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ABSTRACT: Extensive Hydrothermal Activity in the Southern Lau Basin

U. Von Stackelberg

During research cruises with RV Sonne in 1984 and 1987 high-and low-temperature hydrothermal activity was found in the Lau basin. During a French-German diving cruise with the submersible NAUTILE in 1989 low-temperature discharge (40°C) was observed and measured at various locations, especially along the southern Valu Fa Ridge, an active back-arc spreading ridge that is mainly composed of highly vesicular, brecciated andesitic rocks. Near the hydrothermal discharge a great number of vent organisms were observed. Because of the acid character of the solutions the volcanic rocks are strongly altered and corroded, sometimes altered into white fine-grained debris. Most of the near-surface rocks show a strong impregnation or vein-like stockwork mineralization by metal sul ides. Mn and Fe oxide crusts and dead clams were observed at the Valu Fa Ridge extending for a few kilometers, which indicates the widespread occurrence of low-temperature hydrothermal products. High-temperature discharge (400°C) was measured at 15 m high smokers at the central Valu Fa Ridge. A mineral assemblage suggestive of high temperatures is also found in the stockwork deposits and in mineralized xenoliths. According to our model, the extensive occurrence of low-temperature surface mineralization and of high-temperature subsurface mineralization is

explained by the high permeability of the fragmented andesites depressing the hydrothermal front to deeper levels within the spreading ridge.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990