Siliceous stalactites, formed of opal-A laminae that are concentrically arranged around a hollow soda-straw, are common features structures in some of the geyser and hot spring deposits at Whakarewarewa and other geothermal areas of New Zealand. These siliceous stalactites contain diverse biota of bacteria (including cyanobacteria), fungi, and diatoms that lived on the stalactite surfaces, with locally abundant pollen grains, and springtails. The microbes are the templates for much of the opaline silica precipitation and thereby controlled most of the fabrics that form the stalactite laminae.
Siliceous stalactites form at sites of dripping water below the overhanging edges of geyserite rims that surround geyser vents, and along the steep margins of sinter terraces on discharge aprons. Stalactites may hang as isolated individuals or may coalesce to form dripstone draperies along terrace or rim margins. Stalactite growth is controlled by the volume, temperature, and the silica concentration of the water supplied to the growth site. These waters mainly originate from geyser or hot spring discharge. The microbes preserved in the stalactites, however, imply that silica precipitation took place after these waters had cooled.
The growth and coalescence of geyser stalactites to form complex dripstone draperies plays an important role in the lateral accretion of sinter around the rims of many geysers and in progradation of sinter terraces.