Evgeny Gurvich

Low-temperature hydrothermal deposits of Franklin Seamount, Woodlark Basin, Papua New Guinea

Abstract Hydrothermal deposits of siliceous FeMn oxides from a few cm to 7 m in height are widespread at the summit and caldera floor of Franklin Seamount, a 250 m high edifice of basaltic andesite lava tubes, pillows and talus located near the western propagating tip of the Woodlark spreading axis. Some deposits are venting clear fluid at about 30 °C. The hydrothermal constructions are composed predominantly of X-ray amorphous protoferrihydrite mixed with variable amounts of opaline silica and greenish nontronite. Most deposits are covered by mm-thick black crusts of vernadite and feroxyhyte which are mineralised bacterial formations (genus Leptothrix ) of hydrogenous origin. Microbes also influenced formation of the Fe-rich parts of the deposits, as evidenced by filamentous protoferrihydrite pseudomorphs of Gallionella . Blocky inorganic particles of protoferrihydrite and ferrihydrite occur as rare interior phases. Nontronite formed by precipitation from hydrothermal solutions, commonly involving replacement of microbial protoferrihydrite and silica. Patchy and lenticular birnessite, partly altered by biogenic processes to vernadite, occurs within siliceous protoferrihydrite matrices in dark cm-thick outer margins of some edifices. This is considered hydrothermal in origin, reflecting percolation of hydrothermal fluids through porous protoferrihydrite-opal aggregates to more oxidised conditions near the exterior related to mixing with seawater. The different mineralogical associations are reflected by geochemistry. Trace elements in the manganiferous phases were derived partly from the primary hydrothermal solution and partly from seawater with which this was extensively diluted at the depositional site and also deeper within the volcanic structure of Franklin Seamount. Most of the Fe, Mn, Cu, Zn and Pb was derived from the primary hydrothermal fluid. From limited data, nontronitic portions of the deposits contain more Cu, Zn and Pb than the siliceous protoferrihydrite-rich portions. Migration of redox boundaries and redistribution of elements during flow-through processes influence development of the typical zoned structure of the deposits. Uranium-thorium decay series equilibria at both actively venting and inactive sites at Franklin Seamount yield ages not exceeding 43 years.

Sediments of the active rift zone in the western Woodlark Basin and the development of hydrothermal and volcanic activity

Abstract In the western Woodlark Basin within the neovolcanic zone sediments form ponds in fissures and between individual pillows and tubes. A continuous thin sediment cover appears only at the base of the neovolcanic zone. Its thickness increases with the distance from the zone. Data on the lithology, mineralogy and chemistry of sediment cores from the spreading centre in the western Woodlark Basin and adjacent areas show that these calcareous sediments (avg. 44.1 % CaCO 3 ) contain significant amounts of terrigenous, arc volcanic, hyaloclastic and hydrothermal material, the latter constituting up to 15% of the non-carbonate fraction. Two types of sedimentary sequences occur in the studied area. Those of the first type were formed by particle-by-particle sedimentation; horizons of redeposited sediments occur in sequences of the second type. Redeposition of sediments is widespread and is tentatively correlated with cycles of volcanic activity at swells along the spreading centre. These alternate with hydrothermal cycles, which were determined by the presence of hydrothermal matter in sediments. Radiocarbon dating of sediments showed that both cycles are of similar duration, of the order of 10,000 years. They are not synchronous along the spreading axis, each swell having a specific evolution. Based on these determinations we can say that within the studied spreading centre only the Franklin Seamount swell is prospective for contemporary hydrothermal activity.