Small spicular, columnar, and blade-shaped stromatolites are common features of hot spring and geyser systems in the Taupo Volcanic Zone of North Island, New Zealand. These organosedimentary structures can grow in many settings, where water temperature and pH vary widely. Although most common in neutral and alkaline waters, stromatolites also are forming in several acid-sulfate spring and geyser systems in the Waiotapu geothermal area and at Lake Rotokawa. Growth of these stromatolites is partly mediated by a biota that is dominated by fungi and locally, diatoms.
When stromatolites from acidic thermal waters are compared with those from neutral and alkali waters, significant differences in their biota and mineralogy are evident. The biota in stromatolites from the neutral and alkali waters is dominated by prokaryotic bacteria (including cyanobacteria), whereas stromatolites from acidic waters are dominated by eukaryotic fungi and to a lesser extent, diatoms. Stromatolites in the neutral and alkali thermal waters are formed almost entirely of opaline silica, with calcite laminae present in a few localities. Although stromatolites in the acidic systems also are composed mainly of opaline silica, they contain substantial amounts of kaolinite and, locally, sulfur and/or jarosite. In ancient thermal deposits it may be possible to distinguish stromatolites that grew in acidic waters from those that formed in neutral and alkali systems by considering their preserved biota and mineralogy.