MINERALOGIC AND TEXTURAL TRANSFORMATIONS OF SILICEOUS SINTER: EXAMPLES FROM NEW ZEALAND

The Taupo, Northland and Coromandel regions, North Island, New Zealand, have recorded surface manifestations of geothermal systems dating from the Miocene to the present, and hence span ~20 m.y. of geologic history in one of the most volcanically active regions on Earth. In particular, more than four dozen siliceous sinter deposits are known. A preliminary study of 29 grab samples of low-temperature sinter facies from global locations allowed construction of a sinter aging model, showing concomitant changes in both mineralogy and textures (Herdianita et al., 2000). Specifically, silica phase transformations of sinter follow those reported for wood petrification and diagenesis of siliceous marine sediments: noncrystalline opal-A transforms to paracrystalline opal-CT and/or opal-C, and then to microcrystalline quartz plus moganite. This transformation requires moisture, and is affected by factors such as presence of other minerals, organic matter, excess heat, and time. Unassisted, the complete sequence of silica mineral phase change appears to be accomplished in 30,000 to 40,000 years. Subsequent detailed studies of specific New Zealand sinter deposits along neotectonic fault escarpments and in boreholes have shown the same pattern, with mineralogic transformations captured "frozen" in stratigraphic succession. Increases in structural ordering of silica during sinter diagenesis, revealed by X-ray powder diffraction, are accompanied by progressive reductions in porosity and water content, and an increase in particle density. Textures change at a micro-scale along two distinct pathways - fibrous or granular - from an initial starting point of freshly deposited, smooth spheres of opal-A, to a final end product of equant/bladed microcrystals of quartz. Biotic and abiotic inclusions (e.g., microbial filaments, plant molds, sinter fragments, peloids, pisoids) are commonly well-preserved in sinters of varying ages, although patchy obliteration of primary textures has been observed in post-depositional fabrics, such as massive, mottled, diffusely layered, quartzose sinter. Consequently the aging of silica sinter recorded from New Zealand's geothermal areas provides a series of taphonomic windows along a continuum of mineralogic and textural change.