Oncoids, formed entirely of amorphous silica (opal-A), are common on the central to distal parts of Golden Fleece Terrace at the Orakeikorako geothermal site, North Island, New Zealand. The discoid oncoids, which grew in shallow (1 cm) pools bounded by low rimstone dams, are up to 16 mm long, 14 mm wide, and 7 mm high. The core (nucleus) and cortex are formed of alternating porous and non-porous laminae. Porous laminae are composed of complex, but loosely interwoven, meshworks of silicified filamentous cyanobacteria that have relatively little silica precipitated between them. The non-porous laminae have a radial fabric in which filamentous microbes have their long axes perpendicular to the underlying growth surface. Pore spaces between the filaments are partly filled with amorphous silica cement. The morphology of the microbes in these oncoids is well preserved because silica precipitation occurred while the microbes were alive or very soon after death. Early silica precipitation allowed the differences between porous and non-porous laminae to be preserved. The alternation between porous and non-porous laminae may reflect times when microbial growth outpaced silica precipitation and vice versa. Such changes probably reflect variations in the discharge patterns and/or the temperature of the spring waters that flowed across the microterraces during oncoid growth. Periods of rapid silica precipitation may have been a factor that caused microbial filaments to adopt the radial attitude that characterizes the oncoids.

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