ABSTRACT

Amorphous silica and calcite form the deposits in the vent and on the discharge apron of Waikite Spring 100 (WS-100), which is located in the Waikite Geothermal area on North Island, New Zealand. These precipitates formed from spring water that has a temperature of >90°C and a pH of 8.1-8.8. The opaline silica is restricted to areas around the vent where cooling and evaporation of the spring water triggered precipitation. The calcite deposits in the spring vent and on the discharge apron are formed of large (up to 15 cm long) asymmetrical dendrite crystals that are characterized by multiple levels of branching. Branches grew preferentially from the downflow side of their parent branch. All branches have a trigonal transverse cross section except in areas where competition for growth space induced merger of neighboring crystals.

The primary branches of the dendrite crystals are (sub)perpendicular to the substrate even in areas where the discharge apron slopes at a high angle (up to 80°). On the steeper parts of the discharge apron, the plate-like primary branches form the floors of the small terrace pools whereas their distal edges form the rims of the pools.

Growth of these dendrite crystals is attributed to abiotic processes. High levels of saturation with respect to calcite were caused by rapid CO₂ degassing of the sheets of spring water that flowed down the steep discharge apron. Calcite crystals with different crystal morphologies characterize other springs near this spring. The variation in crystal morphologies from spring to spring is attributed to different levels of saturation that are related to the initial PCO₂ of the spring water upon discharge and the rate of CO₂ degassing at each spring.