Morphologic and Mineralogic Transitions From Opal-A to Opal-CT in Low-Temperature Siliceous Sinter Diagenesis, Taupo Volcanic Zone, New Zealand

The opal-A to opal-CT silica phase transformation in New Zealand’s siliceous hot-spring deposits (sinter) is accompanied by re-curring textural changes at the micron scale. Recognition of these changes is essential to extract paleohydrological, paleoenvironmental, and paleobiological signatures from ancient hydrothermal systems, and to recognize diagenetic overprints upon primary depositional signals. We examined 39 samples of siliceous sinter from the low-temperature (, 358 C) palisade microfacies at Orakei Korako and Te Kopia geo-thermal fields, Taupo Volcanic Zone, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weath-ered). Friable to indurated samples were collected from modern, pres-ently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs, to sinter; 3,500 years old. The progressive miner-alogical change from noncrystalline opal-A to paracrystalline opal-CT was traced by X-ray powder diffraction and scanning electron micros-