Sulfide‐impregnated volcanics and ferromanganese incrustations from the Southern Lau Basin (Southwest Pacific)

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Abstract
The Lau Basin is a young back‐arc basin east of the Tonga‐Kermadec trench with active spreading ridges and off‐axis seamount volcanoes. A great variety of hydrothermal deposits associated with the spreading volcanism were observed and /or collected at numerous locations during a German‐French diving campaign with NAUTILE in 1989 and a research cruise with R. V. SONNE in 1990. The spreading ridge is characterized by highly permeable, autobrecciated, andesitic‐to‐dacitic rocks and volcaniclastic sediments. Within these “spongelike” rocks a widespread high‐to‐low‐temperature discharge takes place with sutfide impregnation and Mn‐Fe incrustation forming halos around rare high‐temperature vent deposits of massive sulfides restricted to the occurrence of faults. This deposition due to different temperature and different environment may 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: (1) magmatic stage (T > 1000°C; moderate fO2, at or slightly above the QFM buffer; logfS2 < ‐13), which forms Ti‐bearing magnetite, ferrian ilmenite, native iron, pyrrhotite (first generation), chalcopyrite; (2) early postmagmatic stage (T around 365–345°C; low fO2; fS2 initially at < ‐8 to ‐9, subsequently rising to > ‐8.5; pH > 5), which forms Cu‐, Ni‐, or Co‐bearing pyrrhotite (second generation) and pyrite (first generation); (3) later postmagmatic stage (T 345°C decreasing to around 150°C; fS2 < ‐8.5 and decreasing with decreasing temperature; pH decreasing to < 5), which forms Cu‐, Ni‐, or Co‐free pyrrhotite (third generation) and pyrite (second and third generation), marcasite; (4) low‐temperature stage (at pH 6–8, decreasing to pH < 5), which forms framboidal pyrite, melnikovite‐pyrite, and acicular marcasite. The sulfide impregnation is combined with strong bleaching of the volcanic xenoliths and rock fragments. The bleaching is due to the decomposition of magnetite in the glassy volcanic matrix by the sulfur‐bearing 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°C) hydrothermal conditions by fractionated precipitation from fluids percolating upward through the highly permeable volcanic rocks. During a first phase of precipitation, surface rocks are sealed by widespread Mn‐Fe crusts. A second phase of precipitation occurs along fissures within those crusts. Densely laminated Mn oxides probably precipitated in an open space isolated from the water column. Due to changing environment, fractionated precipitation results in the formation of Mn‐rich or Fe‐rich material alternating in a macro or micro scale. Fibrous and spherical growth structures of the Mn‐Fe crusts are probably due to mineralized bacteria and possibly also fungi.