Cambrian microbial and silica gel textures in silica iron exhalites from the Mount Windsor volcanic belt, Australia; their petrography, chemistry, and origin

Univ. Tasmania, Cent. Ore Deposit and Explor. Stud., Hobart, Tasmania, Australia

Stratiform quartz-hematite or magnetite lenses (ironstones) occur sporadically throughout the Mount Windsor volcanic belt at three main stratigraphic positions, including along strike from and immediately stratigraphically above the Thalanga massive sulfide deposit. The area is ideal for examination of the chemical, textural, and mineralogical changes which occur in silica iron exhalites around ancient hydrothermal submarine vent sites. Major and trace element chemical data for the ironstones indicate they are essentially composed of SiO ₂ and Fe ₂ O ₃ (generally >99 wt %), with very low concentrations of other oxides (e.g., Al ₂ O ₃ , generally <0.5 wt %) and trace elements.Across the length of the Mount Windsor volcanic belt, most of the ironstones have experienced substantial recrystallization at metamorphic grades from greenschist to amphibolite facies. However, a number of samples from the eastern part of the belt are less recrystallized (prehnite-pumpellyite facies) and retain abundant evidence of microbial fossils and the inorganic maturation of amorphous phases. At least three distinct microbial morphologies are recognized; (1) approximately 1-mu m-avg-diam filaments with septa, (2) approximately 3-mu m-avg-diam filaments with no septa, and (3) 20- to 30-mu m-avg-diam filaments with no reconnecting branches. The range in metamorphic grade along the Mount Windsor volcanic belt not only permits a rare insight into the primary textures of ancient exhalites but also a useful view of the textural and mineralogical changes which accompany the progressive increase in temperature up to amphibolite facies conditions.Comparisons with similar modern deposits and experimentally produced synthetic textures suggest that the ironstones formed from the crystallization of silica iron oxyhydroxide gels. These gels formed as a result of mixing between hydrothermal fluid and seawater and conductive cooling of the product. The rare earth element geochemistry of ironstones along strike from Thalanga is compatible with migration of a hydrothermal fluid away from the vent center, followed by its accumulation and cooling in topographic depressions on the sea floor. Microaerophilic chemolithotrophic bacteria and/or fungi probably catalyzed the oxidation of Fe (super +2) to Fe (super +3) . Several metamorphosed ironstones also contain pyrite with delta ³⁴ S values approximately 50 per mil lighter than Cambrian sulfate, suggesting the activity ofsulfate-reducing bacteria at some point during their formation. The temperature of the original hydrothermal fluid was probably between 10 degrees and 40 degrees C, originating either as the in situ product of weak hydrothermal activity in which inorganic sulfate reduction did not occur or as the cooled remnant of a higher temperature fluid which had already deposited sulfides. The inferred mode of deposition for the siliceous ironstones (as a gel) is different from that of many modern iron oxyhydroxide precipitates, which form by settling of particles to the sea floor from hydrothermal plumes. On the flanks of the Thalanga massive sulfide deposit some iron silica exhalites have been encompassed by massive sulfide suggesting that the early low-temperature Fe-Si precipitates were subsequently overprinted by hotter fluids. This provides good evidence for waxing and waning in the life of the vent system.

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