Origins of calcite in a boiling geothermal system

Abstract

The formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling, dilution, and condensation. In this paper we show how these processes control the occurrence, distribution, and stable isotope composition of calcite based on a study at Broadlands-Ohaaki. The two principal calcite occurrences in the Broadlands-Ohaaki geothermal system are: (1) as replacement of rock forming minerals and volcanic glass; and (2) as platy crystals infilling voids. Both are stable over a broad temperature range from 160 degrees C to 300 degrees C. Replacement calcite is widespread and forms through hydrolysis reactions involving calcium alumino-silicates and sub-boiling liquids that contain 0.3 to 0.75 m CO 2 . Platy calcite, in contrast, forms over a restricted vertical interval of a few hundred meters within the upflow zone. It precipitates from boiling fluids through exsolution of carbon dioxide as indicated by coeval liquid-rich and vapor-rich fluid inclusions and its formation in the two-phase zone. Fluid inclusion data help to define the boiling paths of fluids from which platy calcite formed. Homogenization temperatures range from 160 degrees C to 310 degrees C and are consistent within the present geothermal regime. Ice melting temperatures range from 0.0 to -1.0 degrees C and indicate the presence of up to 0.5 m dissolved carbon dioxide. Model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fluid dictates the depth of first boiling. Most fluid inclusion data lie along a model boiling path characteristic of the centre of the upflow zone, in which the rising fluid (initially containing 0.75 m CO 2 ) begins to boil at approximately 320 degrees C and approximately 2000 m depth; data from well Br-18 instead matches a curve in which the rising fluid (initially containing 0.53 m CO 2 ) begins boiling at approximately 245 degrees C and approximately 900 m depth. The shallowing of the depth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water, as it rises and mixes with marginal waters. Calcite precipitates from both shallow formed steam-headed groundwater and deeply derived chloride water, and these waters are isotopically distinct. At Broadlands-Ohaaki, the delta 18 0 values of calcite at 200 degrees C range from 0.5 to 7.5 per mil, whereas delta 18 0 values of calcite at 200 degrees C range from 4 to 10 per mil. Taking appropriate temperature dependent fractionation factors into account, these data indicate equilibration with chloride water (delta 18 0 H2O = -4.5 per mil) and steam-heated groundwater (delta 18 O H2O = -7.0 per mil), respectively. Oxygen isotopes of hydrothermal calcites in the nearby Wairakei and Waiotapu geothermal systems show similar patterns, consistent with the occurrence of both chloride and steam-heated waters there. Calcite formation is explained by a model that describes the distribution of two-phase conditions and aqueous carbon dioxide concentrations in a column of hydrothermal fluid rising through a rock matrix of isotropic permeability. In this ideal situation, platy calcite forms along the inner margin of the two-phase zone, having the shape of an inverted cone, whereas replacement calcite mostly forms in the surrounding one-phase liquid-only zone. The sparse occurrence of calcite at less than or equal to 800 m depth in the central upflow of the Ohaaki sector at Broadlands-Ohaaki is compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling deeper in the system.