SULFIDE-IMPREGNATED VOLCANICS AND FERROMANGANESE INCRUSTATIONS FROM THE SOUTHERN LAU BASIN (SOUTHWEST PACIFIC)

The Lau Basin is a young back-are basin east of the Tonga-Kermadec tre nch with active spreading ridges and off-axis seamount volcanoes. A gr eat variety of hydrothermal deposits associated with the spreading vol canism were observed and/or collected at numerous locations during a G erman-French diving campaign with NAUTILE in 1989 and a research cruis e with R. V. SONNE in 1990. The spreading ridge is characterized by hi ghly permeable, autobrecciated, andesitic-to-dacitic rocks and volcani clastic sediments. Within these ''spongelike'' rocks a widespread high -to-low-temperature discharge takes place with sulfide impregnation an d Mn-Fe incrustation forming halos around rare high-temperature vent d eposits of massive sulfides restricted to the occurrence of faults. Th is deposition due to different temperature and different environment m ay occur simultaneously in a spatial sequence and also successively in a temporal sequence. Sulfide-impregnated volcanic rock fragments and xenoliths were identified as high-Ti basaltic andesites. They contain ore phases that were formed in several stages: (I) magmatic stage (T > 1000 degrees C; moderate fO(2), at or slightly above the QFM buffer; logfS(2) < -13), which forms Ti-bearing magnetite, ferrian ilmenite, n ative iron, pyrrhotite (first generation), chalcopyrite; (2) early pos tmagmatic stage (T around 365-345 degrees C; low fO(2); fS(2) initiall y at < -8 to -9, subsequently rising to > -8.5; pH > 5), which forms C u-, Ni-, or Go-bearing pyrrhotite (second generation) and pyrite (firs t generation); (3) later postmagmatic stage (T 345 degrees C decreasin g to around 150 degrees C; fS,< -8.5 and decreasing with decreasing te mperature; pH decreasing to < 5), which forms Cu; Ni-, or Go-free pyrr hotite (third generation) and pyrite (second and third generation), ma rcasite; (4) low-temperature stage (at pH 6-8, decreasing to pH < 5), which forms framboidal pyrite, melnikovite-pyrite, and acicular marcas ite. The sulfide impregnation is combined with strong bleaching of the volcanic xenoliths and rock fragments. The bleaching is due to the de composition of magnetite in the glassy volcanic matrix by the sulfur-b earing fluids. This process furnished Fe for the sulfide formation. At growth rates up to a few cm/ka, Mn- and Fe-rich crusts formed in low- temperature (5-25 degrees C) hydrothermal conditions by fractionated p recipitation from fluids percolating upward through the highly permeab le volcanic rocks. During a first phase of precipitation surface rocks are sealed by widespread Mn-Fe crusts. A second phase of precipitatio n occurs along fissures within those crusts. Densely laminated Mn oxid es probably precipitated in an open space isolated from the water colu mn. Due to changing environment, fractionated precipitation results in the formation of Mn-rich or Fe-rich material alternating in a macro o r micro scale. Fibrous and spherical growth structures of the Mn-Fe cr usts ave probably due to mineralized bacteria and possibly also fungi.