ORIGINS OF CALCITE IN A BOILING GEOTHERMAL SYSTEM

The formation of hydrothermal calcite relates to the movement of carbo n dioxide in a geothermal system as governed by boiling, dilution, and condensation. In this paper we show how these processes control the o ccurrence, distribution, and stable isotope composition of calcite bas ed on a study at Broadlands-Ohaaki. The two principal calcite occurren ces in the Broadlands-Ohaaki geothermal system are: (1) as replacement of rock forming minerals and volcanic glass; and (2) as platy crystal s infilling voids. Both are stable over a broad temperature range from < 160-degrees to >300-degrees-C. Replacement calcite is widespread an d forms through hydrolysis reactions involving calcium alumino-silicat es and sub-boiling liquids that contain 0.3 to 0.75 m CO2. Platy calci te, in contrast, forms over a restricted vertical interval of a few hu ndred meters within the upflow zone. It precipitates from boiling flui ds through exsolution of carbon dioxide as indicated by coeval liquid- rich and vapor-rich fluid inclusions and its formation in the two-phas e zone. Fluid inclusion data help to define the boiling paths of fluid s from which platy calcite formed. Homogenization temperatures range f rom 160-degrees to 310-degrees-C and are consistent within the present geothermal regime. Ice melting temperatures range from 0.0-degrees to - 1.0-degrees-C and indicate the presence of up to 0.5 m dissolved ca rbon dioxide. Model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fl uid dictates the depth of first boiling. Most fluid inclusion data lie along a model boiling path characteristic of the center of the upflow zone, in which the rising fluid (initially containing 0.75 M CO2) beg ins to boil at approximately 320-degrees-C and approximately 2000 m de pth; data from well Br-18 instead matches a curve in which the rising fluid (initially containing 0.53 M CO2) begins boiling at approximatel y 245-degrees-C and approximately 900 m depth. The shallowing of the d epth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water, as it rises and mixes with marg inal waters. Calcite precipitates from both shallow formed steam-heate d groundwater and deeply derived chloride water, and these waters are isotopically distinct. At Broadlands-Ohaaki, the deltaO-18 values of c alcite at >200-degrees-C range from 0.5 to 7.5 permil, whereas deltaO- 18 values of calcite at <200-degrees-C range from 4 to 10 permil. Taki ng appropriate temperature dependent fractionation factors into accoun t, these data indicate equilibration with chloride water (deltaO-18(H2 O) = -4.5 permil) and steam-heated ground water (deltaO-18(H2O) = - 7. 0 permil), respectively. Oxygen isotopes of hydrothermal calcites in t he nearby Wairakei and Waiotapu geothermal systems show similar patter ns, consistent with the occurrence of both chloride and steam-heated w aters there. Calcite formation is explained by a model that describes the distribution of two-phase conditions and aqueous carbon dioxide co ncentrations in a column of hydrothermal fluid rising through a rock m atrix of isotropic permeability. In this ideal situation, platy calcit e 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 calci te at less-than-or-equal-to 800 m depth in the central upflow of the O haaki sector at Broadlands-Ohaaki is compatible with this model and ap pears related to the exsolution of dissolved carbon dioxide through bo iling deeper in the system.