CSIRO Exploration and Mining, Australia

Ocean Drilling Program (ODP) Leg 193 investigated two sites of hydrothermal activity along the crest of the Pual Ridge in the eastern Manus Basin. A site of low-temperature diffuse venting, Snowcap (Site 1188), and a high-temperature black smoker site, Roman Ruins (Site 1189), were drilled to depths of 386 and 206 meters below sea floor (mbsf), respectively. Although the two sites are <1000 m apart, the (super 87) Sr/ (super 86) Sr and d (super 34) S signatures of anhydrite recovered at both sites are very different. Measured anhydrite (super 87) Sr/ (super 86) Sr ratios vary between 0.7050 and 0.7086; the most radiogenic values occur just beneath the variably altered dacitic cap at both sites. At Snowcap (Site 1188) there is a clear trend downhole to less radiogenic (super 87) Sr/ (super 86) Sr values, from 0.7086 beneath the dacite cap to values of 0.7060 at approximately 100 mbsf, and these low values are broadly maintained to the base of the hole. The measured anhydrite d (super 34) S values vary between 18.1 per mil and 22.5 per mil. For Snowcap, most of the values are less than 20.99 per mil and trend significantly toward lower values downhole. In contrast, anhydrite d (super 34) S values at Roman Ruins are mostly equal to or greater than the seawater value, show no obvious downhole trends. The data suggest a complex interplay among hydrothermal fluid, magmatic fluid, and seawater during alteration and mineralization of the PACMANUS system. These new results significantly expand the subsurface data on seafloor hydrothermal systems and may begin to explain the earliest processes of multistage mineralization and alteration history that typify ancient massive sulfide systems.