins field. Hole 1189B, higher on the Roman Ruins mound, intersected a "Stockwork Zone" with abundant quartz+ or -pyrite+ or -anhydrite veins and breccia infills, from base of casing (31 mbsf) to approximately 110 mbsf, below which an abrupt change occurred to a "Lower Sequence" with interleaved cristobalite- and quartz-bearing assemblages and common preservation of igneous plagioclase. Only two thin intervals of sulfide-rich mineralization were encountered, both below the Roman Ruins chimney field. Postcruise volcanic facies analyses based on logging data and cores with well-preserved fabrics, plus assessments of immobile element geochemistry for altered rocks referred against a local database for glassy lavas, establish that Pual Ridge is constructed from numerous lava flows averaging approximately 15 to 30 m thick and ranging from andesite to rhyodacite in composition, with dacites dominant. Investigations of alteration and mineralization support the concept of a single major hydrothermal event imposed at PACMANUS after accumulation of most of the Pual Ridge volcanic sequence. Different phases within this event, involving pronounced differences in fluid chemistry, created a variety of alteration styles yet to be fully unraveled. Much of the extensive subseafloor alteration may have been completed before uprise of high-temperature vent fluids that formed seabed chimneys.