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MIDDLE ARCHEAN VOLCANO-HYDROTHERMAL SEQUENCE: 3.2 GA DIXON ISLAND FORMATION, COASTAL PILBARA TERRANE, AUSTRALIA
The Dixon Island Formation, which is approximately 350 m in thickness, is composed of Rhyolite Tuff, Black Chert and Varicolored Chert Members. Many black-chert vein swarms in the Rhyolite Tuff Member indicate intensive low temperature hydrothermal activity during deposition of the Black Chert Member which formed 15~20 m in thickness. Many barite, halite and gypsum pseudomorphs and well continuous stromatolite-like biomat bed layer (10~20 cm in thick) preserved within the laminated black chert bed. No evidence of detrital sediments from continental origin in this formation, also indicate that the sedimentary facies is identified as hydrothermal environments at approximately 500~2000 m in depth of the immature island arc setting.
The massive black chert has carbonaceous peloids (0.3 mm~2 mm in diameter) similar with that of in the black chert veins. The massive black chert were well preserved wriggle-, rod- and dendritic-shapes bacterial material. Total organic carbon (TOC) in black chert and black chert vein varied within 0.05 ~ 0.16 % (average 0.1 %) and carbon isotope (delta 13C) of these rocks are -32~ -27 per mil (average 30 per mill). These evidences suggest carbonaceous grains and bacterial shape materials in the black cherts are biogenic which exuberate close to the hydrothermal vent system.
Field observation and geochemical evidence suggest that the Dixon Island Formation is one of the best example preseving of the hydrothermal vent system with biogenic microbial colony on Archean oceanic floor on island arc. The microbial colony might be quickly fossilized by silicification during the hydrothermal activity.