| Abstract |
Sinter Island on Lake Ohakuri (10 m x 7 m, ~3 m above lake level) in the Orakei Korako geothermal area, Taupo Volcanic Zone, New Zealand, contains a large extinct vent, hemispheroidal stromatolites and bedded sinter rich in microbial filaments. Despite its young age (456 ± 35 years BP), this ancient hot-spring deposit preserves the complete diagenetic sequence of silica phase mineralogies, confirmed by X-ray diffractometry and infrared spectroscopy, from opal-A to opal-CT to opal-C + quartz. Corresponding nano- to micron-scale morphologies include spheres and vitreous silica botryoids, bladed lepispheres, and merged irregular silica rods. Incipient ìfuzzyî quartz is represented by rows of criss-crossing rods/blades that are aligned along the c-axis, and showing the typical external habit of microcrystalline quartz. No microbial fabrics are evident in quartzose samples. The deposit also experienced spatially patchy dissolution, resulting in formation of unusual morphological features for some opal-A portions of the sinter. Three intervals of silicification (thin encrusting, thick botryoidal, granular to vitreous cavity-fill) occurred around filament clusters during alkali-chloride thermal discharge to create a moderately dense opaline deposit. Subsequent acidic conditions (acidic steam condensate or ascending acidic spring) ensued within the deposit, whereby patchy dissolution preferentially removed the last-formed silica to expose the underlying filaments in cavities. Continuing corrosion freed silica spheres from the encrusted filaments to accumulate loosely in the vugs. Where touching, the spheres then merged via linking, double-annular necks among adjacent spheres. Further dissolution reduced sphere sizes concavely inward and extended lengths of the connecting neck-points to develop star-shaped patterns with ìt-joints.î The multi-pointed stars eventually broke loose as necks thinned and sharpened during further corrosion, and many likely dissolved completely. Solubility at the negative radius of curvature of the neck between spheres is less than elsewhere on the spheres (Iler, 1979). Therefore, the lower the solubility, the easier it would be for the neck area to dissolve, and it also would be the first to be attacked by dissolution. Acidic conditions also may have resulted in surface spring manifestations, as irregular silica coatings, clays and diatoms accumulated in some cavities of the original sinter substrate. The change to acidic conditions also may have accelerated diagenesis of the siliceous mass, to produce quartz in less than 500 years. |