PRIMARY SILICA ONCOIDS FROM ORAKEIKORAKO HOT-SPRINGS, NORTH-ISLAND, NEW-ZEALAND

Oncoids, formed entirely of amorphous silica (opal-A), are common on t he central to distal parts of Golden Fleece Terrace at the Orakeikorak o geothermal site, North Island, New Zealand. The discoid oncoids, whi ch grew in shallow (<1 cm) pools bounded by low rimstone darns, are up to 16 mm long, 14 mm wide and 7 mm high. The core (nucleus) and corte x are formed of alternating porous and non-porous laminae. Porous lami nae are composed of complex, but loosely interwoven, meshworks of sili cified filamentous cyanobacteria that have relatively little silica pr ecipitated between them. The non-porous laminae have a radial fabric i n which filamentous microbes have their long axes perpendicular to the underlying growth surface. Pore spaces between the filaments are part ly filled with amorphous silica cement. The morphology of the microbes in these oncoids is well preserved because silica precipitation occur red while the microbes were alive or very soon after death. Early sili ca 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 preci pitation and vice versa. Such changes probably reflect variations in t he discharge patterns and/or the temperature of the spring waters that flowed across the microterraces during oncoid growth. Periods of rapi d silica precipitation may have been a factor that caused microbial fi laments to adopt the radial attitude that characterizes the oncoids.