• Calcite;
  • hot spring;
  • opal-A;
  • silica sinter;
  • spicular microstromatolite

ABSTRACT A silica–carbonate deposit is forming from the dilute alkali chloride waters of Pavlova spring, a small thermal pool and outflow channel (85 to <40 °C), situated at the northern extent of the South Orakonui area of the Ngatamariki geothermal field, Taupo Volcanic Zone (TVZ), New Zealand. It is one of a small but growing number of thermal spring features known to yield deposits of mixed mineralogy. At Pavlova, a distinctive, crustose, chalk-white, meringue-like sinter, comprising non-crystalline opal-A silica with subordinate calcite, is actively precipitating both around the margins of and as small islets within the spring, with an average accumulation rate of ≈ 2 mm year−1. Both emergent and partly submerged substrates host the sinter, including fallen pine branches, twigs, needles and cones, gum leaves, grass blades, bracken fronds, pumice, sediment and microbial mats. The sinter is thin (25–35 mm thickness), finely laminated and contains three distinct types of stacked horizons. Submerged basal layers constitute stratiform to undulatory microstromatolites with pseudocolumns, which grew outwards and upwards on narrow twig nuclei. Emergent middle layers comprise discontinuous, spicular microstromatolites (to 10 mm height), with prostrate and erect microbial filaments, silica spheres and silicified mucus, overlain by silicified structures of probable fungal origin. In places, lower and middle sinter layers are capped by white, smooth, convex surfaces that coalesce into subdued, curved ridges, resembling laterally continuous peaks of egg-white meringue. The meringue is internally laminated, with fossilized microbes preserved in thin horizons. Small lensoid masses of calcite crystals nestle between silica laminae throughout the sinter. The near-neutral (pH ≈ 7·2) spring water is a dilute chloride-carbonate type (HCOinline image≈ 470 µg g−1, Cl≈ 600 µg g−1) with low inline image (≈ 50 µg g−1), typical of TVZ thermal fields where deep chloride fluid mixes with CO2-rich, steam-heated shallow waters before discharge. The hot water changed little in composition from 1993 to 1999 and, despite dilution by meteoric waters, contains sufficient SiO2 (≈ 220 µg g−1) for opal-A to deposit at the surface upon cooling. However, the concentration of Ca2+ (≈ 6 µg g−1) is such that the precipitation of calcite is not expected without modification of spring waters. Precipitation occurs by evaporation of thin water films at exposed substrate surfaces, via meniscoid as well as capillary creep (wicking), through porous sinter horizons and across emergent vegetative surfaces in contact with spring water or steam. The height of the deposit above the water surface is restricted by the upper limit of moisture bathing these substrates. Splash and spray are not involved in the formation of Pavlova spicular microstromatolites, as is the case for other texturally similar deposits from hotsprings elsewhere. This young (< 15 years), mineralogically and morphologically complex hot-spring deposit exhibits > 10 times lower accumulation rates than typical siliceous sinters in the TVZ, and deposition of both silica and calcite is controlled by microchemical conditions and local temperature gradients, rather than by bulk spring water chemistry.