Towards a Unified Theory on Calcite Formation in Boiling Geothermal Systems

Abstract

The formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling, dilution and condensation. Replacement calcite forms in rock-dominated environmentswhere sub-boiling liquids contain relatively high concentrations of dissolved carbon dioxide, while platy calcite forms in fluiddominated environments where rising two-phase fluids carbon dioxide through boiling. The distribution of these two occurrences reflects the distribution of boiling conditions within a phase of steady hydrothermal activity. In the ideal situation, platy calcite forms along the inner margin of the two-phase zone, having the shape of an inverted cone, whereas replacement calcitemostly forms in the surrounding one-phase liquid-only zone. The sparse occurrence of calcite at m depth in the central upflow of the Ohaaki sector at Broadlands-Ohaakiis compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling'deeper in the system.