LOW-TEMPERATURE HYDROTHERMAL DEPOSITS OF FRANKLIN SEAMOUNT, WOODLARK BASIN, PAPUA-NEW-GUINEA

Hydrothermal deposits of siliceous Fe-Mn oxides from a few cm to 7 m i n height are widespread at the summit and caldera floor of Franklin Se amount, a 250 m high edifice of basaltic andesite lava tubes, pillows and talus located near the western propagating tip of the Woodlark spr eading axis. Some deposits are venting clear fluid at about 30 degrees C. The hydrothermal constructions are composed predominantly of X-ray amorphous protoferrihydrite mixed with variable amounts of opaline si lica and greenish nontronite. Most deposits are covered by mm-thick bl ack crusts of vernadite and feroxyhyte which are mineralised bacterial formations (genus Leptothrix) of hydrogenous origin. Microbes also in fluenced formation of the Fe-rich parts of the deposits, as evidenced by filamentous protoferrihydrite pseudomorphs of Gallionella. Blocky i norganic particles of protoferrihydrite and ferrihydrite occur as rare interior phases. Nontronite formed by precipitation from hydrothermal solutions, commonly involving replacement of microbial protoferrihydr ite and silica. Patchy and lenticular birnessite, partly altered by bi ogenic processes to vernadite, occurs within siliceous protoferrihydri te matrices in dark cm-thick outer margins of some edifices. This is c onsidered hydrothermal in origin, reflecting percolation of hydrotherm al fluids through porous protoferrihydrite-opal aggregates to more oxi dised conditions near the exterior related to mixing with seawater. Th e different mineralogical associations are reflected by geochemistry. Trace elements in the manganiferous phases were derived partly from th e primary hydrothermal solution and partly from seawater with which th is was extensively diluted at the depositional site and also deeper wi thin the volcanic structure of Franklin Seamount. Most of the Fe, Mn, Cu, Zn and Pb was derived from the primary hydrothermal fluid. From li mited data, nontronitic portions of the deposits contain more Cu, Zn a nd Pb than the siliceous protoferrihydrite-rich portions. Migration of redox boundaries and redistribution of elements during flow-through p rocesses influence development of the typical zoned structure of the d eposits. Uranium-thorium decay series equilibria at both actively vent ing and inactive sites at Franklin Seamount yield ages not exceeding 4 3 years. (C) 1997 Elsevier Science B.V.