CAMBRIAN MICROBIAL AND SILICA-GEL TEXTURES IN SILICA IRON EXHALITES FROM THE MOUNT WINDSOR VOLCANIC BELT, AUSTRALIA - THEIR PETROGRAPHY, CHEMISTRY, AND ORIGIN

Stratiform quartz-hematite or magnetite lenses (ironstones) occur at three main stratigraphic positions, including along strike from and immediately stratigraphically above the Thalanga massive sulphide 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 SiO2 and Fe2O3 (generally >99 wt%), with very low concentrations of other oxides (eg Al2O3, generally <0.5 wt%) and trace elements. Comparison 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. -from Authors