Leg 193 Synthesis: Anatomy of an Active Felsic-Hosted Hydrothermal System, Eastern Manus Basin, Papua New Guinea

Abstract

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 low-temperature 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 (~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 ± pyrite ± anhydrite veins and breccia infills, from base of casing (31 mbsf) to ~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 ~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 sub-seafloor alteration may have been completed before uprise of high-temperature vent fluids that formed seabed chimneys. Prominent alteration-related geochemical differences between Sites 1188 and 1189 include enrichments in potassium, barium, and uranium at Site 1189. Altered wallrocks in the Stockwork Zone of Hole 1189B have lost silica, but Si is more generally conserved at precursor levels. Leaching during hydrothermal alteration did not contribute significant base or precious metals, or barium, to seabed chimney deposits. At both sites hydrothermal alteration involves volume expansion arising from grain-scale dilation imposed by excess pore fluid pressures. Progressive dilation with nonreplacive deposition of sulfides and gangue minerals in open spaces is a dominant process in the two occurrences of subseafloor semimassive sulfide encountered below Roman Ruins. Fluid inclusions in vein anhydrites provide conclusive evidence of phase separation ("boiling") within the PACMANUS hydrothermal system at temperatures exceeding 360°C, somewhat higher than alteration temperatures computed from oxygen isotope analyses of clay minerals but comparable with oxygen isotope temperature estimates for vein quartz. Strontium isotope characteristics of anhydrites from veins, breccia matrixes, and semimassive sulfides imply deposition from varied mixtures between seawater and high-temperature hydrothermal fluids. The latter are more radiogenic (87Sr/86Sr = 0.7050) than fresh lavas at Pual Ridge (basaltic andesite to rhyodacite; 87Sr/86Sr = 0.7036) and so include a component of very deeply circulated seawater. There is circumstantial evidence for a magmatic component in the high-temperature hydrothermal fluid. Hydrothermal alteration of the volcanic sequence at Pual Ridge may have been largely completed before the main mineralizing events. Excess fluid pressures during both alteration and subsequent mineralization suggest a "pressure cooker" model whereby the subseafloor hydrothermal system is largely confined by a cap of impervious volcanics that become sporadically breached by hydrofracturing or tectonic processes to allow seafloor venting and sulfide deposition. Fluid flow within the PACMANUS system, especially that related to seabed venting, is governed by fractures rather than high porosity and permeability of the subseafloor rocks. A vibrant microbial assemblage exists in the higher parts of the hydrothermal system (to ~130 mbsf). Below this the system appears sterile, but temperature limits for viability have not been established. Cultures at 60° and 90°C are dominated by Geobacillus sp. and Deinococcus sp., respectively. Whereas mineralized bacterial cells have been observed, subseafloor biomineralization appears not to play an important role at PACMANUS.