Journal of the Geological Society, London, Vol. 156, 1999, pp. 89–103. Printed in Great Britain. Actively growing siliceous oncoids in the Waiotapu geothermal area, North Island, New Zealand BRIAN JONES 1 , ROBIN W. RENAUT 2 & MICHAEL R. ROSEN 3 1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada (e-mail: Brian.Jones@ualberta.ca) 2 Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada 3 Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand Abstract: Oncoids that are actively growing in some of the shallow-water pools around Champagne Pool, Waiotapu, New Zealand, are formed of amorphous silica (opal-A) with minor amounts of native sulphur. The growth of these oncoids is being mediated by a high-diversity microbial biota that achieves optimal growth in waters that have a temperature of 35–42C and pH of 6.6–6.9. Although this biota is dominated by coccoid, bacilliform, and filamentous cyanobacteria, sulphur-oxidizing bacteria are common in the cortical laminae of some oncoids. In addition, diatoms and silicified pollen grains are present in some cortical laminae. The silicified microbes are superbly preserved with their three-dimensional form and some of their internal structures being apparent, including the sites of sulphur globules in the sulphur-oxidizing bacteria. The microbes probably acted as templates for silica precipitation and thereby mediated the growth of the oncoids. Some cortical laminae are formed entirely of silicified pollen grains that came from the Pinus radiata that grow in the exotic pine plantations around Champagne Pool. By using these laminae as a record of the annual pollination event (typically in September–November), it can be shown that many of the oncoids are <15 years old and that the cortical laminae grew at 0.2–1.0 mm (average 0.35 mm) per year. The average daily rate of silica precipitation (0.5–2.75 m, average 1 m) is high compared to the average size of the microbes (commonly <1 m in length and diameter). This rapid rate of silica precipitation accounts for the superb preservation of the microbes and is consistent with silicification of the microbes within days of their demise. Keywords: New Zealand, hot springs, microbial structures, siliceous sinter, microorganisms. Oncoids are coated grains formed by microbes (cf. Flügel 1982; Peryt 1983) that mediate growth by binding detrital grains to adhesive organic substrates or by biochemically mediating in situ mineral precipitation (Doemel & Brock 1974; Monty 1976; Krumbein & Giele 1979; Gerdes et al. 1993) Although most oncoids are formed of calcium carbonate and develop in marine and freshwater systems, primary siliceous oncoids are known from hot spring systems at Orakeikorako (Renaut et al. 1996) and Ohaaki Pool (Jones et al. 1998) in New Zealand, and El Tatio (Jones & Renaut) in Chile. These oncoids, however, came from inactive systems or systems that have undergone substantial anthropogenic modifications. Consequently, inter- pretations of the conditions under which these siliceous on- coids grew were based on their physical features and settings, or inferences from analyses of the spring waters that pre- and (or) postdate anthropogenic modifications. In the Waiotapu geothermal area on the North Island of New Zealand, siliceous oncoids are actively growing in some of the shallow, warm-water pools that flank Champagne Pool (Fig. 1). The growth of most oncoids is typically mediated by a low-diversity biota, irrespective of the environment in which they are growing or the composition of the material being added. Growth of siliceous oncoids from Orakeikorako and El Tatio, for example, was mediated by a low-diversity biota that was dominated by the filamentous microbes tentatively assigned to Calothrix (Renaut et al. 1996; Jones & Renaut 1997). Growth of the Waiotapu oncoids, however, was medi- ated by a highly diverse microbial population that includes coccoid, bacilliform, and filamentous cyanobacteria and other bacteria including sulphur bacteria. These microbes are remarkably well preserved. We can therefore provide some insights into the structure of the microbial community that inhabited these waters and controlled the formation of the oncoids. Using oncoids collected from six localities around Champagne Pool (Fig. 1) in February 1997, this paper describes the microbes responsible for their formation and the environmental conditions under which they formed. By describing and illustrating these superbly preserved microbes relative to the environmental conditions in which they grew, we can provide information on the parameters that govern the formation of siliceous oncoids in a modern hot-spring system. Methods Siliceous oncoids were collected from six localities (A–F) around Champagne Pool (Fig. 1). We measured the field temperature and pH of water samples collected at sites of oncoid growth. The water samples were analysed at the Institute of Geological and Nuclear Sciences, Wairakei Research Centre. Large (7 5 cm) thin sections were made of selected oncoids so that their overall fabric could be determined. Powdered samples of some oncoids were analysed by X-ray diffraction (XRD) analysis in order to determine their mineralogy. Small fractured samples of oncoids, mounted on standard SEM stubs, were examined on a JEOL 6301F Field Emission Scanning Electron Microscope (FE-SEM) with an accelerating voltage of 2.5–5.0 kv. An attached Princeton Gamma- Tech analyser, capable of light element detection, was used to establish 89