Petrography and genesis of spicular and columnar geyserite from the Whakarewarewa and Orakeikorako geothermal areas, North Island, New Zealand

Domal masses of geyserite, which surround many geyser vents in the Whakarewarewa geothermal area, are formed largely of spicules, spicule columns, and shrub columns. The non-branching spicules and the branches of branching spicules, individually up to 3 cm high and 1 mm diameter, have a laminated core encased by siliceous cortex. Silicified microbes are rare in the core but common in the cortex. Silicified microbial mats and pseudodendrites are found in the crevices between neighbouring spicules. Shrub columns, up to 5 cm high and 1.5 cm in diameter, are formed of opal-A that was precipitated around a three-dimensional, branching, shrub-like microbial (?) structure. The shrub branches, which are hollow with scalloped walls, do not contain any evidence of the original microbes or minerals that formed them. Silicified microbial mats are present between the columns. Microbial boring and etching by acidic steam led to local diagenetic degradation of these columns. In the geyserite mounds at Whakarewarewa, spicules and spicule columns are common, whereas shrub columns are rare. Interbedding and intercalation of spicular geyserite with shrub columnar geyserite indicate, however, that these different morphologic entities probably formed under similar environmental conditions on the mounds around the geyser vents. Petrographic evidence shows that the spicules, spicule columns, and shrub columns grew through a combination of biotic and abiotic processes.