Vapor‐Dominated Zones Within Hydrothermal Systems: Evolution and Natural State

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Abstract
Three conceptual models illustrate the range of hydrothermal systems in which vapor‐dominated conditions are found. The first model (model I) represents a system with an extensive near‐vaporstatic vapor‐dominated zone and limited liquid throughflow and is analogous to systems such as The Geysers, California. Such systems can evolve within low‐permeability barriers without changes in boundary conditions or rock properties, given an adequate supply of heat. Their scarcity in nature may be due to the need for a long‐lived, potent heat source and for a low‐permeability aureole that remains intact for significant lengths of time. Models II and III represent systems with significant liquid throughflow and include steam‐heated discharge features at higher elevations and high‐chloride springs at lower elevations, connected to and fed by a single circulation system at depth. In model II, as in model I, the vapor‐dominated zone has a near‐vaporstatic vertical pressure gradient and is generally underpressured with respect to local hydrostatic pressure. The vapor‐dominated zone in model III is quite different, in that phase separation takes place at pressures close to local hydrostatic and the overall pressure gradient is near hydrostatic. A relatively large number of high‐temperature systems in regions of moderate to great topographic relief are similar to either model II or model III; however, in most cases there are insufficient data to establish a single preferred model.