ABSTRACT

Ohaaki Pool was the main hot spring in the Broadlands-Ohaaki geothermal area before recent anthropogenic modification. The alkaline Na-HCO₃-Cl water, which discharged at 95°C with a flow rate of 10 l·s^-1, flowed down a broad discharge apron into the Waikato River.

The discharge apron was inhabited by thriving microbial communities that included Calothrix, Phormidium, and numerous Synechococcus. These microbes mediated the construction of columnar stromatolites around the edge of Ohaaki Pool, oncoids in the discharge channel, and intercalated stratiform stromatolites, "Conophyton", and coccoid microbial mats on the distal part of the discharge apron. All the microbes were variably replaced and encrusted by amorphous silica while alive or shortly after death. Consequently, fabrics in the siliceous sinters around Ohaaki Pool are controlled by the growth patterns and composition of the microbial community. For example, the Calothrix-dominated community gave rise to stratiform stromatolites that are characterized by alternating erect and prostrate laminae. Conversely, the Synechococcus-dominated communities formed mats that produced dense, white siliceous laminae. The Phormidium-Synechococcus community constructed the "Conophyton".

Platy, skeletal, and blocky calcite crystals are found in, around, and between the siliceous stromatolites that formed around Ohaaki Pool, the proximal part of the discharge apron, and in the oncoids. Although minor calcite is found locally in the stratiform stromatolites on the discharge apron, there is no evidence that microbes played any role in calcite precipitation.