| Abstract |
Steep Cone Spring, Yellowstone National Park, has a small sinter mound of 7 m high and 60 m wide. Microstructure, mineralogy and chemical compositions of siliceous deposits collected from five locations of the mound have been examined. Siliceous deposits are essentially composed of sub-micron to micron size spherules of non-crystalline silica (opal-A). There are variations in silicified microbial structures and chemical compositions of amorphous silica along the flow channel on the mound. In the high-temperature (>73"C) vent areas, especially in the boiling pool (94"C, pH = 8.3), microbial microstructures composed of filamentous silica and ftamboidal pyrite are present. In the intermediate- and low-temperature areas (73-25"C), where macroscopic cyanobacterial mats predominate, accumulation of amorphous silica spherules on surfaces of microbes are common and various microbial structures are well preserved in silica matrix. The concentration of Si02 is low (88-97 wt YO) in vent areas and increases to distal terrace areas (91-98 wt %). In contrast, A1203 (-5.3 wt %), CaO (-0.3 wt %), NazO (-2.3 wt %) and K20 (-0.9 wt %) show higher concentrations in vent areas than those of distal areas. Some pool rim geyserite have exceptionally high concentrations of FeO (-1 1.9 wt %) and Ti02 (-1 .O wt %). These characteristic morphological and chemical zoning patterns may apply to ancient siliceous deposits. |