The Fluid Inclusion and Mineralogic Record of the Transitionfrom Liquid- to Vapor-Dominated Conditions in The Geysers GeothermalSystem, California

Abstract

The Geysers, an active vapor-dominated geothermal system related to a hypabyssal granitic pluton, was liquid dominated from its inception at 1.2 to ~0.3 Ma when the modern vapor-dominated regime formed. As the hydrothermal liquids boiled off, a vapor-dominated heat pipe developed in which the flow of steam was balanced by downward percolation of condensate. A record of the transition from liquid- to vapor-dominated conditions is contained within late-stage quartz and calcite crystals that are common in the upper part of the steam reservoir and overlying cap rock. Samples from three localities were examined in detail. Quartz from these veins is characterized by corroded crystal faces, solid inclusions of contemporaneous calcite that are generally etched, coeval high- and low-salinity fluid inclusions, primary low-salinity liquid-rich inclusions, and abundant vapor-rich inclusions. Primary vapor-rich inclusions up to 2 mm across were trapped mainly on rhombic and prism faces of skeletal quartz crystals during periods of rapid growth. These inclusions nucleate crystals of CO2 that disappear by sublimation rather than melting, at temperatures ranging from –92.9° to –56.7°C. Liquid-rich fluid inclusions from veins near the top of the modern steam reservoir yield homogenization temperatures ranging from 304° to 188°C. These temperatures commonly exceed boiling point to depth relationships based on the paleoground surface, suggesting that pressures were initially greater than hydrostatic. Temperature-salinity relationships indicate that the early, shallow liquids had salinities of 1.5 to 2 wt percent NaCl equiv and that boiling began by ~300°C. As the fluids boiled off, salinities of the residual liquid increased to 3 to 4 wt percent NaCl equiv. Trapping of low-salinity fluids (0.0–0.5 wt % NaCl equiv) interpreted as condensate in primary liquid-rich inclusions suggest vapor-dominated conditions had begun to develop by the time temperatures had dropped to 290°C. The primary origin of these inclusion fluids indicates that they were capable of depositing quartz and calcite. During the evolution of the vapor-dominated system, the hydrothermal fluids were at different times, but at the same location, undersaturated, in equilibrium, or oversaturated with respect to quartz and calcite. The hydrothermal fluids initially deposited quartz and calcite as they boiled off. Dissolution of these minerals occurred as the condensate, which was initially undersaturated with respect to quartz and calcite, percolated down. Subsequent deposition of these minerals occurred as the condensate, or mixtures of condensate and boiled liquid that leaked back into the fractures, vaporized during periods of intermittent pressure reduction. The top of the vapor-dominated system migrated downward with time as the overlying fractures were progressively sealed off. Today, pressures within the steam reservoir are vaporstatic and temperatures range from 240° to at least 342°C.