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Martin Van Kranendonk, Geological Survey of Western Australia (Australia)
Pascal Philippot, Institut de Physique du Globe de Paris (France)
Simon Bodorkos, Geoscience Australia (Australia)
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Results of detailed stratigraphic, petrographic, and zircon U-Pb geochronologic studies from outcrops and drillcore through the c. 3.49 Ga, stromatolitic chert-barite unit of the Dresser Formation (Warrawoona Group, Pilbara Craton) point to deposition within an active volcanic caldera affected by syn-depositional growth faulting and repeated pulses of hydrothermal fluid circulation.
Six members are recognised in the unit. At the base, thinly bedded carbonates were deposited under deep- to shallow-water conditions (members 1-2). Minor diamictite and felsic volcaniclastic sandstone point to periods of felsic eruption and tectonic instability. Shallow-water carbonates display evidence for microbial binding of carbonate sediment and contain coniform stromatolites, suggestive of phototrophy.
Up-section, bedded carbonates are disconformably overlain by volcaniclastic conglomerates and polymict diamictites deposited during periods of tectonic instability caused by growth faulting, as well as bedded carbonates (members 3-4). Pulsed, repeated episodes of itense hydrothermal fluid flow accompanied growth faulting and precipitated both silica ± barite ± sphalerite and pyrite. Barite clasts in sandstone and diamictite beds highlight the repeated nature of hydrothermal fluid circulation during sediment accumulation. Barite grains are rimmed by pyrite indicates fluctuation between magmatic fluids and recirculated seawater. Hydrothermal fluid circulation resulted in replacement of the carbonate sediments by hydrothermal precipitates, including pyrite and barite-silica-sphalerite. Putative signs of life in the hydrothermally-dominated parts of the unit include stratiform and domical stromatolites. Petrographic observations of local relics of carbonate between pyrite crystals in stromatolitic laminates indicate carbonate protoliths, supporting a biological origin for stromatolites.
Unconformably overlying members 5-6 consist of coarse sandstone passing up to thinly-bedded carbonate with local, minor felsic volcaniclastic sandstone. Clasts of finely laminated carbonaceous material are present in primary micritic carbonates, providing further evidence for early life. A 10 cm thick bed of felsic volcaniclastic tuff within finely bedded carbonates near the top of the unit has yielded a maximum age of deposition of 3481 ± 4 Ma, confirming earlier Pb-Pb age data.
Textural analysis of jaspilitic "cherts" near the top of the unit reveal haematite as tiny crystals within recrystallized carbonate rhombs in carbonate beds affected by hydrothermal silica alteration. Unaltered diagenetic pyrite crystals in the jaspilitic cherts are used to infer that haematite did not result from oxidation, but rather to result from alteration of Fe-carbonates by mildly reducing, silica-rich hydrothermal fluids. This has important implications for the origin of jaspilitic cherts throughout the early Archean record and supports an anoxic Archean atmosphere.
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